Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

7041540 Buick 455 goes lean at WOT

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Cliff Ruggles:
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........

lightning boy:
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.

Rhett:
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.

Rhett:
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....

Cliff Ruggles:
"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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