Cliff's Quadrajet Parts and Rebuild Kits
General Category => Quadrajet Carb Talk and Tips => Topic started by: tayto on August 26, 2018, 07:26:37 PM
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Cliff we talked last Sunday on the phone about selecting a roller cam for my 350. To refresh your memory it's a 350 bored .030" over with flattop pistons. squish will be in the in the .035-.038
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What are the rest of the engine specs?
Static compression ratio?
What brand?
What cylinder heads?
Type of vehicle?
Auto or stick shift?
Converter stall speed?
Rear gear ratio?
Intended use of the vehicle, etc?
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What are the rest of the engine specs?
Static compression ratio?
I think with flattops bored .030 over, .035 squish and 64cc heads its in the 10.2-10.3 range
What brand?
SBC 350 roller block
What cylinder heads?
191 TBI heads, 64cc
Type of vehicle?
1986 GMC 1500
Auto or stick shift?
Auto
Converter stall speed?
1800
Rear gear ratio?
Has 3.08s but I will be going to 4.11s
Intended use of the vehicle, etc?
Hauling/towing, want a nice driver
My answers are in BOLD. I need cam to have a fuel pump lobe, I do not want to retrofit an electric pump. There is a cam grinder locally that can grind to my specs. but i wil have a reduced base circle and have to run longer push rods.
We talked about the 350 TPI cam, 202/207 @ .050 .404/.4 114.5 LSA. These apparently have a fuel pump lobe on them even though they were used in a fuel injection application. You had mention when we talked about something in the 210 duration and no less than 112 LSA.
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Will your cam grinder to a HR grind or are you talking flat tappet?
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Hydraulic roller
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I'd have him grind one based on the 350 TPI profile with about 6-10 additional degrees of seat timing. This will reduce octane requirements at the higher compression ratio and still idle smooth, and have pretty much the same street manners as the 202/207 TPI cam would in a 350 build around 9 to 1 or so compression.
Another option is to just use the 202/207 grind an put high ratio rockers on it. This will add some effective duration and probably less cost than have a custom cam ground for it.......Cliff
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I will already be running 1.6 fill roller rockers, unfortunately they are the self aligning type and I am limited to higher ratios...
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I found the correct TPI roller cam with fuel pump lobe on eBay for $100. The local cam grinder only chargers $175 to regrind cams, maybe less depending if there is less grinding involved. With the TBI what should I shoot for for lift? The TPI cam with 1.6 rockers is .430/.442 should I shoot closer to .500 if I get a custom grind?
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I found a set of 1.7 Scorpion aluminium full roller rockers, would this be a better choice rather than regrinding the cam? Or maybe regrind and use both?
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1.7 ratio rockers on the 202/207 TBI cam would be a home-run, IMHO......Cliff
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Thanks for the help cliff. Will let you know how engine turns out in the next few months. get block back from getting decked and crankshaft balanced next week then can start assembly.
FYI, apparently the 90-92 "773" TPI cam is the same duration and LSA but has about .010" more net lift VS the the 87-89 "643" TPI cam that I purchased. I have a feeling the later cam does not have a fuel pump lobe but have not seen one to verify.
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Cliff, finally got short block assembled, will be degreeing cam in the next few weeks. Do I want to install it "straight up"? due to it being a factory cam, I have no cam card. are the duration @ .050" enough for me to degree correctly?
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Yes, I always install them "dot to dot" and in most cases don't have to move them, but I do use factory timing sets, not aftermarket as they tend to be more accurate....
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I am using the "heavy duty" GM timing set that came on the roller motors.
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Cliff, I have measured my deck to piston height on my short block. I took a measurement on the left, then right of each piston added the values and divided by 2 to get an average. It varies cylinder to cylinder from .0125" to .0165" in the hole. Probably what I get for using cheap summit brand pistons or budget Eagle rods. Originally I was going to run the .028" GM head gaskets you had suggested but this would put my quench between .0405" and .0445". After talking to you I have found some Cometic .023" headgaskets on summit that would put my quench between .0355" and .0395". I talked to the machinist that decked my block and he thinks I am being too picky and should just run the the GM head gaskets and "call it a day". Usually when I succumb to this pressure I end up with something I am not happy about...
What do you think?
BTW, my compression is basically bang on 10:1 with the .023" headgaskets.
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Tight quench improves combustion efficiency. The engine will need less timing and fuel to make best power, plus it will run cooler.
The higher compression makes more power at every RPM as well.
There should be lower cost shim type gaskets available. GM used .020" steel shim gaskets for decades, I've even seem them already coated......Cliff
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I can only find a steel shim .020" headgasket made by Mr. gasket p/n 1130G. I thought running under .035 put me in the danger zone of piston to head contact?
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Some engine builders shoot for .025" with steel rods. I shoot for .035" with my engines so I sleep better at night!....
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You think the Mr. gasket .020" will work ok? I have not been able to find a GM part number, they might not even make them anymore? The Cometic gaskets are pricey but i lile thay they are multilayered abd reusable.
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The Mr. Gasket 1130G will not work with my heads as they are the "lightweight" heads with extra exhaust bolt... I found some copper shim gaskets that are .021" https://www.summitracing.com/int/parts/sce-511152/applications/year/1987/engine-size/5-7l-350 (https://www.summitracing.com/int/parts/sce-511152/applications/year/1987/engine-size/5-7l-350)
(https://static.summitracing.com/FileServices/images/usercontributions/xlarge/-Mr-Gasket-1130G-20168133502169.jpg?rep=false)
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Cliff one more question concerning the HD timing chain, do you find they stretch much after "break in" like the bicycle chains do? should i advance my cam with an offset key? think you can get them in 2 and 4 degree offsets.
I degreed the cam, even without a cam card. I found the ICL @ 120* and LSA @ 115* if i'm understanding correctly the cam is already 5* retarded? i installed dot to dot with #6 @ TDC.
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Those chains don't require additional advance to account for chain stretch.
120 ICL? I would think it would be a lot further ahead than that. What are the cam specs?
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Internet says 202/207 @ .050" duration. .269"/.277" lobe lift. 114.5* LSA.
This is what I found
Intake
Lobe Lift: 0.262"
Open @ .050" 18* ATDC
Close @ .050" 42* ABDC
Duration 204*
ICL 120*
Exhaust
Lobe Lift 0.265"
Open @ .050" 34* BBDC
Close @ .050" 8* BTDC
Duration 206*
ECL 111*
LSA 115.5*
I plugged numbers into an online cam calculator and it's almost like my Intake and Exhaust are backwards? I did all measurements on cylinder #1, intake lifter furthest away from the front of the engine and exhaust closest. This was my first time degreeing a cam, I think I made some errors along the way...
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Try checking the intake open point at .050" and see if it's close to 18 ATDC......
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when i checked the intake @ .050 it was 18* ATDC
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How far are your pistons down in the hole? If it hasn't been decked they are usually .02-.025 down. Felpro makes a .015 gasket I think the part number is 1094.
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If the intake open point checked right on the money then the cam is installed correctly.....Cliff
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How far are your pistons down in the hole? If it hasn't been decked they are usually .02-.025 down. Felpro makes a .015 gasket I think the part number is 1094.
block has been decked. pistons are .0125" to .0165" in the hole. i am going to bite the bullet and buy the Cometic gaskets as I don't want to tear down the block again. with .023" thick gaskets that will put my quench in the .035 to .040". i am not comfortable going under .030" quench with .015" gaskets.
If the intake open point checked right on the money then the cam is installed correctly.....Cliff
I guess it's right on the money then, thanks Cliff!
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The ICL method can be confusing and difficult to obtain good results especially if you are going back and forth with the crank instead of in one direction only.
As many engines as I've done over the years I just check the intake open point of the #1 intake lobe and leave it alone if it's at or close to where it needs to be.
I do NOT use double roller or multiple keyway timing sets, only factory link belt type and source out the 3/4" wide sets for my engine builds. This makes moving the cam around a little more difficult, but well worth the effort, IMHO.
EVERYONE out there since WAY back in the 1970's when I got into this hobby runs right out and buys a "double roller" timing set like it's some sort of upgrade. Most are just OK some are a little better and some are just plain JUNK. The ones that fall into the decent category will have billet sprockets, seamless full roller chains and multiple keyways to move the cam around. I still woln't use them here in my own engines.
One of my mentors ran a very successful Stock Eliminator Camaro and told me decades ago that the factory timing sets and much stronger than the roller variety, and they have constant tooth contact, absorb some harmonics, and will last the life of the engine.
So jump ahead to 2000 when I built my first 455. The company I sourced all the parts from highly recommended that I purchase a 9 keyway Rollmaster timing set, so I went off course from what I typically do and bought one. It did fine for a few years but I was driving the car almost every day and racing it as often as I could get it to the track.
I was racing in Northern Ohio one evening and the car started to slow down, WAY down, but oil pressure was fine, fuel pressure was fine, no internal noises, etc. I put the car on the trailer and pulled the engine over the weekend. Figured I freshen it up and restore power. Turns out the ONLY thing wrong with it was that the very expensive double roller seamless timing chain had failed. It was so loose you could just about walk it right off the gears!
I replaced it with a Melling 3/4" wide set and the engine was fine till I replaced it in 2009. The new engine has a Melling timing set in it and just happened to have the water pump/timing cover off recently to repair a coolant leak and the timing set looked perfect........FWIW......Cliff
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Cliff, ended up selling TPI cam/rockers to a friend that needed them for his TBI truck. I found a replacement cam from Howards Cams, it has 10* more duration than the TPI cam (202/207) as you recommended previously... It also has a fuel pump lobe because I am not running a e-pump.
Specs:
266/270 @ .006"
213/217 @ .050"
.485"/.495" lift w/ 1.5 rocker
I can get the cam either 112* or 114* LSA? What would be best for me? My truck will be used for hauling/towing. believe my transmission guy recommended/sold me a 1800 stall converter for what I was doing.
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Tough call there. Wider LSA will idle better and slightly broader power range and peak cylinder pressure slightly. I think you'd be OK either way......Cliff
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I'm running tbi heads, apparently they start to run out of steam around 4500 rpm if that helps?
will the 112* help more with my higher compression?
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It will add some overlap but also close the intake early if you put it at 108ICL.
Sort of a trade off, but I think I'd go 112LSA with those heads to pull power down slightly and tighten up the power curve for the RPM range.....
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The GM845 cam would be a good choice for you as well if your running a late model block with a step nosed cam. they ran alot of these in 350's making good power. they have a fuel pump lobe.
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The GM845 cam would be a good choice for you as well if your running a late model block with a step nosed cam. they ran alot of these in 350's making good power. they have a fuel pump lobe.
I believe cam is discontinued but crane still grinds it. Too bad crane thinks their cams are made out of gold, over $600 canadian, howards cam is $380.....I was also told by a fellow that played around alot with TBI heads that they really like straight pattern cams and to not have more than a 5* split in duration. The Howards cam fits this bill and is 10* more than the TPI cam thay Cliff recommended.
Cliff, I assume the 108 is already ground into the cam when installed "dot to dot". I could figure out when the intake closes and check my dynamic compression. I could install it "straight up" on a 112* ICL??
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they are $219.00 at jegs.
I had the 847 cam years ago and it was a steel billet core.
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not sure what part number you are searching but the crane # is 109821 and it's not cheap.
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Sorry, your're right thats the wrong part #
However, they are still available at GM performance for $300.00
part # 24502586
214/224 488/500 lift on 112 l/s angle
these are nice cams on a hot 350
I believe they are on a billet core.
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I wasn't even aware that GM made any roller cams on billet cores. Do they specify this someplace in the specifications?.....
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that's the LT4 hot cam, duration is 218/228. I actually HAVE one brand new but its SUPPOSE to go into my buddies boat....
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The LT4 "Hot" cam is excellent. I've used them here a number of times in the 1987 and later blocks. It's a drop-in for over 1hp/CID depending on heads used.
I customer recently used my basic "recipe" to build his 355. His used Trick Flow heads, 64cc chambers, quench at .035", flat top pistons, GM LT4 cam, high ratio rockers, factory iron intake, Q-jet, HEI and good set of headers.
He brought the vehicle here for custom tuning as the Q-jet he built for it was down about 15 HP to the shops dyno mule Quick Fuel carb. I found a few issues with the Q-jet and got it up to par for the combo. We took the vehicle out and it was an "animal". Very impressive for a "little" 355 build and had excellent street manners as well.........Cliff
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I am worried it may be to much cam for the TBI heads? The heads don't need much over .450-.500" lift. I could just run 1.5 roller rockers and I will need to upgrade my valve springs.
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The early Vortec heads were excellent for low and mid-range power.
I've used them on 350's for towing applications and they make the grade well there. Never tried or even considered using them in a "high performance" application where upper mid-range and top end power was part of the equation.......Cliff
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The motor is for towing /hauling that is why I went with the TBI heads. If you think the LT4 hot cam is a good choice then I will use it especially because I already have it. My compress is 10:1 so I am definetly not "stock" any more. If not I will go with the Howards cam as it seems to match nicely with what we originally talked about. Thanks Cliff.
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Towing/hauling requires a cam with smooth idle and strong power right off idle and in the lower RPM range. The LT4 cam even in a 350 build even at 10 to 1 compression will be a little "soft" below 2500rpm's but really come on strong in the 3500-4500rpms range with those heads....IMHO.
I'd rather see you use a smaller cam and pull the power down a bit. It's not going to need a lot of timing any place but will make great power and be fine with custom tuning on pump gas.
WAY back when my dad's 1991 350 HD truck engine cracked a piston at 240,000 miles we bored in .030" and put flat tops in it, and the stock early style Vortec heads. I replaced the flat cam with one we had here from a late 1980's 305 IROC Z-28 engine. Going from memory it was a pretty small cam without much lift. It ran flawlessly with the higher compression and made butt-tons of low and mid-range power. It also kicked the fuel economy up to about 16-18 average and it would get over 20 at times on the highway if you drove it like you had some sense.
It was pretty darned impressive to all of us at it's towing capabilities. He had a huge steel horse trailer and would load it up with 4 mules and all the gear, plus put another trailer behind it with his covered wagon on it. The truck was a 4 x 4 HD 3/4 ton with a NV-4500 trans and 3.73 gears. It yanked that deal around with great authority for a "little" 350 engine.
Prior to the engine upgrades it did OK but you were really having to run the "bag" off of it to get up to speed or in hilly country. He ran with a bunch of "high rollers" with big diesel rigs and was able to keep up with them without a lot of trouble with the new engine. Just amazing to all of us how much better one of these engines does with some compression in it and cam/heads targeted for the RPM range it's going to be used in.......Cliff
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sounds good cliff, i will stick with the Howards cam 213/217 112 LSA and keep the hot cam for the boat.
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Cliff, I am getting around to rebuilding my carb. Before I went to the Howards cam posted above, you sent a rebuild kit with 73 Jets, 50C rods and an orange spring. Will this be ok with a cam with about 10* more duration?
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They will be fine. The big deal with cam selection is idle tuning. Once off idle all of these engines make good vacuum at light load and require very similar fuel delivery. The vacuum production at idle varies considerably, and with larger cams we need larger idle tubes, DCR's, and bypass air......
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does anyone know what is float height & other specs for this carb? #17086249
the literature cliff gave me only goes up to 1980, this carb is 1986 (non-CCC)
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Factory float height on this unit is 13/32" +- 2/32".
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thanks keith, you have any other specs for it?
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never mind I did some serious searching and found it in this document:
https://quadrajetparts.com/pdfs/4009B.pdf
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well, it only took 2 years but i finally have the engine together. had some issues with rocker studs (not enough thread engagment with my roller rockers) so i had to pull them off and have them machined and tapped. machinist took pity on me and allowed me to do them myself on his machine. in exchange I had to do the exact same to a set of camel hump heads for him. Was there all day but it was a good learning experience. I take the engine to the dyno on Monday morning. if successful it will be going back in my truck which is currently waiting for its new drivetrain.
Cliff, I had tried to text you awhile back but the machinist had recommended to advance the cam a degree or 2 to compensate for chain Stretch. dont know if this was the right thing to do or not but I did it and we will see how it turns out. (cam has 106 ICL vs 108 ICL). will keep you posted in dyno results.
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Well, almost a year later and back on the road. Had no oil getting to my rocker arms last summer which I discovered on a road trip. Limped car home and tore engine apart and went back to machine shop. after some testing we found oil was pouring into the top of the intake valley from the lifter bores. Found GM had drilled some weird counter bore 3/16” into the lifter bore. Block was out if an 87 GMC,had roller lifter provisions bit came with a flat tappet cam. Anyway, having read various things the machine shop decided it would be best to just get a new block. Got engine back in January and started picking away getting it back in. Probably would have had it on the road back in April, but a LOT of screwed up and/or late parts orders. Anyway it's on the road again. BTW, cliff I set the cam to the cam card and DID NOT advance the cam 2*. It actually didn't seem to make any difference on the engine dyno.
Now onto why I posted. Need some help with idle tuning. Carb #17086249 cliff's kit with 73 jets and 50C rods. 355, .034" quench, TBI heads, 213/217 112 LSA cam, on the engine dyno made max power @ 26* total timing. Currently have initial @ 7* (+19 = 26* total). Haven't messed with dist curve yet. Currently making about 15" of vac at idle. My idle screw is turned in so far it is almost coil bound, so i am thinking I should add some idle air bypass? Also the idle is not consistent,it floats. You can actually hear it when sitting at a stop light with the windows down,it will sound ok then the tone changes and drones then goes back to normal and repeats. I did not measure anything when I rebuilt carb 2+ years ago, but everything is STOCK aside from the jets&rods.
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here's the weird counter bore on my old block if anyone is interested
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Nice dyno results Tayto! I always figured TBI heads would make excellent low-end torque with the right cam.
Sorry to hear of your idle issues. Are you using vacuum advance? Do the mixture screws have any effect?
And I would NOT run more than 24° total timing under any kind of load with those heads...at least not with American 87-octane gasoline.
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I have vac advance, currently on ported.I think I have 6-10 degrees,I have not messed with it since last summer. Have control with mixture screws,can definitely slow and speed up the engine. I dyno'd it with (canadian) Shell 91, current have 89 in there and it doesn't seem to mind it @ 26* all in... It's been very hot here lately 95-100*f and no audiable detonation.
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Opened idle mixture screws from 0.063" to 0.089". Upon taking carb out of service found i forgot to tighten the tube nut for the brake booster vacuum supply. Got my idle screw backed out a bit, and in-gear RPMs down to 550. Still have that random surge,all though it seems to be less now. Find it hard setting my idle mixture screws as the surge moves the needle. They're currently at about 5 turns out, but as I turn them in vacuum goes up slightly but hard to distinguish,sitting at about 12" in gear currently. Tried tipping in the choke and surge is still there. My timing curve starts coming in around 800 rpm. Not sure where to go next.
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I'd try stronger distributor spring(s). In almost all cases fluctuations and surging in engine speeds at idle are the timing is not ROCK solid below 1000rpm's.
I see this a LOT as nearly every troubled vehicle that comes in here for idle issues and carb work has some attempt with cheap offshore distributor spring/weight kits to get all the timing in early.
Every offshore distributor I've seen being sold these days also uses something similar and most do NOT pull all the timing out when the engine returns to idle speed.
I actually LOVE those parts, they have helped keep my wallet full for at least 40 years now by charging folks to remove all that junk and put the stock/correct parts back in place!.......
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I have 2 extra GM distributors, will check tonight for stiffer springs. I took truck on a little road trip Yesterday and got some rattle going up about a 1km Long steep incline. about halfway up i went from heavy throttle and back off then went WOT and it shifted down to 2nd and i heard rattle. Sounds like a tip in issue, but wondering if I may have to lower my total timing. I currently have no vac advanced hooked up as I am trying to only do 1 thing at a time and not confuse myself.
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Found a set of springs out of one of my parts distributors. Curve starts at 1050-1100 rpm. Timing mark doesn't move at idle now. Seems to have solved my light throttle tip-in detonation problem (REALLY noticable in 4th locked up at lower speeds), but still have a surge @ idle,it is slight but you can see it in the vac gauge and hear it. makes setting idle mixture a chore. I tried moving the chole closed still does it. Tried changing my initial from 0 all the way up to 15,still does it. It seems LESS with lower idle rpm, but that may be my imagination.
I notice when i swapped the springs and fired up the vac gauge was rock solid but as the engine warmed up it began moving as described above. truck has been sitting just over a hour before firing it up so it wasnt stone cold.
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Did you ever measure the idle airbleeds in that carb?
Some of the later carbs will have pretty large idle bleeds. I've seen a few applications where I was able to clean up idle surging, running a little rough, etc by going a little smaller with the IAB's even when I had some control with the idle A/F ratio with the mixture screws.....
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Cliff,had a busy weekend and didn't get around measuring orifices in the carb.hopefully will have time tonight after work. should I pull the idle tubes and measure them as well? thanks.
BTW, the heavier springs seem to have done the trick on my tip-in detonation problem and same with lugging in 4th. not like i like to do this but it happens from time to time until i re-gear. havent had a chance to recheck the heavy load heavy throttle detonation i experience but
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Idle Discharge Holes: .089" (I assume this is a another term for "idle mixture holes" ?)
Idle Bypass Air: .046"
Lower Idle Air Bleeds: .075"
Upper Idle Air Bleeds: .067"
Idle Channel Restrictions: .046"
Idle Tubes: .033"
Main Air Bleeds: .086"
Main Jets: 73
Primary Metering Rods: 50C
Accelerating Well (main casting): ??
Accelerating Well (air horn): .051"
Secondary Air Bleed Tubes: .070" (??)
Secondary Metering Rods: DR
Hanger: I
Fuel Inlet Seat: .135"
Float Level: 13/32”
Air Flap Open Distance: 1.400"
Secondary Flap Spring: 3/4 turn
Choke Pull-off Release Time: stock
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For sure that carb needs opened up some for what you are doing.
Did you happen to check the vacuum at idle speed, down around 700-750rpm's?
At a minimum I'd open up the idle tubes and DCR's and see how it responds. A little goes a long ways but .035-.036" on the idle tubes and .052" on the DCR's should really help things out......
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Cliff, opened up the idle tubes .036" and DCR's .052". Surge still there but definitely seems less. I will have a better feel for it tomorrow on my drive to work. Before I took carb off this evening was 14-14.5" hg @ 700 rpm in park. After about the same,maybe moreso 14"hg. Needle still vibrates but only swings about 1/2"hg or less. In gear (600 rpm) only about 13" hg. I figured i'd make more vacuum but i guess its the cam? Idle mixture screws only about 1.5 turns out. I think my APT is 4 or 4.5 turns up. Orange PP spring.
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You should see more sensitivity from the mixture screws adding some idle fuel.
With some set-ups you may have to close down the lower IAB's some to smooth out idle and get better control with the mixture screws.
I would run it for a for days to see if it likes what you did before making any more changes.
Did you happen to glance into the primary to see if there is any "nozzle drip"?
Advancing the camshaft may also be contributing to the idle deal. I'm not a fan of cams with advanced specs and early intake closing. If you want to tune an engine with "quirky" idle quality fine one with a Comp XE cam in it. I've had so many troubled engines brought up here for idle issues using those cams I hope to never see another one.
Not exactly sure why folks go that direction, but for sure short seat timing, tight LSA and advanced intake lobe positions can make idle tuning difficult in some engines........FWIW.....
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Cliff, the surge is definitely a lot better after driving it for the last 2 days. It's still there but much more stable and idle seems quieter. Cam was degree'd a 107*, cam card says 108* but it was more like 107.5*. I checked before and didn't see any nozzle drip but will check again. Since the mods my idle screw has been backed up a little. Also found it seems to take off with less throttle idle since mods.
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That's all telling you likes more fuel to the mixture screws and transfer slots, which most combo's do.....
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Cliff, what's the next step with closing up the LIABs? Use a check ball to peen them smaller then redrill? Or should I open the idle tubes or DCRs more?
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I tried that with mine, but couldn't get good results & I used 6-32 brass set screws.
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sounds good, i'll see if i can source some locally. Did you just use red loctite to secure them? what size should i drill them to? currently @ 0.075"
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At .075" i would NOT touch the lower idle airbleeds.
For these units i often use .038" idle tubes and .052"-.055" DCR.
Also, i would not use less than 650 rpm´s idle speed in gear (A/T).
And, don´t be afraid using full manifold vacuum to the vacuum advance, some engines really like this, you may try this before anything else.
JMHO
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The lower IAB's can have a major impact on idle tuning with some engine combinations.
For "mild" engines making good vacuum you may be fine leaving them at .075" as Kenth mentions.
For engines that use cams with a lot of overlap, tight LSA, or low vacuum at idle you may not be able to effectively tune them without making the lower IAB's smaller. Keep in mind that the lower IAB's are the first place to "vent" the fuel supply passages to the mixture screws.
Creating a large leak there can reduce the signal so much up to the DCR's that you can't make the idle tubes and DCR's big enough to get good sensitivity from the mixture screws, especially if there is a pretty decent size Upper IAB venting the passages again.
I've tried tuning some pretty heavily cammed engines without making the Lower IAB's smaller and was partially successful, but it required huge idle tubes and DCR's, and even then the idle quality wasn't all that great.
I've had much better success with some set-ups that have low vacuum at idle by making both the Lower and Upper IAB's smaller vs huge idle tubes/DCR's.
Keep in mind here that some of these engines required this simply because the engine builder and/or owner made poor parts selections in terms of compression and camshaft. Any time you start pulling the LSA down, increasing overlap, and longer seat timing the static compression ratio becomes a pretty big player in how well the engine will idle. IF you find your engine needing a lot of timing and fuel at idle speed the cam simply isn't all that well chosen for what you are doing more times than not. I'm talking here about street engines.
For race engines designed to make big power in a narrower RPM range and where idle quality isn't a big deal narrow LSA and long seat timing cams are pretty much the norm and the end user expects low vacuum at idle, rough idle, stinky exhaust, poor street manners, etc........Cliff
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cliff, suggestion for LIAB size to start with?
kenth, i believe i am 600 rpm idle in gear. much more and i found the engine trys to run-on on hot shutdown.
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Are you already using full manifold vacuum to the vacuum advance?
If not try, engine will run smoother and this will often cure run-on (diesling) at shut off because the engine will run with more closed throttle blades.
It certainly wont hurt anything.
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I am running ported. will try full vac the next or 2 and report back,I suspect my in gear vacuum will go up a few points.
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Timing can be a big player with idle quality.
Some engines like a LOT of timing at idle, others not so much.
I would get the engine fully warmed up, adjust the idle mixture screws for best idle quality and highest vacuum.
Then loosen up the distributor so you can turn it. Advance the timing till it runs dead smooth and highest RPM's, then start retarding it until it just starts to slow down and the exhaust note develops a slight deep/heavy sound.
Take a look at where the timing is at. That's going to be pretty close to the ideal setting for your engine. If it's back around 10-14 degrees there will be no need to run manifold vacuum to the advance at idle.
Most well thought out engine combos will not like, want, need or ever respond well to a lot of timing at idle. This assumes the tuner has the carb set up to deliver adequate fuel to the mixture screws from rich to lean, and that there is no nozzle drip from the carb so adequate bypass air.
Almost every engine I've built here in the past 25 years or so has been fine with about 10-14 degrees timing in it.
I've tuned some troubled engines brought up here that needed a lot of timing at idle. In every single case the cam was simply poorly chosen for the total combination of parts (low vacuum at idle speed). I consider running the timing clear off the scale at idle a "crutch" for poor parts selection and just as often poor tuning skills.......FWIW......
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I have found it is an undeniable fact that most 1967 and earlier engines used full manifold vacuum for the vacuum advance. 26-30° timing advance at idle was the norm. These engines were in no way a result of poor engineering, this was a way to make the engines work as efficiently as possible. In 1968, there were legal requirements for reduced exhaust gas values and one way to achieve this was to delay the ignition, even to the negative side of the scale, so that the cylinder heads would be heated up at the end of the combustion cycle by the mixture still burning when the exhaust valve was opened to reduce NOX. This did nothing to improve the efficiency of combustion, quite the contrary. When the throttle blades opening became larger with later ignition, idle bypass air was introduced in the Quadrajet to cure problems that arose with the later ignition, e.g. run-on/dieseling. With a hotter camshaft than standard, however, it can be difficult to get enough vacuum to fully activate the vacuum advance and then it can be advantageous to use a ported vacuum source for the vacuum advance and help the carburetor deliver a sufficiently combustible mixture and add more idle bypass air to avoid having the dampers open more than necessary and that the dieseling at shut-off. If anything, i think the "crutches" started 1968, and in the end the level of needed "crutches" to have a proper running engine depends on the skills of the engineer/designer/tuner.
JMHO
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You ever see how much timing a TBI 350 idles with? About 30 degs. till you pull the tan wire.
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I agree with Kenth. ALWAYS connect the distributor to full manifold vacuum; "ported" vacuum is just a primitive emissions aberration.
And I've seen either 23° or 30° idle timing for stock TBI 350s on various squarebody (and Camaro?) forums.
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It is NOT always possible to use manifold vacuum to the advance.
For some combinations you aren't going to have enough vacuum at idle to effectively use it. I run into that all the time here tuning engines that don't make enough vacuum to employ the vacuum advance.
In these scenarios you MUST locate a vacuum advance that is either adjustable for when it applies all the timing or one that has a really "low" rating on it.
There are also engines out there that are already making enough vacuum at idle with reasonable advance in them that adding more has the throttle plates almost or completely closed and they will not idle down to the desired idle speed. I run into that scenario on occasion as well.
The most common scenario for troubled engines brought here for custom tuning is that the engine builder or whoever chose all the parameters LOWERED the static compression ratio (for pump gas) then installed some whiz-bang modern lobe profile cam to try to bring all the power back.
Yes, companies selling short seat timing, fast ramp, tight LSA camshafts advertise that those camshafts will idle better, improved throttle response, better fuel economy, more power everyplace, walk on water, tuck you in at night and cook breakfast for you in the morning. It's marketing genius but in the real World lowering compression hurts power, vacuum, throttle response and fuel economy all else being equal, and closing the intake valve early and adding overlap isn't going to do much beyond killing off upper mid-range and top end power and put a little "attitude" in the exhaust note.
You'll also find yourself requiring a LOT of timing at idle and fuel to the mixture screws to make those engines happy. You will also need to speed up the mechanical advance curve as well. Here in lies the problem that I see. About 80-90 percent of the customer base installs some POS distributor advance weight/spring kit, unhooks the vacuum advance (most add too much timing now that we are running so much initial and bringing in all the mechanical right off idle).
This makes for a final product that doesn't idle well, lackluster power production, and all sort of tuning issues because the owner/tuner doesn't have the skill sets to improve the carburetors idle system to supply adequate idle fuel at the much lower than stock readings and some of the mechanical spark curve is in at idle so idle tuning is difficult or impossible right to start with.
I make a nice living sorting these things out and have for decades.
The FIRST thing I do with 99 percent of the vehicles brought here for tuning is to remove the distributor cap and replace the cheap Chinese weights and springs with OEM parts. Once I get the timing under control and NOTHING in below about 900-1000rpm's I move on to the carburetor an get the idle system up to par for the engine combo.
We put the carb back on, set the timing, hook the vacuum advance back up (in most cases it wasn't being used) and guess what? The owner comes back from their first test drive grinning from ear to ear, well, at least until I unload their wallet!
I started a thread on some of these tuning sessions and will be adding to it, time permitting. Right now I'm so busy I don't have time to reach around and scratch my backside........Cliff
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switched to manifold vac over a week ago. have about 12 or 13* @ idle now, from 7*. with my latest fillup looks like i picked up 1 or 2 mpg. definitely seems to like it at idle better, vac needle isn't vibrating like it use to and with the throttle plates closes vac slightly increased too. 16" in park, 14" in gear. still would like to try and close up the lower IAB but have not been able to source small brass set screws locally. i will probably be calling you soon Cliff there's some other stuff i would like to order as well.
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"switched to manifold vac over a week ago. have about 12 or 13* @ idle now, from 7*."
I'm confused? Timing at idle went from 7 degrees to 12 or 13 degrees?
Then below that you mention that vacuum increased to 16" in park and 14" in gear......
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Yes my timing at idle went from 7* to 13*.
My vacuum @ idle in gear went from 13" to 14". I assume this is because i was able to close the throttle plate more. Hopefully I'll have time this week to call.
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Are you saying that adding manifold vacuum to the advance increased timing at idle from 7 to 13 degrees?
That's only 6 degrees if my math is correct. Most engines will like a bit more than that at light engine load and some like it at idle speed.
The BIG myth with all of this is that all engine like, want, respond well to and run better with a lot of timing at idle speed.
In reality the better we do with compression, squish distance, cam selection, etc, the LESS timing the engine will want everyplace. They will also require LESS fuel as they are more efficient.
Tuning for a living I get to see all sorts of troubled engines combinations. The trend in recent years is to LOWER the compression ratio then install camshafts with advanced intake lobe positions, shorter seat timing, and tighter LSA.
The thought process there is to bring back some of the lost compression, power and efficiency from lower the static compression ratio. Sounds good in theory but doesn't work well in actual use. At least not if you are trying to make optimum power on pump gas with these N/A engines.
I run higher compression instead, then use cams with more duration at the seat, later intake closing and wider LSA. The engine result is an engine that idles well without a lot of timing in it, and has a broad/flat/strong power curve (torque).
Combine that with good flowing heads and the end result will run as good as or better than a lower compression engines with smaller a smaller cam, then reward the end user with a LOT more upper mid-range and top end power. I'd also mention that even with more power output octane requirements will often be less than a lower compression engine doing it the other way.
This simply happens because efforts to increase dynamic compression early in the RPM range spike cylinder pressure high even if you have a relatively low static compression ratio. SO basically having peak VE occurring lower in the RPM range increases the chance for detonation/ping all else being equal. For these engines I like to push peak VE up higher in the RPM range which makes detonation less likely as the events are simply happening much quicker so not so high of a "spike" in cylinder pressure.
If you don't think this works well below is a clip of my car at the track. Notice how well the engine idles with only 10 degrees timing in it. The static compression ratio is 11.3 to 1 and it runs fine on currently available pump fuel with zero issues anyplace..........Cliff
https://www.youtube.com/watch?v=6zVdoLR-VzM
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My question is where to source the springs/weights for the HEI? The distributor I am running I don't remember where it came from, I put it in because the Mallory Unilite was burning up modules suddenly when I had no issues the previous 10 years, and it left me stranded multiple times. The HEI has been reliable for 3 years now except the mechanical advance now needs help.
I don't want the cheapie spring kit. With my Kauffman 74cc heads/041 cam/406 Pontiac/4 tubes/Pypes 2.5" with X-pipe the information I have found says my timing should be around 32* by about 3000 RPM. I found out I was only 26. I had to advance my base timing to 14 to get my desired 32. I have ported vacuum advance that adds another 10.
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I´d say you´re right on the money with your timing.
Remember, Pontiac used 15° initial for the RAIV.
Only thing i would add is a stop for the centrifugal advance so the weigths don´t override max advance.
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Good advise. Installing a positive stop for the advance in an HEI is not optional...IMHO
IF you install lighter springs the weights can and will ride out past the "flats" on the center cam and add more timing at high RPM's. I discovered this fact on the dyno decades ago following an engine built here with a timing light to verify total timing on a dyno "pull".
There are several different methods folks recommend to provide a positive stop. I use the MIG and a round chainsaw file here. I can also closely control the exact amount of advance added with this method......Cliff