Cliff's Quadrajet Parts and Rebuild Kits
General Category => Quadrajet Carb Talk and Tips => Topic started by: Macadoo on July 03, 2018, 08:39:13 AM
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Hi folks, new member here :)
I just bought Cliff's book. I thought I knew a lot about these carbs, but wow!
So, a couple questions, if I may. I just broke-in my first ever engine build. An Olds 455, 030 over, Wiseco HC pistons (9.8:1 CR, if my numbers are correct), Howards cam (.512 lift in and out, 227/233 duration, 110 LSA), Edelbrock performer intake, Hedman long tube headers, holley fuel pump with regulator (althoug it doesn't seem like the gauge is consistent), 200-4r OD trans with lockup, 3.73 rear gears.
I'm, of course, having major idle problems with the Qjet calibrated for my old small block. I'm through most of the book and it all makes perfect sense. I have a couple of carbs to play with. The one on the car now is the original '71, #1705... but I've started a rebuild on a 1981, 800 CFM from a checker cab. #17081287
Q #1. The book says to go with a float level of 1/4" if the stock setting is unknown. I've looked up the stock setting but I doubt the checker cab had a high performance 455 (461) under the hood, lol. The paper says to set it at 15/32 for an '81 455. What's the best compromise here?
Q #2. Although the pictures in Cliff's book are excellent, I still can't seem to figure out where the upper air bleeds are on this '81 carb. I think they're in the air horn, recessed in a shallow channel, but these holes don't go all the way through. If I understand the book correctly, there's a good chance I don't have to mess with them but I'd at least like to measure them.
Q #3. I've never had this wild of a cam but from what I've read, a properly tuned Qjet shouldn't idle erratically. I really have to crack the primary blades to get it to idle. The carb that's on it now (the '71) has the idle bypass circuit in place and hoes drilled into the primary blades. Yesterday, just to get me by, I pulled it and enlarged the idle screw holes to .1065 but that didn't have any effect. I guess what I'm asking here is, by following recipe #2 from the book (with the '81 carb), will I get it to idle well enough to run AC without having to add a solenoid to bump up the idle?
Anyway, nice forum you have here. Very informative.
-Mac
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A 455 with that cam on a 110LSA will have a somewhat "ratty" idle and require idle system modifications.
Opening up the holes under the mixture screw do nothing without increasing idle fuel to them.
Can't help much not knowing all the parameters of that unit, but I would identify what you have, measure all the players, and make improvements to the areas that need help.
Being a 1981 emission calibrated unit it's going to need larger idle tubes, DCR's, and possibly a little smaller idle airbleeds.
It will also need larger main jets, and I'd recommend using our custom primary metering rods and smaller secondary metering rods.
We sell rebuild kits with tuning parts for any carburetor part number as a "package". This will include the necessary high flow N/S assembly to keep up with your 455, HP accl pump, small float, secondary cam/spring, new PP retainer, gaskets, small parts, filter/spring, jets, metering rods, PP spring and secondary metering rods.
We can also include idle tubes (pre-drilled if requested) with the kit.
This will take the guesswork out of setting the carb up so basically "plug and play"......Cliff
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Thanks Cliff. I appreciate the input.
Do you recommend a 1/4" float level? And "custom" primary rods? Are these different than the B (pre 75), and K, M, and P (post 75) rods? I've been through the book but I haven't been around this website yet.
Happy 4th!
-Mac
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We offer full taper 50C rods for the single main airbleed later units. They taper from .050" all the way to .036".
We also have custom .044" rods that taper from .044" to .026". Those have a gently taper on the first part of the upper section, then quicker down to the .026" tips.
Since you have a fuel pressure regulator I'd set the float at 9/32" and pressure at 7 to 7.5 psi with our high flow .135" N/S assembly. That should keep up with that engine, if not get the .145" seat......Cliff
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Hey fellas, it's been a little while since I posted. But I went through Cliff's book and used recipe #2 on an '81 Q-jet with APT. I installed it, grabbed my neighbor, and ran down the road. When I punched it, it scared us both, it was so fast. However, it still wouldn't idle worth a squat and had a hard time starting. I found that the mating surface on the Edelbrock intake was all jacked up and uneven and sucking air. I got it flat now but that Quadrajet is now warped, I'm sure from bolting it to the uneven surface (or maybe it was to begin with).
I have another that looks very close to my original '71 Olds carb but the fuel bowl was replaced and has no numbers. D'oh!
Cliff, can I post pics of the carb and gaskets here? Maybe you could post the part numbers of the gaskets I need? I don't need the entire rebuild kit since this was a carb I bought a couple years ago from a rebuilder. Now that I know MUCH more about these Q-jets, I can tell this was a decent build for my old small block. I just need to match it to my BB, now. I'll need the gaskets and some odds and ends.
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Thought I'd go ahead and add some pics.
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Air horn gasket
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Base plate
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Main body, top
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Main body, bottom
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Air horn, bottom, has six tubes.
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Our basic kit will have all the gaskets, plus the high flow N/S assembly and HP pump at no additional charge. A much better bargain than buying the gaskets separately and you'll need the better N/S assembly and pump for the big 455 anyhow......Cliff
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Sounds good, Cliff. But I still don't have a carb number to plug in.
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Doesn't matter I can see enough in the pictures to provide the correct gaskets, the N/S and pump are universal......
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Thanks Cliff, that helps.
I've been reading the Q-jet tuning article by Lars Grimsrud, particularly the table that contains the factory jetting for different motor combos. It seems that the higher compression motors from the late 60s were setup leaner (and meaner?) than the 70s EGR units. The EGR motors ran very rich with jets as big as 77. For example; a 1974 Vette with a 454 BB motor ran 75 jets with a 39 rod (manual trans).
My question is; how does a more aggressive cam change the jetting? More lift with more duration would move more fuel, would it not? I was thinking of installing 74 jets with 49B rods (this is the 1705 era carb) but now I'm thinking that would be too lean. I don't wan to burn my new valves :-\
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Jet and metering rod sizes BY THEMSELVES don't mean a thing as far as rich/lean is concerned. It's the COMBO of jet/rod AND air bleed size that matters. It's all explained in Cliff's book which is a must read for any attempt on using a Qjet outside it's original application. Once you'll read it ALL, then you will see the light.
Frank.
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Correct. That article is really only useful to determine metering area as it doesn't take into account other items with the carb that effect fuel delivery across the load/speed range.
It is not possible to compare what it used in one carb vs another for jets/metering rods and there are a dozen other items that effect end fuel delivery to the engine.
For example, a 1969 Chevy carb part number 7029207 will delivery more fuel to the engine using .065" main jets and 36B primary rods than a 1970 7040205 using .077" main jets and 49B metering rods.
This is because the 1970 model uses HUGE main airbleeds vs tiny MAB's in the 1969 unit.
Idle fuel is also a significant player in fuel delivery and not really factored into the literature you have been looking at. The idle system feeds fuel down to the mixture screws and transfer slots. Some transfer slot is exposed at idle and as the throttle angle increases more slot is exposed and fuel delivery increases supplementing the main system in the "normal" driving range.
When we supply rebuild and tuning kits here I look at the carburetor number being used, how it is set up by the factory, then put the correct parts in the kit for the application. Very few if any other folks in this industry can do that for you, and I can recommend how to set the idle system up as well for any carburetor part number used on any engine combo.......Cliff
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Thanks for the input and explanations, fellas. I've been through the book a few times now and, at this point, my notes, highlights, and comments make it hard to read, lol.
I discovered that the first Q-jet I modified ('81 APT), using Cliff's book, was pretty warped. It accelerated very well but still wouldn't idle. I had another laying around (a 1705 era) so I was having a go at that one when I, and this is hard to admit, accidentally drilled through one of the main fuel nozzles while cleaning up a burr. :-[ I have yet another '81 APT but it is more warped than the first. This seemed more than coincidental so I checked the intake's mating surface. Yup, it was rounded over (the only part I bought for the new 455 that was used). I've got it flattened and am back working on the first '81 APT carb. I know there are un-warping services but I thought I'd give it a go. It's pretty darn close now. I can't feel any rock of the air horn with a single gasket in place but I might use two gaskets for good measure. Or will that interfere with the power piston/ rod hanger?
Anyway, I'm just doing all this to learn and have fun. I'm hoping the Q-jet I'm finishing up now runs because I'm not a big fan of the Holley I have on there now. It's my first Holley (a gift from a friend) but I'm not very impressed. But to be fair, I haven't tuned it. It's "out of the box" stock.
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We spent some time last year with a brand new out of the box Holley Street Avenger.
It was installed on a good friend of mines new SBC engine for his work truck. I had NOTHING to do with the engine build or anything used on it. A local Chevy "guru" did all the work and picked all the parts.
It was a 400 SBC with early double hump heads on it and unknown Comp XE camshaft. From day one it didn't run for chit anyplace, HORRIBLE fuel economy and quickly developed a history for being very unreliable.
He uses the 1 ton Chevy dump truck for his tree service and pulls a chipper with it, and hauls heavy loads, etc.
Fuel economy with the new engine topped with the Edelbrock intake and Holley carb is gallons per mile instead of miles per gallon! Takes about 20 dollars worth of fuel for a local 10 mile round trip to and from a job site. A trip to Columbus 50 miles away is over $120 worth of fuel.
The previous stock 350 2bbl engine was about 3 times better.
Anyhow, from day one the POS electric choke on the Street Avenger did not work correctly no matter where you set it. So it ended up getting wired open. He's been left walking a dozen times and I've grabbed a BFH and screwdriver to rescue him when he breaks down close to here at least half a dozen times. The original engine builder/guru finally kicked my friend to the curb and we had the truck brought here with a roll-back last time it left him walking.
I had a 1973 Olds motorhome Q-jet that was never claimed in the back room, installed it with no modifications anyplace other than an electric choke and it is FLAWLESS! Fuel economy is back where it should be, and it has been dead solid reliable now for many months.
I had the fuel bowls off that Holley half a dozen times and replaced the N/S assemblies several times as well. I did notice that this new fuel was de-laminating the shiny material it is made of, but it also has fundamental issues that go WAY past that problem. By far and above it's the biggest POS I've worked on in many years and I would avoid them at all costs......FWIW......Cliff
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Vindication is worth A LOT! I'm tired of my gear-head buddies asking me why I'm building a Quadrajunk for my new motor. It may take me a few tries but once I get it, they'll see why ;D
Is there a good place to shop for non-warped Q-jet cores. I even checked my original 1971 carb. This was a one owner car and the only time I messed with the carb was to remove it and it, too, is warped. Pretty badly warped.
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Warping isn't really a big deal provided that you have a good fit between the parts.
I checked our books back in January and we had 12,900 work orders since going full time here in 2003. That's how many carburetors have been thru our shop, but all were not full rebuilds, some will have been here just for bottom plug repairs, correcting issues with them, coloring/plating work, etc.
It is rare to find one that doesn't show some warping across the front due to tightening of the long bolts to the intake. We simply make sure the fit between the parts is good. In 99 percent of the cases where it wasn't we correct it my running the front of the airhorn across a bench mounted belt sander for a few seconds. We then check the fit and continue this process until the top is slightly flatter than the main body.
We also use much better gaskets that are .015" thicker and good to go. I actually have one of the good unwarping tools and have used it a few times on carbs that were excessively warped. It will pull the main casting back as well as flatten the airhorn.
I still don't see a little warping as a problem and it woln't be provided the builder take the steps needed to make sure the fit between the parts is good and the gasket used is effectively sealing things up.
As far as the Quadrajet having a bad reputation it is well deserved for the early models as they are pretty much HOPELESS for high performance work. The early plunger inlet valve models (1965-66 and some early 67's) are difficult if not impossible to use for high HP applications.
Even the improved versions thru 1968 and any later variety with the early hinge pin location have limitations when it comes to using them on engines making big power in really fast cars.
By 1969 they had sorted out the issues with the early units which included moving the hinge pin location to provide superior leverage for a smaller float on the N/S assembly. They also added POE for the secondaries and got rid of the weighted air door design and used the pull-off to keep the flaps from whipping open too quickly going to full throttle.
They also had the idle bypass system in place by 1969 which is far superior to drilling the throttle plates for big camshafts.
Even with all the improvements folks had been removing factory Q-jets and tossing them aside for aftermarket carburetors thinking they were improving engine performance when in most cases they were not......Cliff
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Seldom mentioned in conjunction with this topic are how many excellent factory intake manifolds hit the scrap pile in favor of aftermarket aluminum ones. Just like the excellent factory carburetors the engineers spent a lot of time with intake manifolds to make sure they provided the best power production over the engines potential RPM range, good idle/off idle characteristics, and efficiency in the "normal" driving range.
Sure you'll read all sorts of advertising about ditching the stock intake for something better that will make more power, but about all that happens in 90 percent of these scenarios is that you remove some weight from the front of the car and kill off some low end power while shifting power to higher rpms where the engine doesn't spend much time.
I get a LOT of attention if/when I take my car to our local track and make a few runs with it. Last time I went there on a Friday night "test and tune" it was really hot/humid. In full street trim on DOT's I ran low 7's at 96mph and never even cooled the engine down. Yep, drove it right off the street, dropped the air pressure in the rear tires to 18psi and started making runs. I made 5 runs about 20 minutes apart all in the 7.18-7.24 second range (1/8th mile), aired the tires back up and drove the car home. In staging folks were coming by to check out the combination since I was running WAY quicker than the street cars in attendance and faster than a good percentage of the full race cars that were there.
The look on their faces when they spotted the factory intake and Q-jet sitting on it......PRICELESS!.......Cliff
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Thanks for the info and great stories, Cliff. I've read through these posts a few times. The wife and I just got home from a 3 day weekend at Route 66 Dragway so your last anecdote really hit home. There was a car show in conjunction with the races and, yet again, I had to defend the quadrajet (even though I'm still working on mine and have the Holley on the motor). Of course now everyone is all about EFI. Which would be great if I could afford it, I suppose.
I've hand-milled the base plate and main body so that all three parts fit pretty darn close. I can't feel any movement with the gaskets in place. I'v gotten the circuitry and idle tubes tuned to the recipe #2 specs and will start assembly soon. For right now I'm using parts I had laying around or gleaned from the other carbs because I want to see if I have any air leaks before ordering jets and rods. The gaskets (for this particular carb) are in fine shape but I'll probably replace anyway. I've installed 49B rods and #77 jets (a bit rich for recipe #2, I think) along with a G-hanger ans either the short tip CS rods or the long tip CV rods. Again, just until I know the carb isn't going to leak air.
It was a good weekend at the races and we brought home 2nd place for the GM A-body category. I haven't won an award in years so that was a nice surprise. :)
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Oh, and yes, I scrapped the stock EGR intake for an aluminum Edelbrock. Rookie mistake, maybe, but I don't think my back could manage a cast iron anymore, lol.
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Hey Cliff, based on my engine/cam specs, which power piston spring should I try first? I have all these parts from last summer, when I just started to look into all of this (but before I bought your book). I've never been able to get it to idle good enough to get a decent vacuum reading.
Cam: .512 in and out, 227/233 @ .050, 110 LSA
CR about 9.8:1
Dual 2.5" exahaust
Cast iron Heads ported and polished
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I'm a little confused and not even sure which carb you are using and how it is set-up.
Above you mention 77 jets and 49B rods which would be for a 1974 or early model, not the later Q-jets with the APT system located directly in front of the power piston. B rods are longer and used in many Buick, Chevy and Olds' carbs thru 1974.......Cliff
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Sorry Cliff, I've been working on three different carbs, lol.
This is the '81 APT carb from a Checker cab. I thought I read in your book that the longer rods could be used as long as the APT is set correctly but I may be mistaken. I'll read over that again.
The gasket pictures I posted earlier were for the carb that I ruined by drilling through the main nozzle. I wanted to use that one because I knew that it was recently bushed. It turns out that all three were recently bushed so I'm back to the '81 APT model.
I apologize; I'm trying to do too many things at once because I'm running out of summer.
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Yeah, I misread the sidebar about the rods. My bad. Again, I just need parts in there so I can test the carb for leaks before buying parts. I'll go with the P rods I have and 74 jets, for now.
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Correct, "B" series rods will be too long in the later APT models.
You can use the later K, J, L or P rods in earlier units if you team them up with different style Rochester jets.
Rochester Quadrajet main jets have the actual orifice much lower in the jets than some other options that are out there.......Cliff
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Thanks Cliff. One of the "rebuilt" M4MC units I purchased last year had M-rods. Aren't those marine?
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The "M" series rods are shorter and used in the later Q-jets with the APT system in front of the power piston. For the most part they were used in the single main airbleed models with few exceptions......Cliff
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Okay, I have the '81 Checker Q-jet (M4MC) installed and it runs great! Very crisp throttle and hard launches. The idle is MUCH better now that the mating surfaces are all sealed up. It is, however, dropping too much when put in gear; from 1100 in park to 700 in Drive. The torque converter is a 2500 but the brand doesn't get great reviews (it came with my 200-4r).
For the carb, I used 74 jets with 56P rods which actually measured at 048 at the widest so maybe they were machined? Half way down the taper they measured 44 so I raised the APT in an attempt to get that .030 range. The only thing is; it pings at WOT, where as the Holley did not. So I must be running a little lean. Initial advance is right at 14* (recommended for my 455 setup) but I haven't checked "all-in" with the HEI. Truth be told, I haven't had the guts to stand next to this engine I built myself with it running at 4k RPM :-[
I'm wondering if your modified primary rods would be a better/richer fit.
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P metering rods should taper apprx .010" on the upper section unless they are worn down some.
I do NOT put used metering rods into our builds here, most are worn down and often flat-sided, and the jets are typically "egged" out some as well.
Pinging at WOT could be sucking the bowl low and going lean, may not be a tuning issue.
Our "M" series metering rods are full taper with .036" tips and designed to be used in the later model single MAB models.......Cliff
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The 74 jets I bought new last summer and never installed. The rods are questionable, as they came with the "rebuilt" carb.
I'll replace the rods and play with the fuel pressure.
Thanks for your patience, Cliff. I'm happy with my progress.
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Okay, well, this is embarrassing but I was told at a car show that I was cranking down on the distributor bolt too hard. I went out this morning to figure out the pinging and the bolt was loose and the dizzy had jumped forward.
I started from scratch, set the initial at 14* with vac disconnected, played with the fuel mixture screws for best vacuum, and locked it all down.
I went out for a test, shifted from 1st to 2nd (200-4r) and hammered it. The darn thing broke loose at the 2-3 shift! I laid a short patch in 3rd gear so I must be getting close.
Cliff, would the shipping really be just $5 for a couple rods? I'm looking at the 44s and maybe the secondary rods.
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Shipping to where?
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61753
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Flat rate shipping this month for any shipments in the USA no matter what parts are purchased.
I would recommend installing new jets with the metering rods, unless you bought them from us. Currently there are 4 different jets on the market being sold for Quadrajet carburetors and 3 styles do not work.
We sell the correct jets made to OEM specifications so the depth of the orifice in them is where it's supposed to be.
We can also so USPS First Class shipping on very small items for even less if you call the shop directly to order, usually around $3-5 dollars if the total weight is under 1 lbs and they will fit in an envelope of small package.......Cliff
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I called and ordered the parts today (main rods, jets, and secondary rods). The dude I talked to was really cool and we chatted for a while. He thinks, and I tend to agree, that the 110 LSA is at least part of my idle problem. But I'm stuck with it for now so I'll make due.
The other day I accidentally knocked off a vac hose (direct to manifold) and the idle sped up and smoothed a bit. When I did the mods I drilled the idle bypass in the baseplate (and plugs in the main casting) to .106. The fellow I talked to said I need at least .110 for my motor. But .004 doesn't seems like much. Should I go more than .110?
Carb #17081287
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Okay fellas, I'm having trouble nailing down my tune for this 461. Seems like I get it close and then I lose it. I think a lot of it is the 110 LSA on the cam. I can get it to cruise very smooth and to break loose at the 2-3 shift but it pings at WOT. So I dial it back just a little and it becomes a turd. It likes a quick timing curve but the mark bounces around at idle (800 rpm in park). So I put in stiffer springs (two medium) and it becomes a turd at part throttle. (HEI)
Today I tried limiting the mechanical advance to about 12* and bumping up the initial to 21* and it runs "okay" but I know there is more power there. I realize that's only 33* total so I need to trim my mechanical limiter a little. And it's a little hard to start when hot with 21* initial. No kickback, just a little slow turning over (Summit mini starter).
I'm still dialing in the Q-jet's APT but I've got it close. No stumble and good cruise mileage (18 mpg - ish with my 200-4r and 3.73s). That's using Cliff's tip-in method at 2200 rpm. But I can still pull the AC vacuum hose and increase the idle (manifold vacuum).
Idle tubes are at .038
Down channels .055
Idle bypass air is at .120
Jets: Cliff's 73
Metering Rods: Cliff's 44
Mixture screws are 3 turns out giving me a steady 14 inches of merc
I do have a 1" spacer under the carb for clearance reasons. That may be part of my low part throttle power issue. I could probably get away with a 3/4" but not much less.
Engine runs strong and pulls HARD but only if I push the timing past the safe zone.
Thoughts?
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Is the spacer 4 hole, fully divided, or open?
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Hey Cliff,
It's a four hole phenolic. It wasn't real flat when I bought it but it sanded flat pretty easy. But I don't think I can take a 1/4" off by hand. Although I do have a wide belt sander at school.
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Oh, and the carb/spacer are sitting on an Edelbrock performer dual plane manifold.
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A small update: my dial-back timing light is a POS. I put my old light on it and, although I can't see the timing mark since it's so advanced, the nicks and other marks on the balancer are rock-solid steady. I'm making a timing tape and will get it on tomorrow. I was real close to pulling the distributor to have a look at the gear.
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I've done a lot of testing with spacers and have found them for the most part to be a waste of time/funds.
They create issues by raising the carb up higher, like hood clearance, choke tubes, throttle cables, etc.
On my 455 making over 550hp I did some drag strip runs back to back with 4 different spacers. 4 hole, fully open, fully divided and semi-open (divided in the front and open between the secondaries.
I also drove the car on the street with all 4 of them to get a "feel" for how well they worked there as well.
The 4 hole and full open were horrible at the track, slowed the car down everyplace and the fully open induced a huge stumble going to full throttle.
The fully divided did OK, and I ran the highest MPH with the semi-open.
However, when the smoke cleared and dust settled, the quickest ET was with no spacer at all, just a 1/4" thick factory gasket.
I also did some intake testing. The Edelbrock Performer was a "turd" on my engine, and couldn't even make a full throttle pass as the runners are smaller in cross section than a stock one. The engine hated it. The RPM did fine, and I also tested a Holley Street Dominator, Torker and Tomahawk.
All the single plane intakes ran slower everyplace than the dual plane, and required a 1" open spacer to work well. The Tomahawk ended up running the highest MPH but slowed the car down considerably in 60' times. Once again the factory iron intake ran the quickest ET, so it was making the most average power.
Folks in the Pontiac World cried "foul" because I had modified the iron intake by opening it up under the carb to the same apprx size/shape as the RPM intake.
For SBC builds it is hard to beat a stock iron intake to at least 450hp. They are excellent, low profile, very well made, etc. Even so we see them tossed out all the time for aftermarket intakes and folks think they are making more power.
A few years ago I did some back to back dyno testing with a 428 Pontiac engine. It had aluminum heads on it flowing 260cfm, 10.7 to 1 compression, and custom ground 236/242 hydraulic roller cam in it. I tested a factory aluminum HO intake, Edelbrock RPM, and my "modified" iron intake.
The HO made 487hp, the RPM 491 and the iron intake 497hp. Keep that in mind when you buy a Performer, Performer RPM or even the larger Air Gap version for you little SBC engine build make considerably LESS than 500hp.
Looks like you may have found your tuning issue. No real need for timing tape, I just measure over with a tape measure and put a mark at 36 degrees with a paint marker. You can reference off of that mark when setting the timing, etc..........Cliff
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Thanks Cliff, that's all surprising info. But the older I get, the more I understand that reality and "sales pitches" are rarely the same.
But at the same time, there's no way I could lower a cast iron intake onto my motor by myself, especially with the motor in the car. Even so, I may keep an eye out for a pre-EGR iron intake on Facebook.
However, I'm stuck with, at the very least, a 1/2 inch spacer because of clearance issues. The bracket I have to use for the 200-4r TV cable interferes with the back runner and needs to be raised.
I haven't seen a semi-open spacer offered on Summit or Jegs so I may have to make my own from hardwood. I've done it before, it's not difficult.
I've already made the timing tape so I'll go ahead and use it (until it falls off). A fellow on the ClassicOldsmobile site made a printable template for the Olds balancer that is spot-on.
What's your take on moving the timing from the mechanical more to the initial, as I've done? A lot of talk on the internet claims that can be beneficial for large cams. I know it has increased my idle rpm, allowing me to back off the idle speed screw. I was too far into the transfer slots before the change. Right now I have a *park* idle of 1100 with no nozzle drip. In gear it drops down to 850.
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Update:
The timing tape was a big help. My dial-back light was jumping all over the place. My old style light and the tape was nice and steady. I've got 19* initial and 17* at the mechanical for a 36* total.
But I'm having trouble with the tip method for the APT. I set it at 2100 rpm and tipped in the choke. RPMs dropped so I lifted the power piston a half turn and tipped in again. RPMs still dropped. I kept turning the screw counter clockwise and kept dropping RPM. I got to about 6 turns out with no difference. So I tuned it all the way in and back out 2 turns. The tip in didn't drop RPMs at that setting but it didn't increase RPMs either.
It runs okay at this setting but wants to stall after a short WOT run. As in every time. Choke is set at 1/4 with 7 psi fuel pressure. Float needle is installed but not through a hole in the choke.
Cliff, what would it cost me for one of your custom Q-jet builds?
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1100 RPM is too fast for idle speed and you are seeing too much RPM drop when the trans is placed in gear. I suspect some of the mechanical curve is in and falling out when the additional load is placed on the engine.
I don't like to run that much initial timing either, and you shouldn't have to with your combination of parts. For most engines we build and/or tune here we set the initial timing around 10-14 degrees and they will idle great and not "buck" the starter on hot restarts.
We sell custom units outright, no core required, pretty long wait time for them at the moment. Price would depend on exactly what you wanted, and they are all fully restored with plated parts and colored castings.....Cliff
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I can verify the mechanical coming in at the 1100 *park* idle with a timing light, correct? So, Put it in Drive, the mechanical advance that's also increasing the idle speed in park, drops out and lowers the idle speed in gear even more. Kind of a viscous cycle.
But if I take the *park* idle down to, say, 800 or 850, putting it in gear, idle will drop to a point where it feels like it wants to die and the engine rocks in the bay like it wants to jump out of there.
If I wiped a lobe during break-in, it would act like a a sticking valve and show up on a vacuum gauge test, yes?
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Huge drops in RPM placing the trans in gear is usually distributor related with some timing falling out. This happens if/when aftermarket spring/weight kits are used, or if the stock springs are getting weak or stretched out. I've also seen worn pins and weight holes cause this.
Insufficient idle fuel to the mixture screws can aggravate the scenario, so when a load is placed on the engine timing falling out and not enough fuel as the vacuum signal is reduced causes the huge drop in RPM's and idle quality getting worse.......Cliff
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Man, this is like herding cats, lol. Thank you, Cliff, for your patience as i get this ironed out.
I am running an MSD spring and weight kit, as you say; aftermarket. One light spring and one medium. Although I can't see my tach, it sounds to me like the curve isn't all the way in until around 3000 - 3500. I'll see if I can get the wife to sit in the car while I test this, this weekend.
Do you think I need to open up the idle tubes and DCRs a little more? Currently they are at:
Idle tubes are at .038
Down channels .055
Idle bypass air is at .120
Mixture screw holes; I can't remember exactly (I'm at work), but on the high side of recommendation, maybe a little over. One coil clipped from springs.
Jets: Cliff's 73
Metering Rods: Cliff's 44
Again, I can still pull the AC can vac hose from the intake and the idle speed increases maybe 200 rpm. This is why I took the idle bypass air to .120. Is this to be expected?
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You will get some idle speed increase from pulling a vacuum hose unless there is a serious lack of idle fuel available, then the engine will act worse and/or RPM drop off.
I use that method more to see if it needs more idle bypass air than any other reason. Just yesterday was testing a Marine carb, they do not have idle bypass available.
To simulate a smoother idling Marine engine and to get rid of the nozzle drip we were seeing I move the vacuum advance to manifold vacuum and pulled off the vacuum supply to the choke pull-off. This allowed me to lower the throttle angle and stop the nozzle drip we were seeing and get control with the mixture screws.
Before you can do any idle tuning you have to verify that no timing is being added at idle speed, it has to be rock solid below about 900 rpm's. If it falls out any at all it makes idle tuning difficult.
To this day I don't know why folks think you have to have the mechanical advance "all in" early in the RPM range. That's more of a "crutch" fix for engines that don't have enough compression for the cam choice, really tight LSA camshafts, or just plain poor choices in general for the combination of parts.
The distributor in my engine is pretty much untouched since the mid 1980's when I first placed it in service. It's been in 4 different engines and even dyno tested against an MSD billet with a 6AL box. The dyno runs were nearly blueprints of each other with a very slight advantage to the HEI. I suspect this is from how precise the tips of the pointers are on the pick-up vs the broad/wide ends on the MSD unit.
I just had it out a few weeks ago to check the status of the bronze gear and cleaned things up. It's a Pontiac HEI with stock weights, stock springs, stock center cam, stock 990 module, and stock coil. The only modification done to it is adding a positive stop so it can't continue to add timing at really high RPMs (common problem with factory HEI's and compounded when you put lighter springs on them).
It starts adding timing right off idle/900rpms and smooth/steady timing curve all in around 3000 rpm's. I add another 14 degrees from the vacuum unit via ported vacuum. Base timing set at 10 degrees, 30 total.
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Excellent info, once again. I'll report back when I get this stuff nailed down.
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Sounds great.
I will add that at least 50 percent if not more of the problems we see here with tuning is the distributor and not just the carburetor.
It's ALWAYS the same thing, some "goofy" offshore spring weight kit added to the factory distributor at some point and the excellent factory center cam, weights and springs are long gone.
Those kits not only use less spring tension they change the geometry of the parts and allow the weights additional leverage on the center cam to bring in timing early and easy. Problem is that it is often difficult, if not impossible to keep some of the curve out at idle speed with those parts.
Nearly ALL of the aftermarket HEI's we've seen in recent years have the same problem. I'd also add that I reject most that show up here due to low quality of the internals and lessons in humility trying to get a decent spark curve with them.
I use and prefer stock distributors and stock components.
Case in point. Recently we had a Buick 455 Stage I car brought here for custom tuning. Many hands had been on it, and it was NOT making the grade anyplace. Biggest complaint lacking in power (wouldn't even spin the tires).
As soon as we removed the air cleaner the "lacking in power" was clearly evident as the carb "builder" used the wrong choke pull-off (2bbl) and it wasn't allowing the secondary flaps to open.
We went thru the carb and it was a little "hacked" so fixed all the issues with it and recalibrated it for the application. We installed the carb and the owner took it for a drive, came back smiling ear to ear, so power was restored. The idle quality was a little "funky" and I noticed it "bucked" the starter slightly on a hot re-start I we put a timing light on it. At idle timing was over 1" off the scale and hardly any advance when you revved it up.
Removed the cap/rotor and low and behold a "goofy" spring and weight set-up allowing nearly all the advance it an idle speed.
We installed factory weights and springs and took a look at the curve with the timing light and re-timed the engine. Another quick test drive and the owner says that it's just a "touch" down on power right off idle.
I removed the factory springs, opened up the "eyes" just a tad, one more test drive and good to go.
With that simple modification we saw NO advance at idle speed, and the timing started advancing at 900rpm's and all in around 2800-3000rpm's. No more "bucking" the starter on hot restarts and idle settled now nicely every single time you let off the throttle, just like it should be doing.......Cliff
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Okay Cliff, did some checking this morning.
There is no timing difference between 800 and 1100 RPM so it's not advancing at that high idle, probably due to the stiffer springs. However, when revving, there is a quick jump that starts at about 1300 RPM up to 28 degrees, and then slowly advances after that up to 37 degrees at about 4000 RPM (although this is still by ear. I need to find a helper to read the tach). Certainly NOT a smooth timing curve.
As far as the timing "settling nicely" when letting off the throttle; I have little experience with what is to be expected but it seemed to settle a little slowly.
I did save the stock weights, center plate, and springs. However, this is not a stock distributor. [Full discloser]: it's one of the eBay cheap aftermarkets.
If I look for an original GM distributor, it would need to be Oldsmobile, correct? Since they turn counter-clockwise. May be a little hard to find.
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I've rejected every single aftermarket HEI that's been send or brought here for help.
I would obtain a stock Olds HEI and completely/correctly rebuild it. At most it may need a lighter spring on one side or you may get what you want by opening up the "eyes" in the springs slightly.
Always weld in a positive stop for the advance for exactly how much timing you want it to add and so it doesn't keep adding timing at high RPM's.
The current timing curve you have is too slow to get things started. The timing should start moving right off idle and smooth/steady to full advance and not slow up and some point, then take a lot more RPM's to get all the timing in.......Cliff
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I'm not sure I follow. Advance starts right off idle, then a smooth steady curve all in by 2800-3000. And that's where I would want the lock, yes? No more advance after 3000. No quick jump and then slowing way down of the curve, right? Might that be the MSD weights and center plate? I'll play with it in the interim.
I'll start the hunt for a factory unit. Might make a fun winter project.
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No advance till 1300rpms could use a little help. I like to see the timing start immediately with RPM's right off idle or at least by 1000rpms if not 900.
No need to split hairs with this sort of thing but as engine speed increases timing needs to increase so it has sufficient opportunity to effectively burn the mixture and maximum efficiency of using the heat energy to push the piston down.
The vacuum advance, being a load sensing device helps us at very light engine load and light throttle openings, but when you go to heavy/full throttle the mechanical advance is adding the timing, and if there is a delay in when it starts there can also be a hesitation/stumble/bog or slight lack of power until it starts advancing the timing.
You probably wouldn't notice it at 1300rpm's but I've seen troubled distributors with rusted weights, worn/rusted pins, etc cause issues that acted like carburetor problems when it was simply a spark advance issue.......Cliff