Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
7041540 Buick 455 goes lean at WOT
Rhett:
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...
Cliff Ruggles:
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.......
Rhett:
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).
Cliff Ruggles:
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.....
Rhett:
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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