Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Fuel pressure issue? Comes and Go's

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BARRACUDA1968:
Hello, I have a Y88 78 Trans Am 4 speed car.  Original engine and carb.

My fuel pressure comes and go's?

I've replaced the tank, sending unit, all rubber lines; had the carb rebuilt with Cliffs kit and installed 2 new fuel pumps.

I have a fuel pressure gauge hooked up to the car and taped to the windshield.  Basically the car runs out of gas after running it wide open for maybe 3 or 4 seconds. 

I'm on my second mechanical fuel pump, just an over the counter piece.  That's all I can find that matched the original fuel pump.  I'm trying to stay away from an aftermarket pump because the car is original.  I think the pump is made by airtex?  I got it at CARQUEST.

It will idle between 4 and 7 psi just fine for a while then drop to 2 psi and just lay there.  Reving the engine doesn't change it.  Then if I drive it down the road it will sometime go to normal pressure then after a while it will drop down under 3 psi.

I double checked the lines again today and all seems fine.  I unhooked the feed line and blew air through it and installed a new rubber line back by the tank.  When pressure is down I have loosened the gas cap and no air.

It just seems like something is restricting fuel flow or vapor locking??  I am at a loss at this point.  Maybe the fuel pumps I'm getting are junk but 2 in a row seems unlikely.  My motor is basically stock so a mechanical pump should run it fine.

I do NOT want to through on an electric pump.  I want to figure this out.

I am running a return line and a vapor line to the canister.  I unhooked all the line from the canister just for kicked and plugged them, still the same.

Throw some ideas at me,  I've tried everything I know to do.

Thank you,

Brett

Malibu72:
          I have read on other sites of problems, usually with too high a pressure output, with that particular brand of fuel pump. I am assuming that when you replaced the sender that you also replaced the filter sock. I would purchase a AC Delco pump. Also make sure you try a new fuel filter.  Hope it helped.

429bbf:
welcome aboard . im not familiar with your fuel system but this is what i would do. i would unhook the fuel inlet at the pump and put a gallon or whatever i could get under the hood. directly to the fuel pump.this should tell you if its the pump or the supply coming to the pump.one key thing to remember (pumps dont make pressure) they produce volume and it takes some sort of restriction. ie needle and seat ,fuel line size etc.to make pressure. fwiw

BARRACUDA1968:
Thanks for the information!

Yes I installed a new sock on the sendor.

The only place I've been able to find an AC Delco pump is Butler Performance.  I talked to Tech and they couldn't tell me who made the pump and how good is really is.  It's about $85 and the pump I'm using is $36.  Again the only over the counter pump I can get from 3 different auto parts stores.  They all carry the same pump.

Tell me more about this needle and seat.  I know what it is and I understand it but is there a way to adjust it on the carb?  IE can I set it to withstand more pressure.

When my fuel pressure go's down the car seems to idle a little high and run rich which would seem opposite to me with less fuel pressure but if the pump is blowing through the needle and seat I guess I would have less pressure and shoving more fuel in the motor?

The guy who built the carb is a competent builder.  He has a special tool he made so he can take this plug out of the top of the carb and then he has a short looking screwdriver with a slot in the end of it to adjust something?  I'm thinking float level possibly?  It's toward the front of the carb on top.

I was under the assumption that these stock type pumps pressure between 4 and 6 psi so I really shouldn't need a regulator.  When I do have fuel pressure I am in that range.  Then the pressure go's away and sometimes comes back and sometimes not.

I have responsive idle air screws as well.

I can try the fuel pump without the main feed line to the carb and see how it pumps.  Not sure if this tells you anything but I can take the main fuel line off the inlet on the fuel pump with the car level on the hoist and some fuel will run out and stop.  I do not have to plug the line.  I figured fuel would just run out but it doesn't.  Maybe thats normal.  Just trying to give as much info as possible.  It's probably something very minor I'm missing.

One other thing, I am not running a fuel filter right now.  My carb builder said not to run the filter on the front of the carb so I plan to install an inline filter.  Just need to figure out where I am going to put it at this point.  Right now I'm just trying to get things lined out.  I doubt this would make a difference but I'm throwing it out there.

Thanks for the help so far.

429bbf:
not sure where to start. your rebuilder is adjusting the APT (adjustable part throttle ) this sets the height of the metering rods in the jets.i think if you hook an electric pump to the carb with the engine off you may find your problem.if the needle and seat are bad you should be able to see the gas run into the engine or out the vent.its sounds like your pump is not putting out a constant volume.if you use the electric to feed the carb you will be able to narrow this thing down.hope this makes sense

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