Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Fuel deliver, and a brass float

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Gizmo:
Hi Guys.

I'm running the bigger quadrajet ( lump in primary venturies, off a 454 ) on a 350sbc, on a victor junior manifold, some head work, medium cam. On the dyno we were getting just over 400hp at the engine. The carb has tuned up well, is smooth, good idle, good economy, great response, no bog or flatspots. But having trouble with the fuel supply.

Mechanical pump, 3/8 line all the way to the tank, with a electric pump at the tank to help ( switch on dash ). I've seen two problems. One we noticed on the dyno, under power the carb would suddenly flood and pour fuel out the breather and down the carb. I think this was caused by a vibration, had a spark plug lead off. Next run it was ok, but is the float that sensitive to vibration?

Second problem is fuel starvation under power. Seams a bit randon. Even with the electric pump on, under power in 2nd it would starve and die, and take a few seconds to recover after I back up as the bowl refills. Try again, and its OK.

If I take the line off the carb, there is fuel gushing out, and a pressure guage shows 6 pounds. Carb built in fuel filter seams ok, I can breath through it. There is no return line. What should I look for?

Also, mine has the brass float, does this need to be taken in account when setting float levels, as all the guides I've seen are for the plastic float.

Thanks.
Glenn ( Australia )

quadrajam:
My guess would be fuel boiling in the fuel pump and line to carburetor.  Make sure the fuel
line is not touching anything hot or maybe insulate it.
A fuel return line may fix the entire issue. You almost have to run one nowdays with
this crappy gasoline.
  As for the brass float, I prefer the phenolic floats but if it runs as good as you say otherwise
dont dwell on it.

Carefull with those fuel leaks around loose plug wires, that could have been bad.

QJ

Cliff Ruggles:
Never use a brass float in a Quadrajet.

What is the carburetor part number?

What has been done to it?  Fuel inlet seat size, etc?

What are the engine specs, CID, compression ratio, head casting used, specs from the cam card?

Couple of things I've learned about fuel delivery over the years.  Using an electric pump before a mechanical pump is just putting a stumbling block in front of it.  Despite all sorts of success stories I've read on other Forums in not one single instance have I been able to overcome a fuel delivery issue with a "pusher" pump to a mechanical pump. 

Electric pumps are free-flow, mechanical pumps start and stop fuel flow with each stroke of the pump.  Mechanical pumps are engine mounted so have a LONG supply line which is NOT a good scenario for a really fast car with a high HP engine in it.  It is ALWAYS better to "push" fuel instead when it comes to this sort of thing.

"High Performance" and "high output" mechanical pumps do NOT like to be dead-head even though many folks making and selling them tell you they can be used that way.  Here I ALWAYS provide a return system for any high output fuel pump, whether electric or mechanical.  It takes load off the pump and can and will prevent intermittent flooding issues.  For electric pumps running a return will make the brushes in the motor last at least 100 times longer as they work pretty hard in a "dead head" scenario......

Gizmo:
Hi Cliff

Its a 17080512 quadrajet. Choke removed ( its hot here ), new bushes, etc. Some fiddling of the secondary needles and flap spring to clean up the bog. Drilled and taped a plug above the power piston adjustment, adjusted for good highway millage. Sits on a VJ with a home made adapter, nicely blended in. Dont know the needle seat size sorry, whatever it came with.

The motor is a 350 +60 thou. Heads are 441 castings, standard valves, some port tidy work. Cam is a 280 deg 444 lift inlet, 290 deg 467 lift exhaust, standard rockers. Block decked 100 thou. Flat top pistons. Exhaust extractors, 2.5inch dual exhaust. TH400 with a 2.5k stall.
Cant give you the compression ratio, I built the motor 30 years ago and long forgotten, but suspect over 10:1.

I'm tempted to run a 1/4 return line back to the tank, from a T piece just before the carb. At least that would get any air or vapor out of the line.

I'll order a new plastic float, any recommendations on needle and seat?

Glenn

PS. sorry, uploaded wrong photo, there is no fuel regulator these days.

Gizmo:
Correct photo, and the car if wondering what we drive over here.

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