Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: Rhett on May 07, 2024, 08:50:43 AM
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Hey all - I've got a Buick 540 carb that goes lean at WOT, above 4900 rpm. I firdt saw this on the dyno where the carb was being fed direct, above 4900 the GPM went down while the RPM went up, and the HP went flat. We tried a Holley spread bore and the HP continued to climb. I didnt have an O2 sensor there, but on the car at WOT it reads 13.5 from a tailpipe bung. The car is a 4000 lb GS, 3.73 gears, 35 deg total timing stocker with with ported iron manifolds and a 228/239 114 cam. It idles great, has crisp throttle response through the range, pulls to 5700 and smells great. If I have detonation I cant hear it, the plugs look clean and show just slighly lean on the porcelain. Car ran 12.67@106.5 with no fuel delivery issues first time out. SInce then, I chaged the needle/seat from .135 to .145, and the primary jet/rod combo from 76/44 to 77/45. Cruise got a little fatter @ 14.5 but no significant change at WOT. This carb already delivers way too much fuel, but the car loves it and still gets 13.5 MPG. My main concern is hurting the engine at the high RPM. Carb spcs are as follows and the back to back Q-Jet/Holley sheets are attached. Interested in your thoughts, thanks in advance.
Fuel Inlet 0.145
Float Level 0.250
Idle Tube 0.036
Idle Channel Restriction 0.055
Accel Pump Discharge 0.033
Main Air Bleed (upper) 0.070
Main Air Bleed (lower) 0.070
Idle Air Bleed (upper) 0.067
Idle Air Bleed (lower) 0.070
Mixture Hole 0.090
Primary Jets 0.077
Primary Rods 0.045
Secondary Well restriction 0.039
Secondary POE Restriction 0.059
Secondary Tube Restriction 0.036
Secondary Rods .00300 CC
Secondary Valve Opening 1.287
Secondary Hanger F
Idle Air bypass 0.194
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Just to update, I have reviewed the similar post with the Chevy 468 and I did bring my needle/seat out to.152, no significant change noted..
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Have you checked your fuel pressure? What pump do you have?
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No, but it showed up with the fuel hooked direct from a 6 lb electric pump on the dyno too. Honestly, engine feels great in the car. I'd never know if not for the 02 sensor..
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I guess I wouldn't worry about it.
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That'd be fine except I'm giving up 30 hp to a workbench Holley, and we pride ourselves on that not being the case...
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what was the vacuum reading at WOT?
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2-3 inches going by memory? Not quite zero is what I remember.
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I should have had a vac. gauge when I had mine dynoed. I guess I could do it in car.
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What size speadbore Holley?
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It was a 6211 800 CFM. And a leaky, uncalibrated POS that wouldnt idle too
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looks like it's too fat to me. What makes you think it's lean?
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It should be fat, way fat but doesnt act like it. plugs, tailpipe, smell all normal if not slightly lean. The thing that concerns me is with the Q-Jet the fuel GPM went backward, and the HP levelled off where as with the Holley it kept climbing. Yeah the 02 sensor shows lean at WOT, but cant count on that cause its too far back in the system to be entirely accurate. I have another 540 that has a stock bypass, 76/44's and .067/.070 idle bleeds and .052 ICR and it wont idle at all on this engine. It likes the bypass, but I always suspected too much bypass is contributing to a lean condition. I keep thinking a .135 needle/seat was causing the fuel turbulance/stand off at high volumes, but a .145 and a .152 seat didnt change the condition.
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This is what i would have started with:
Fuel Inlet 0.135"
Float Level 13/32"
Idle Tube 0.039"-0.040"
Idle Channel Restriction 0.062"-0.064"
Accel Pump Discharge 0.033
Main Air Bleed (upper) 0.069
Main Air Bleed (lower) 0.039
Idle Air Bleed (upper) 0.054
Idle Air Bleed (lower) 0.080
Mixture Hole 0.095"-0.100"
Primary Jets 0.074
Primary Rods 0.045
Secondary Well restriction 0.039
Secondary POE Restriction 0.059
Secondary Tube Restriction 0.036
Secondary Rods 0.0527 CV
Secondary Valve Opening 1.287
Secondary Hanger F
Idle Air bypass 0.110"
JMHO
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Well I can still set the other 540 up that way and see how it likes it..
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What fuel pump are you using? Fuel delivery system to the pump?
6psi is marginal fuel pressure for the .135" seat for you dyno runs. If you saw power loss or falling off around 4900rpms you were sucking the bowl really low.
If going to a larger fuel inlet seat doesn't help then it's telling you either fuel pressure is too low and/or fuel delivery inadequate for the power level.
I've never once in all the years of doing this been outran anywhere by a Holley or any other type of carburetor on the dyno or at the track, provided we took steps to keep the Q-jet full on hard pulls or hard runs........
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Just trying to help. Hope you get it figured out. Usually if you see BSFC numbers higher on the top end of a dyno pull you can lean it out a little and it will gain power.. It would have been nice to see EGT temps from the pull and we would know. Being a little lean at the top end doesn't cost 30HP, but being too fat does.
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So I said 6 but it could have been 7, it was regulated for Quadrajet use on the dyno and feeding the carb direct. This same thing happened with this carb on a different dyno a few years ago, but it ran so well in the car I just dismissed it as a fluke. In the car, this was running 12.4's-5's and never missing a beat and given the build and HP thats exactly what I was expecting, it was putting out the MPH that I would expect from the Holley pulls (435hp/516tq). It wasnt as lean at WOT as it is with this cam, but I never really bothered to spend a lot of time finding out either. The car has a AC pump and stock fuel lines.
Fast forward to this build, and the car is again on it's way to putting out the MPH I would expect from the new cam (441hp/544tq) and while it's somewhat lean at WOT (13.5 if you trust a tailpipe bung), it runs great, idles clean and low, pulls good vacuum, great throttle response, clean fuel delivery to 5700 etc..I just dont want to leave anything on the table.
I've got two other 540's, one built and testing, one ready to bubild. This car likes a lot of fuel and a lot of bypass with my current carb, but I may be covering for too much bypass with too much fuel because I just stairstepped my way into the current config till it ran right. I'd like to take a more scientific approach with the next two, and make sure I kick that Holley's ass.
Lightning Boy, I'll throw some sheets up from the dyno sessions three years ago.while I dont have EGT's, there is more info there. I couldnt run the air flow meter cause we coulnt cobble it to the air cleaner base at the time and clear the dyno throttle linkage.
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and Cliff BTW I see the fuel pressure from a couple years ago was regulated at 6.5 or so.. and back then too the lbs per hr reverses significantly. I wouldnt think that would happen if the bowl was looking for more fuel unless there was turbulence in the path. I never could figure whether that was a chicken or egg condition, but the RPMS were still climbing even if HP levelled off....
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"So I said 6 but it could have been 7, it was regulated for Quadrajet use on the dyno and feeding the carb direct."
Even with a small float and .145" seat those carbs will EASILY take 8-9 PSI fuel pressure w/o issues.
When you reduce fuel pressure you kill volume to the bowl, especially since all of it has to pass one N/S assembly.
The rumor or myth that Q-jets can't handle high fuel pressure comes from the early designs with the short hinge pin and huge float. Those are tough units to keep full and will give troubles with higher fuel pressure and larger fuel inlet seats.
First thing to do when you go lean at WOT and high RPM's is to kick up the fuel pressure. If that doesn't get it install a larger N/S assembly until the problem goes away. If you can't correct it with those moves fuel delivery is not adequate for the power level.
I'd also add here that you MUST have adequate fuel delivery and keep the bowl FULL on hard runs before any tuning can be done to get the A/F where you need it to be.......
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Thanks Cliff, was thinking of a Holley red pump in line with the mechanical pump, wired to the trans kickdown switch as a trigger on a dedicated 12v relay. I ran this weekend up at the drags, was like 90 degress so the car was a dog but I pulled a couple plugs at the top of the track.. car should have been pig rich at that temp but was actually somewhat lean...
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I've NEVER been able to get that set-up to work.
A mechanical pump is not "free flow" and just becomes a stumbling block for a good high flow electric pump.
I've cured fuel delivery issues on really fast cars with powerful engines in them more times that I can count be using a high output free flow electric pump (fed by a sump on a stock tank or fuel cell) and 8an lines fittings everywhere with a good return style regulator.
Even better these days are to use an in tank pump with baffles and push ALL the fuel.
Even though I've heard about folks getting away with much less, I've never been able to with any of my cars. It's a topic where you'll read about folks running deep into the 10's with stock lines and a stock mechanical pump, where others can't get out of the 14's doing the same thing.
My car hit a "brick wall" at 12.30-12.40's. It was fine slower than that with a stock pick-up in the tank feeding a Holley Red pump right in front of the tank pushing thru a Carter 120gph mechanical pump.
I ran into a "brick wall" with ET and MPH and decided over one Winter to just ditch all that and start over. I removed the factory tank and welded a CE rear sump on in, then a Comp 140gph electric pump behind the tank, and 8AN lines/fittings everyplace. I also installed a return system and high-flow fuel filter (before the pump).
The best runs I'd ever made with the car prior to the new fuel system were 12.30's at 109mph. The FIRST run with the new fuel system and not other changes were 12.0's at 112mph!
I never knew anything was wrong and even tried an 850 Holley DP carb before making those changes and it ran no quicker than my Q-jet. All I knew was that the engine went "flat" just past 5000rpm's with either carb on it.
After the changes the engine revved to and past 5500rpm's and still pulling really hard with no indications or going flat/loosing power.
So I was not sucking the bowl or bowls empty, just not keeping them completely full up near the shift point.......
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Thanks Cliff, a lot to consider here but it's still not getting me to understanding the delta between the Holley and Q-Jet, and in car vs on the Dyno, and fuel rates at say 4800 vs 5200 even with a regulated constant fuel supply. Even with a .135 needle and seat, what could cause the carb to call for and allow for .60 GPM at 4800, then have that fall off to .56 through 5200-5300 rpm and stop any significant HP gain, when the Holley on the same pump/feed will continue to increase fuel use in a linear fashion straight through to 5500 RPM and .63GPM while continuing to gain HP? Even if the bowl is not getting enough fuel, I cant see how it would allow any less than peak flow ..Type of fuel filter? some kind of turbulance in the filter housing or inlet path? I suggested the electic pump on the car not because I think it needs it, but to take fuel starvation/fill rate out of the equation as I go forward. Car pull strong to 5500 on the mechanical pump and stock lines as it sits, but to your point could I be leaving something on the table and not know (and probably am).
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If you don't keep the fuel bowl full on hard pulls or hard runs it will go lean at high RPM. The Q-jet only has one N/S assembly to feed all the fuel to the carburetor fuel bowl. It's a relatively small fuel bowl so A/F would very quickly fall off if the fuel level in the bowl started to fall some.
Did you try a larger fuel inlet seat? A .145" will flow a LOT more fuel than a .135". I typically limit the use of the .135" seat to about 450hp. From there on up I'll go to the .145" seat.....
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I did go .145, then I even tried .152 with no noticable change. But in my mind, it doesnt matter cause at one point on the dyno with continual electric fuel feed, that .135 seat was able flow .60 GPM AT 4800. RPM's continue to climb, fuel demand stays the same or increases, bowls are emptying but fuel flow decreases to .54 GPM. If the bowls are emptying, and the seat CAN handle .60 at minimum, why wont it continue to do so? We know the dyno fuel feed can give us up to .63 GPM or better, and HP will continue to climb with the Holley. So what about my Q-Jet inlet path could cause the fuel rates to reverse during continued increasing demand? It does it consistantly, I see the same thing on dyno sheets from three years ago, but this time measured in lbs/hr (267 lb/hr down to 202 at 5000) . I never chased it down back then cause on the car, the carb was a champ and clearly not falling off at 4400 rpm. We dont race dyno's, but the stats are trying to tell me something, I'm leaving something on the table.
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Time to focus in on the secondary set-up.
QJ
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Given fuel wont make it in the front door, adjusting the back door seems a little premature?
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I've been thinking about trying a 540 carb with no primary rods, and wanted to see if the group had any suggestions on set up:
-Upper and lower air bleed sizing
- OK to eave the primary piston in place? (this is a trial set up, I dont want to solder a good core shut)
- Primary Jet sizing
- Bypass sizing - this car loves a huge bypass
Anything else I need to consider? this is on a 440/540 Buick 462 stocker, 5700 rpm max, 3.73 gear and 3000 stall, 4000 lbs. Will I lose day to day driveability?
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I would leave the main airbleeds std.
Power piston left in the hole w/o spring.
Use Rochester 2G .065"-.067" jets w/60° approach (short taper).
.110" to .138" idle bypass air.
If more needed for correct position of throttle blades to off-idle slots, holes of proper size may be drilled 1/4" from front edge of throttle blades.
This wont lean WOT mixture as larger holes in throttle plate will.
HTH
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Just a thought Rhett, have you checked the secondary tubes in the airhorn? Sometimes they're pressed it too far blocking some flow.
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Just a thought Rhett, have you checked the secondary tubes in the airhorn? Sometimes they're pressed it too far blocking some flow.
I'll check that! But honestly I think theyre where they need to be, this car runs pretty hard.
Kenth - I couldnt edit my post but had seen where Cliff writes just about the same thing in his book. I've got a set of 67's I can try so thanks for the tip. I think that carb is alreay at .11 bypass. My go-to 540 never had nozzle drip, just wanted huge bypass for idle quality. Even with that, still idles down to 700 rpm, which seems strange to me..I figured it wouldnt idle to below 1200 with .19
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Here's the config of the spare 540 I put together:
Fuel Inlet 0.152
Float Level 0.250
Accel Discharge 0.033
Idle Tube 0.046
Idle Channel Restriction 0.059
Main Air Bleed (upper) 0.070
Main Air Bleed (lower) 0.070
Idle Air Bleed (air horn) x
Idle Air Bleed (upper) 0.067
Idle Air Bleed (lower) 0.070
Idle air baseplate 0.110
Mixture Hole 0.090
Primary Jets 0.067
Primary Rods N/A
Secondary Well restriction 0.040
Secondary POE Restriction 0.060
Secondary Tube Restriction 0.037
Secondary Rods .044 M DA
Secondary Valve Opening 1.295
Secondary Hanger F
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Looks like a good start! :)
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Nah I tried it last night and it was a plug fouling pig. Looked good on paper anyway. I obviously made a lot of changes at once including going to a M4M float so when I have time I'll go back and make one change at a time and sneak this thing up on final build specs. In the meantime I'll go back to my "go-to" 540
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What is the fuel pressure?
You may have overloaded the inlet valve due to the excessive .152" opening and high adjusted float and got an uncontrolled fuel flow.
Did you check for nozzle drip at idle speed in gear (A/T)?
A .135" inlet seat and 11/32"-13/32" float setting might work better.
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Yeah I checked that, it actually wasnt the idle that was a problem (although It could have used more bypass), it was the cruise/primary power/WOT circuits that just were overwhemed. On both the 540's, the .152/M4M float combo seems to work well at .250, the airhorn gasket just weeps a little.
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Unnecessarily high fuel pressure, an unnecessarily large needle valve and an unnecessarily high adjusted float will invariably work together to cause an excessive fuel level in the float housing resulting in all circuits delivering fuel too early and causing the problems you describe.