Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: dlphil10 on August 26, 2015, 11:12:45 AM
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Background:
I have recently rebuilt a 350 Pontiac motor. The specs are as follows:
350 with 0.030" Overbore
061 heads which are supposedly 72cc heads but published values are known to be a little off
Stainless steel valves: 2.11" Intake 1.77" Exhaust
Crower 60420 Cam: 270/276, 210/221 @ 0.050" lift, 0.421"/0.446" lift, 112LSA
Automatic Car (TH400)
Carburetor: 7029262 which from a 428 motor
The carburetor was rebuild using cliffs kit that had the most components and the throttle shafts were bushed using components from this site.
I believe the primary jets are 72s and the metering rods are 46s
I have just broke in the cam with 40 minutes total run time at 2500 rpm +/- 500 rpm. Brad penn break in oil was used and only the outer valve springs were used to aid in cam break in.
Problem:
Whenever I was breaking in the cam the car had a very slight stumble but it didnt seem too regular. I was told to proceed with breaking in the cam, then tune the car afterwards. When I went to tune the setup, the idle was a bit rough(in park). The timing was set around 10-12* and the idle was ~550rpm-600rpm. I got the vacuum at idle to be around 15inHg by backing out the idle screws, but now I think that my expectations of a higher idle vacuum were incorrect and misguided me into possibly leaning out the idle. Whenever I was breaking in the cam I did notice I had around 20-21inHg of vacuum strong and steady.
I dont think my vacuum advance is engaging at idle, but could that cause this issue (there is that slight stumble at high vacuum cruise scenario). I ordered a crower adjustable unit just in case.
Could an incorrect power piston spring cause this problem? (I was expecting a lot more vacuum at idle and chose the spring that seemed most like the stock spring in length and tension)
I know thats a lot of info, so let me know if you have other questions.
Thanks!
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Also note that the spark plugs are slightly tan, not as tan as I have normally seen, but I attributed that to the cam break in with little load. Is that an incorrect assumption?
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It should also be noted this casting has idle bypass. I couldn't imagine that the build would use the same air as a 428 car, so I set the primary throttles so the slots aren't exposed at idle.
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Pretty lean jet/rod combination for what you are doing. For sure I would install smaller primary metering rods to add some fuel to the part throttle part of the curve. The idle system should be adequate for that engine combination. Make sure to remove the triangular metal cover over the vent tube, there is a tiny screw under that cover that control idle air to the upper bleeds, which can be used to fine tune the idle system as well.....Cliff
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Thanks Cliff! What would you suggest for the primaries? Should I go down to about a 42?
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The screw I believe you are talking about is fully seated (the small screw behind the tube)
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I apologize for all of the additional posts...
I just test drove the car and the carb seems to be fair. The only big issue is a bog upon part throttle. It doesn't have as much power as I believe it should have either. Could this be due to the lean jet/rod combo?
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I found two quadrajets in my dads stash that are in need of rebuilding. One is a 67 and the other is a 69 or 70. Both are chevy quadrajets he has acquired. One pair of rods are stamped 44B, and the others are stamped 42B. The rods stamped 42B are stamped in the middle of the rods while the rods stamped 44B are stamped towards the tips of the rods.
Would these rods be suitable candidates for my build?
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I don't like using used jets and metering rods for custom tuning. Very few used jets will pass a pin gauge test, they are almost always "egg" shaped and worn some. Same with used metering rods.
I would call the shop when you get time and get new jets and metering rods for it and PP spring for what you are doing. Since you already have good rebuild parts in it should be good to go and nothing else needed to get it dialed in....Cliff
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Primary rods, jets, and secondary rods are ordered. In the mean time, do you think its an issue that the screw (Idle air bleed screw im assuming) is fully seated?
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Jets and rods won't affect my idle correct? The idle still runs pretty rough lIke a misfire. If I put my hand over the air horn primaries it smooths up. More fuel? Turn out that air bleed screw on top of the airhorn?
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If it smooths out when you cover the top some, it wants more idle fuel. Turn the bleed screw all the way in. Fine tune the mixture screws. If it still want more increase idle tube size and DCR's a small amount.....Cliff
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I had a chance to install the new 72 jets, new 42 rods, and new DA rods shipped from Cliff. The power difference is night and day, and the setup runs a lot more smoothly.
I replaced the idle tubes and I think it has helped some. The idle mixture screws are backed out around 3.5 -4 turns currently and the screw at the top of the air horn is seated all the way.
There still is a slight "Flub in the idle" and the off idle hesitation is barely there. It chokes down real quick and then picks back up quickly. I think it might still need more fuel, or vacuum advance tuned. The crane vacuum advance is in the second slot before being all the way locked out. I should also note that I have not drilled out the mixture screw openings, and I only have aorund 0.040" idle bypass. Should I follow the example in the book and drill the idle mixture screw holes out to 0.090"?
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Did you open up the DRC's any?
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No I have tried to avoid modifying things if I can help it but it seems that might be unavoidable. I never would have thought the 350 would be as demanding as it is, especially since the carb was made for a 428.
I didnt measure the DCRs because I didnt have anything to measure them that well. It should be a stock setup for the idle circuit, besides changing out the idle tubes and mixture screws.
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If it is set to about 1 -1.5 turns from lightly seated it loads up at idle, but it seems to run better the more I turn the screws out.
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Do the same idle test you did earlier by putting your hand over the front of the carb at idle speed, or gently closing the choke flap. If the engine speeds up or smooths out, it still wants more idle fuel. Next step would be to open up the DCR"s slightly......Cliff
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I finally got to where I can work on it more. It runs without spark knock under load or cruising so I'm assuming the slight flub and irregular spark knock at idle is due to the lean carb setting. What idle down tube restriction size would you recommend for my setup?
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.046" to start with, if that doesn't bring in enough fuel open them up to .052".....Cliff
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So I've got it to where it appears to not want any more fuel by the hand over the choke method. However, it still seems to have a slight miss, it was cleared up considerably with the carb adjustment. Could a warped air horn or bad air horn gasket be causing the issue? All cylinders are firing evenly, the plugs all look good and exactly the same. Manifold vac is rock steady at roughly 14 or 15inHg.
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The 350 is a small CID engine, so it's seeing that cam as a pretty big camshaft. In any and all cases, give the engine the fuel it wants in all areas to be happy. If it's lean at idle, open up the idle system accordingly.
Compression ratio, cam timing, and ignition timing play a HUGE role in idle quality, and idle fuel requirements.....Cliff
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I solved a similar problem by accident. My heater wasn't putting out hot air on a freshly rebuilt 350. My carb stumbled upon stepping on the gas and did several adjustments to the carb with no sign of improvement. I was frustated. So one cold morning I covered the grille with tape and plastic to help the heater. To my surprise, my heater put out hot air and the stumble miraculously went away. Just before a full stop, I thought my engine was going to die but it was because it started idling much more smoothly. Going from a 160 to a 195 degree thermostat was the fix. Carbs will work better with a hotter engine while keeping the carb itself as cold as possible. Something to consider.
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I'm not even sure why they sell 160 degree thermostats, and yes, heat is your friend with these things.
Just curious if the heat crossover in the intake is also blocked off?......Cliff
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It's nonexistent, Cliff. I have a Vortec engine.
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For a wet flow system it will work best fully warmed up and heat soaked. That may take a while when there is no heat crossover in the intake, and even longer if the intake has an air gap under it.....Cliff
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I have no air gap either (wish I did), but sure enough, it takes longer than average to reach operating temperature. Once there, it performs like a champ. Those q-jets are fantastic. Thanks for the response, Cliff.
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With a wet flow system a cold intake just doesn't work well. When the heat crossovers aren't open to heat the intake, it takes quite a while for engine heat to get the intake warm enough so fuel doesn't glob up on the walls and require a richer mixture from the carburetor.
To this day I don't know why folks want to block off the crossovers, it simply works poorly for anything street driven.....Cliff
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Maybe this is an issue for me too. My cross over is intact, but my coolant typically runs like 160-170. If I idle after cruising a bit it can creep up a bit.
I am also wondering two things uncarb related. I am wondering if this carb is too much for my build or if my rings are fully seated.. I did a compression test and all cylinders were 120psi. Seems low... Even why I was diagnosing the failure of that factory muscle 041 comp cam, the compression was 120psi to 150psi. This cam should close the intake more quickly than that 041. Seems to me those heads are far from 72CC.
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Your compression is even on all cylinders, and that is great. I think your next diagnosing step is trying a 195 degree t-stat. It's cheap and quickly done. Then, go from there. Basics should never get overlooked.
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It is a 195 thermostat.. thats what gets me. I do have a 3 core radiator. I wouldn't have thought it would be over cooling though. Would a weak radiator cap keep it from getting hot?
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No. The cap is just a pressure relief device and provides access to the coolant circuit. It is designed to keep a certain pressure thereby raising coolant boiling point.
You've got a head scratcher, man. I am sure you'll figure it out.
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Would it idle a bit more rough if the compression is lower like observed. Should I go for a cam that closes the intake valve sooner in the cycle?
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120 psi is really low for cranking pressure. Did you degree the cam when it was installed?
That cam should make a lot more cylinder pressure in a 350 engine with 72cc heads on it......Cliff
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I originally had bad 46 heads on the motor when it had the comp cam factory muscle and I fixed the 061 heads as a replacement. The mistake I made was not CCing the heads. However, even as the 041 cam went and wiped a single lobe, the same head combo had at most 150psi cranking pressure. Thats what is weird.
The current 60240 cam was installed and degreed at 110 degrees. Do you think 2 degrees off (ICL of 108) will make a large difference in cranking pressure?
I did find my throttle cable was not fully extended as I was using a different year intake/throttle cable setup. It was keeping me from full throtle by about 3/4-1" so I am wondering if rings are not seated. I have verified that cross hatching is still present on the cylinder walls and that the cam is in good shape.
Thoughts? I sure do appreciate it.
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120 psi cranking pressure is pretty "low" for a N/A V-8 engine. It indicates low static compression combined with late cam timing or cam too big for the application.
We like to see at least 150psi cranking pressure for these engines, most of the ones we build here are around 170-180psi.....Cliff
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Thanks, I need to do some checking and verify some specs.
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I pulled the motor and have checked it out. The heads are around 72CC-73CC. I did find that the pistons are around 0.041" in the hole. This is not the only issue, but I would say this is one of the main issues (lower compression and poor quench)
I found a 400 block that I think would be a better choice. I see D dished pistons in my future.
Thanks for the help, I appreciate it.
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.041" in the holes with .040" or so head gaskets just will NOT work well. Not only does that lower the compression nearly half a point, combustion efficiency is very poor, as you were finding out.
For our engines here, we shoot for .035" quench, and NEVER over .040" for any reason.
Finishing up a Pontiac 455 Super Duty engine in the shop this weekend, pick of deck height attached.....Cliff
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Thats sweet, is the deck machined down to be flush with pistons with zero deck, or are there other ways?
What head gasket do you use to get 0.035" quench?
Given my beginner status, whats the max CR you would recommend to stay on top of the tune? I would like to reuse my heads on a 400, but still run pump gas. That would put me around 10.5:1 - 10.2:1. Im going to have to find a relatively large dish I feel. Is 9.5:1 still pushing it for a beginner wanting to play it safe?
Thanks for the information, I'm trying to do this right.
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not a pontiac guy ,i don't know what they like for compression . I'm a ford guy and they like about 10.7 or so and they run great on 91 pump gas with about 14 degree advance .maybe pontiacs just can't handle the compression (just joking )i don't want to be throw off the site. there are a lot of pontiac guys on here including the guru cliff.fwiw
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cliff nice engine. i built a bbf 20 plus years ago. the flattops stuck out of the block about .010 i used 72cc heads and .035 gasket . it had over 220 cranking compression. a little much . everything went could until i pushed the valves up through the heads . the idiot i hired to do the heads didn't put hardened seats in but charged me for them , long story short it ran about 10,000 miles and started missing . shortly after it started . once the lifters pumped up the valves wouldn't close. i ran it on pump gas but it liked to knock a little. had to use octane booster. fwiw
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We shoot for zero deck then use the stock Felpro head gaskets that crush at .039".
Hardened seat sure work well with this new fuel, nothing in it to keep the valves from receding into the heads like the old fuel with lead.....Cliff
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I'm not sure the hone was good on the motor. It had a really random idle which I'm thinking had to be ignition because a vacuum gauge showed a steady 17 inHg at hot idle (~22inHg cruising). The decent vacuum also makes me think that cam wasn't too bad for this motor.
I had an original 350 2 barrel that ran great, just got a lot of miles on it. Its compression should have netted around 8.8:1 in terms of actual calculated SCR. I think decking a block 0.041" seems extreme so im having my builder take a solid 0.023" of the deck and go ahead and take it out to 0.040" pistons. Im also going to reuse the same cam as it seemed to be fine. This should put me around true 9:1 and around 7.1:1 DCR (around 63deg. intake closing). It might not be the most efficient or all out motor, but it should be close to efficient as a stock build, no? This should be good to get more done to the car and then go for the 400, 428, or 461.
Thoughts on how the carb should be tuned?
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I would try it as-is and see how it works. Should be pretty close for what you are doing.....Cliff
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What power piston spring should I use? I'm guessing it'll make around 15inHg in gear. Dark blue? I think I may have the orange spring in it now, for whatever reason.
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With 15" vacuum at idle any PP spring will be down. The stronger ones lift the rods earlier, and can improve throttle response slightly with some set-ups.
The orange spring should be fine for what you are doing, but you may be able to make improvements tuning in that area.....Cliff
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So I shaved the deck down 0.020" and went to an 0.040" over bore, but same heads and cam. It should be around 8.8:1 - 9.0:1 SCR. I know thats still a large cam for the setup, but I would have expected different results.
I am now getting around 11inHg in park and if I put it in gear it dies. If I put my hand over the carb it picks up rpms and will make ~15inHg. Sounds like I should replace idle tubes again with larger ones and drill out DCRs? Can DCRs and idle tubes be englarged while they're installed? Would the carb sitting for a year and a half cause issues?
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They make long precision bits to drill idle tubes in place. Dcr's are easily accessed to no problem opening them up a tad in place.
Sitting shouldn't be an issue unless a mud-dabber crawl into it and muddied it up some......
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Alright so I went through the carb and changed the idle tubes to the 0.037" ones you sent (thanks!)
It idles so much better at around 12-13inHg, but we took it for a drive and it will NOT accelerate... Heavy and medium throttle makes it slow down. It only wants to speed up with light throttle and very slowly at that. Do you have any ideas on whats wrong?
Air horn idle bleeds are ~ 0.050"
Air horn main bleeds are ~ 0.070"
Lower air bleeds are ~0.070"
DCRs are ~0.050"
Mixture screw holes are ~.100" (They were like this already)
Accelerated pump hole ~0.027"
Orange power piston
42 rods and 72 jets
Float ~0.25"
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Power piston stuck or hanger bent down to keep the rods from coming up out of the jets high enough??
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I changed distributor springs and it's much better but I'm still thinking carb issues. The power piston moved freely when I installed it and the rods seemed to be at the right height. After tweaking the dist. springs it will bog under moderate load and when I back off it a little it speeds up and I can give it more throttle. It's consistently around 12inhg in park amd 8-9 inhg in gear.
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I looked at the carb and the primaries are open well into the vertical transfer slots and and past the horizontal transfer slot. I can open up the idle bypass air and close the primaries more, correct? Would having the primaries closed further bring up idle vacuum?
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Are you seeing any nozzle drip at idle, if not it doesn't need any more bypass air?
Makes no sense why the engine would not accelerate the carb has plenty of jet in it, and metering rods are 4 sizes richer than what it came with.
Maybe a fuel flow or delivery issue, filter in backwards or something odd going on?......Cliff
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Is nozzle drip very noticeable? I've looked down at the nozzles with a flashlight several time and haven't seen anything...
The acceleration is a lot better. I have it idling around 700 in gear and I noticed it runs much better with the mixture screws turned out at least 1 turn from highest vacuum.
I don't have good off the line acceleration and I'm wondering if that could be from having the primaries open too far. I'm trying to maintain the idle speed, but see if regaining transition slot could help. If I give it gas it chokes then picks back up. The accelerator pump shot is strong and healthy. A vacuum gauge shows when the throttle is cracked vacuum plummets to 5inHg.
Maybe it's just the highway gears and automatic that I'm fighting?
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Plugs look good creamy tan color.
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Is your problem coming off idle at very light throttle openings or going quickly to heavy throttle?
Two different scenarios as "normal" driving doesn't use the accl pump or PP for enrichment.
More bypass air would simply add more air to lower the throttle angle but not add any more fuel anyplace......Cliff
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Something that just came to mind was that carb every commercially "remanufactured"?.....
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I don't belive it was ever commercially remanufacturered. What would be something to look for to indicate that?
The problem is a bog at practically all off idle. I think it's worse at light throttle. There's a slight grade to climb as I back around in the drive way. When I go to put it in drive to go up the hill it stumbles and I have to give it gas to pick rpms back up.
On the idle bypass I was wondering if I don't have enough off idle enrichment from being far into the transfer slot.
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Most commercially "remanufactured" carburetors are mixed/matched parts and all the finish typically gone from them.
The mixing/matching is what gets you into troubles.
If and when EVERYTHING inside is correct for what you are doing, and there are still issues, time to look for fundamental problems like the wrong power piston (very common with anything commercially remanufactured). Not using the original top and bottom will also cause running issues.
Arms bent on the power piston causing the metering rods to be too deep in the jets, etc......Cliff
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Do you have a list that shows what number air horn and throttle plate should be with the 7029262 to verify?
I tweaked valve timing and ignition timing and now it idles in drive at 14.5inhg. The plugs are only faintly tan as shown in the attached. This is too light isn't it? This was with the mixture screws out 4 turns. I was thinking maybe I should open the DCRs to 0.055 if that would help.
I also was thinking of changing to the light blue power piston spring to get more gas off idle. What do you think?
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Impossible to read plugs on this new fuel.
Always tune for best results. If it needs more fuel the engine will tell you. Start with idle, set the mixture screws for best idle quality and highest vacuum a the leanest settings.
Test heavy part throttle next, and dial in the jet size which provides the best power at heavy part throttle, climbing up steep grades, passing w/o the secondaries, etc.
Then tune the part throttle for best off idle at light throttle openings, steady cruising, etc.
Last tune full throttle with the secondary metering rods.....Cliff
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Good to know, thank you for confirming spark plug reading is difficult. The plugs look white, but more gas makes the idle unstable and causes it to be a little shaky. I know I need to get my advance curve coming in faster, but I think it still needs more gas in part throttle.
If I ease into the gas it'll do good, but it bogs with part throttle stabs. Heavy part throttle seems good (~5-7inhg). I'm thinking this would be helped by changing the power piston since this lines up with the orange spring that I have. If my drive idle vac is 13.5in hg. Would it be ok to use the stiffest spring? I have the long green, orange, and small light blue spring that is shorter than the green. I belive it came wth the plain spring, but it was lost.
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Yes, you can use any of the springs for fine tuning they will all be down with that much vacuum at idle.....Cliff
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Awesome. I know there are two light blue springs in the kit, is there a length difference between the weak one and strong one? The one I have is shorter than the green spring.
Would the power piston spring be the first thing to change for part throttle tuning, or should I be look into fattening up the idle circuit to free up more of my transition slots?
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The power piston will be down for most light throttle/normal driving. You can increase fuel to the main system by adding idle fuel with larger idle tubes/DCR's. This can help with transition issues and being too lean right off idle as well. Just be very conservative when tuning in that area as it does increase fuel to the mixture screws and exposed transfer slots at idle and you can put too much fuel there and loose control of idle A/F with the mixture screws......Cliff
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Light throttle seems to be good. Plus, when the car is warm and in park, the idle screws turned all the way in don't stall the motor. They definitely choke it down though. So I don't think the idle circuit needs any more fuel correct?
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Correct, if you can slow the engine you have control of the idle fuel. There is usually enough transfer slot exposed so that fully seating the mixture screws may not kill the engine clear out. That fact varies from one Q-jet to another due to how much bypass air is used, how the idle system is set up, and the length and location of the transfer slots. If you take a look at a few dozen baseplates from different Q-jets you'll quickly notice that the specific location and length of the transfer slots can vary considerable from one to another.
This is why I always recommend to be very careful and conservative with idle system modifications when setting up your Q-jet for a non-stock application. I would also avoid swapping parts around from one unit to another for the same reason. The parts may look the same but there are minor differences in them you may hot see that will effect the end result, sometimes negatively.........Cliff
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Well that is good news. This baseplate looks like it matches woth the bowl. No passages are blocked and the transfer slots are long. There's even one horizontal slot as well. Maybe I could drill out the idle air bypass a bit and regain more of my transition slots? I believe they are only 0.040" right now. This may help with my part throttle response too correct?
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If you aren't seeing fuel coming from the nozzles at idle (nozzle drip) then it will have enough idle bypass air. This assumes that you also have control of the idle fuel from rich to lean with the mixture screws.......Cliff
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I switched to the green power piston spring and that seemed to help power at part throttle. It also really seems to transition through the pedal amd the other carb circuits pretty nicely. No bog when the secondaries open. Really the only thing is a slight hesitation sometimes off idle amd a minor flat spot off idle. Power piston gives healthy squirts on the outer hole of the lever. I'm thinking I may need a bit more timing off idle honestly.
The only thing that makes me concerned is the tiny black speckles on the porcelin. They aren't heavy or shiny, amd there isn't a lot of them. Plugs are white after idling still, but 1.5 turns out is where the vacuum stops increasing. I ran them out to 6 to see if I could get the plugs to change, but they didnt. It seems to have the fuel it wants so I don't know if the specks have been on the plugs when I got them or what.
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I can back out the mixture screws to 6 turns and it doesn't really bog. It may lower the vacuum a quarter to half an inch after 1.5turns out but it doesnt drop anymore. Could that be an issue? It really seems to have a better response at 1.5turns out though.
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The idle mixture screws are set for the best idle quality at the leanest settings, then move on to something else.
This is done by backing them out far enough so it idles great, then slowly turning them in until you hear a change in the engine speed or very slight drop on a vacuum gauge or hand held tach.
Move them back about 1/4 to 1/2 turn or so until the engine smooths back out. Repeat with the other side and balance them if you are running an open gasket under the carb or single plane intake.
So basically turning them in until you hear the engine slow is finding the "lean drop", then back out till it idles the best and you have the best A/F ratio at idle for your engine. Of course make sure you do this with the engine fully warmed up and fully heat soaked.
If you are unable to establish good idle characteristics in this manner and can continue to back out the mixture screws with zero effect (doesn't go rich), then the idle system is marginal for what you are doing and may need a tad more fuel made available to the mixture screws via larger idle tubes and/or opening up the DCR's slightly.
Keep in mind when doing this that you need to first check for "nozzle drip" to make sure ALL the fuel at idle is coming from the idle system. Also be aware that increasing fuel to the mixture screws for better idle control also puts more fuel to the transfer slots. If you are having issues or lean symptoms right off idle during transition, and/or minimal fuel at part throttle or limited control with the APT system, adding more idle fuel will help that scenario as well.......Cliff
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Thanks for that explanation, that helped.
I opened up my DCRS to 0.052". Then I had bad nozzle drip so I opened the idle bypass from 0.040" to 0.080". I also noticed the screws holding the baseplate and gasket had worked a little loose. It's better, but has nozzle drip at 750 rpm idle. At 700rpm the edge of the nozzle looks wet, but there no visible drip.
Sonce I just barely got the drip to stop at 700rpm should I add more idle bypass air and open up the idle circuit since it's lean? The idle tubes are 0.037, DCRs are 0.052, mixture screws are around .1. The idle vacuum is 14ing. It seems like that should be enough for it..
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Sounds like you are getting close.
A good way to decide whether to add bypass air vs idle fuel is to remove a small manifold vacuum hose and re-adjust the idle speed.
Then look for nozzle drip and tweak the mixture screws to make sure you have full control from rich to lean.
If you do not additional idle fuel is needed. After you add some idle fuel then do the same test to make sure no nozzle drip and full control with the mixture screws from rich to lean......Cliff
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I remember the original motor would bog when I turned out the mixture screws on the 2 barrel. I seem to only get this one to bog down with turning the screws in. Turning them out does make it flubby amd erratic at around 3 turns out so it makes a difference. Is that enough or should I expect a clear bog when riches the mixture?
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Should I increase the DCRs to 0.055 as the next step? It definitely doesnt want more air. I loosened the tiny screw in the air horn over the air bleeds and it breaks up. If I unscrew it just a bit, I can get a little better response from the screws, but still not enough to get full rich indicated by a drop in rpm.
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I don't quit follow. If you open up the upper idle airbleed screw in the airhorn it will lean out the idle system. If the engine responds well to this and you start to see some control with the mixture screws then it's already got plenty of if not too much idle fuel.
What do you have the timing set at? Are you using vacuum advance? Manifold or ported vacuum?....
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Initial timing is set at 15deg, 13 vacuum advance(attached to manifold), 35 total (20deg mechanical). New DUI distributor with plugs gapped at 0.050". Idle vacuum is 18.5inHg in park at 900 rpm amd 15inHg at 700rpm in drive.
When I backed out the air bleed screw it did get worse, hut then backed out the mixture screw to compensate and the control with the screws seemed a little better. Unplugging a vacuum line makes it worse so it's not overly rich. Covering air horn chokes it.
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Why so much timing at idle? Few engines respond well to that much timing as it can cause an unstable idle and loss of control of idle speed and sensitivity with the mixture screws....
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I had it running at 12deg initial and the hesitation was a lot worse and it would backfire through the carb. I bumped timing to 15deg and acceleration is smoother in general. WOT is nice, tuning the air door opening rate is still a process but almost there.
I can still significantly slow the engine by turning mixture screws in. Manifold vacuum rises after about 1.5-2 turns out, (1turn out from lean drop) but then doesnt do much if I continue to unscrew the mixture screws. Should I be looking for a drop in rpm associated with being overly rich?
Also should I bump up the size for the accelerator discharge? It's currently sitting at 0.028"
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Ok so I got the car back out today. It was warmer than in the past. It got up to 67 today. I noticed my secondaries wouldnt open all the way so I adjusted the linkage to do so with them on the car.
Good news is after heat soak I got the idle at 625rpm and backed the mixture screws out to when now I can remove the trans vacuum line amd it picks up 50 or so rpm. They're out 4ish turns, but it seems smoother than before. The car is popping on part throttle now before the power piston opens, but I'm guessing I may have too much timing now that its richer, but I didnt have time to mess with it anymore.
Now that the secondaries throttle is adjusted, the linkage keeps popping off going down the road. Any tips on keeping it in? I dont have the correct 4 barrel throttle cable or linkage, but I cant be the only one that didn't replace the 2 barrel throttle cable.
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when rebuilding the carb you set both primary and secondary plates to open 90*. they usually go over centre or not far enough. you can file the base for primary and bend the pins so they have a positive stop. not sure what you've done to the linkage but i've never had that issue. the only time i had an issue was with a marine carb, there was no way to set a positive stop so i just tweaked the linkage on the bench so it was just right. this is all outlined in cliff's book.
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are you referring to the secondary air door on the top of the carb?
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It's the throttle linkage for the actual secondary butterflies, not the air door. Air door opening has been adjusted via the spring and opening distance.
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Attached is a picture of the linkage where itconnects to the secondary shaft. The cam/lever on the primaries shaft has enough play in it that it allows the linkage to travel toward the primaries in the slot at the secondaries and slides out. I'm assuming this happens when the throttles whip shut.
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it's installed upside down. you might have to disassemble carb to fix it. i usually install that linkage as an assembly when i reinstall the throttle shafts.
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Well that makes a lot more sense with the stop on the shaft... That's the way I got the carb.. Thanks for helping to clear that up. Now to figure out the off idle studdering.
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15 degrees initial timing is probably OK, but not adding another 20 degrees, especially at idle.
The only engines I see that like that much timing have too much cam and not enough compression in them.
35 total plus another 20 from the VA is also a LOT for one of these engines. I usually shoot for 42 to about 48 degrees, initial plus mechanical plus VA, and for most builds I use ported to the advance, not manifold.........Cliff
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I should be right on then as I've got 48 total at cruise. 15initial, 20 mechanical and 13 vacuum advance.
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Is it generally acceptable to reuse the base plate to main casting gasket when I fix the linkage? Looks like I'll be raising the float back up to .25in while I'm in there as I lowered it a bit a while back.
I noticed the engine sags when vacuum hits 7inhg and recovers at 5inhg. 7inhg corresponds to the power piston opening. Is there a great way to measure the proper power piston hanger height?
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as long the gaskets dont rip you're ok. i wouldn't reuse them if they are old and crusty.
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Gave the QJET some TLC this weekend. He res my list of improvements:
1. Float moved from 9/32 to 1/4
2. Accel pump replaced although the old cup was I'm decent shape.
3. Blew all channels out, none were plugged.
4. Verified powerpiston hanger height minimum height by measuring length from hanger to top of jet lined up to the beginning of the thickest part of the rod. The rods were down right at minimum height, keeping them down in the jets as far as acceptable. I adjusted the hangers to be 0.015" above that to get them around nominal height and even on both sides within 0.002".
5. Secondary hangar height raised 0.015" to 0.645".
6. There was a strip of silicone in the fuel filter that got removed.. no clue how it got there.
7. Fixed the secondary throttle linkage and verified full opening
I haven't been able to drive it yet, but hope to do so soon and get back with you all.
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Car fired up after 2 pumps. It hasn't done that in a long time. I noticed my nozzle drip was back so I backed off the idle. I was able to gain full control of the mixture screws at 650rpm.
Throttle response is terrific upon tip in and part throttle. It seems like it has a slight sag when the power piston starts to lift, but it clears up with just a touch more throttle. I may switch from the green spring to the light blue one to delay the release.
I have a slight bog upon heavy/full throttle that clears up if I let off then get back on it. Sounds like I just need to tweak the secondary spring. It probably wouldnt hurt to adjust the kickdown switch either. The biggest ailments now are likely the stock torque converter for the 2 barrel 350 and the 2.78 rear gears.
Thank you so much for the help!