Fuel delivery problem.

[piano_man9009]

Ok i give up, i'm asking for help.
My Cherokee with a '75 401 in it runs pretty great, until it needs much gas. When i floor it, at about 45-55ish it usually starts to sputter and feel like its just not getting any gasoline. If i don't punch it as hard i can of course work my way up to a higher speed, but i wasn't even able to get on the interstate the other day when the onramp was uphill.. didn't have it floored.

I have dual tanks, it does it from both tanks. Both tanks ventilate properly (well maybe properly is a stretch, but i know its not getting any kind of vacuum in the tank:D). I bypassed the tank switcher, and still does it. replaced all the hoses, replaced the fuel pump from a carter m6505 to an airtex 6505. replaced the fuel filter. cleaned out and reset the floats on my edelbrock 1400 carb. noticed the primary jets are .003 undersized from stock, i assume because i bought it from colorado, i haven't changed those.

The engine has 10,000 miles on a rebuild, perfomer intake, 600 cfm carb, try-y headers, and a 268H comp cam. I would assume thats not enough to need a high performance fuel pump? I even rigged up a cheap electric fuel pump on it, i think it was supposed to put out about 50 gpm, i had it clear up on the driver's side wheel well, the only difference was when it started to sputter, it would completely die. Wasn't easy to start again afterwards either.

I rigged a pressure gauge that i could hold in the cab, when i smashed the pedal, i could watch the pressure slowly fall and then it would of course cough and sputter and all but die when the fuel pressure hit zero.
before i checked the fuel pressure to make sure it was a fuel problem, i replaced the coil and pulled the dizzy and did some work on it too.

Anybody got any bright (and inexpensive) ideas?

[jeeper54]

Try it with the gas cap(s) off? Sure pick up tube and sock are good/clean? I'm mechanically challenged so that's all I can think of.

[Wayne]

Try putting the electric pump on the frame close to the tank.

[Snowbird_Hunter]

Try putting the electric pump on the frame close to the tank.

Would you add it to the line, or use it to replace the mechanical? I was wondering about this when I ran across this thread :D

[Greenfish]

Hmm.. what mechanical fuel pump are you running? You shouldnt have to run anything but a good mech pump.

EDIT.. ok I just re-read and you said that already.

[piano_man9009]

Try it with the gas cap(s) off? Sure pick up tube and sock are good/clean? I'm mechanically challenged so that's all I can think of. I've tried it with the gas cap off, and when i pulled the factory tank it didn't have a sock to get dirty


Try putting the electric pump on the frame close to the tank. I had the fuel pump all ready to go on the frame right by the fuel pump, turned it on... nothing! apparently when i dropped it about a foot, it knocked a wire loose or something inside, now its trash.

Would you add it to the line, or use it to replace the mechanical? I was wondering about this when I ran across this thread :D When i had it on the wheel well, i only ran it through one or the other. I was planning on trying different things when i got it back by the tank, but thats not gonna happen now :(

I have thought about getting another electric pump to try on the frame like i wanted to, but i really think i'm not having problems with the pump, i should have more than enough pump already for what i'm using. By the way, if anybody is looking to get a new good mechanical pump, the airtex 6505 is MUCH bigger than the carter m6505. Idk much about how those pumps work, but i assume it would have a bigger diaphragm and can push more fuel?

[tgreese]

You should be able to get gas to siphon out of the tank without any pumping. All you need to do is place the outlet lower than the level in the tank. Connect a piece of fuel hose at the pump end and see if you can make the gas flow. If you can't siphon, you have an obstruction or an air leak in the path from the tank to the pump.

[Greenfish]

I have thought about getting another electric pump to try on the frame like i wanted to, but i really think i'm not having problems with the pump, i should have more than enough pump already for what i'm using. By the way, if anybody is looking to get a new good mechanical pump, the airtex 6505 is MUCH bigger than the carter m6505. Idk much about how those pumps work, but i assume it would have a bigger diaphragm and can push more fuel?

I agree that the mech pump should be more than sufficient for your application. I am running a mech pump on my cuda with a 550hp 440ci motor and it feeds it just fine.

[FSJ Guy]

Double check the float settings. If the float is set too low, you might run out of gas at WOT even with a good pump.

[piano_man9009]

i've ran different line from the pump to a 5 gallon tank in the cab and it still did it, so i know the problem isn't behind the pump.

I'll double check the floats, but that still wouldn't explain the drop in gas pressure in the line.

I noticed last night that the outlet hose barb i took off the old pump and put on the new one is 5/16'' and i have 3/8'' hose hooked to it, i'll get a 3/8 barb and see if that was just restricting it too much.

[tgreese]

...

I noticed last night that the outlet hose barb i took off the old pump and put on the new one is 5/16'' and i have 3/8'' hose hooked to it, i'll get a 3/8 barb and see if that was just restricting it too much.

More likely the too-small fitting is leaking air. A 5/16" line should be plenty big enough.

[piano_man9009]

More likely the too-small fitting is leaking air. A 5/16" line should be plenty big enough.
maybe it was, but it was on the outlet side, so it would only be leaking gas out, and i haven't seen any coming out. My old fuel pump has the hole inside looks like only 5/16'' anyways, and then opens to the threads, so a 3/8 fitting won't help any. i'll put it on just to get a better hose connection though.

I've literally tried replacing everything from the tank to the hose up to the carbuerator. Why on earth would i still be losing pressure in the line? if it were a carb problem, or electric problem, i wouldn't be losing that pressure. :banghead:

[piano_man9009]

Done some more work to it, but still haven't got it fixed. When i let gas out of the line as soon as i turn it off, there is a decent amount of air (or gas vapor or something) that blows out of the line along with the gas, but i never really have a problem with it vaporlocking at low speeds or stopped or anything.

That engine compartment is HOT. it'd like to burn my hand even when i touch the top of the hood. the temp guage never reads too hot, and i've never had it boil over or anything like it overheated, and i've got it full of antifreeze, and it flows fine.

[fasteddy]

I'd try wrapping the headers with heat shield header wrap. I think you are vapor locking the fuel pump. Might also insulate the tank>pump fuel line...

[piano_man9009]

I'd try wrapping the headers with heat shield header wrap. I think you are vapor locking the fuel pump. Might also insulate the tank>pump fuel line...

You could be right. Though the pump is a fair distance from the header tubes, but i'll try putting some wrap on the pump to see if that makes a difference at all to keep things cheaper at first. the headers are ceramic or chrome coated or whatever, so i wouldn't think they put out nearly as much heat anyways, thought they look pretty old and like half of that has worn off.

But here's another head scratcher, doubt if it has anything to do with the fuel problem, but i was checking my vacuums today, The manifold vacuum on the carb acted just like i assume a ported vacuum is supposed to work (no vacuum at idle, regular at everything else). The ported vacuum on the carb acted like it was supposed to, except at idle, instead of giving no vacuum, it gave a steady 10 inches. my vacuum advance unit also acts like it has a hole in the diaphragm, and doesn't move at all, i'll be getting a new one of those soon.

[piano_man9009]

Put a cheap electric fuel pump on today back by the rear tank with the mech pump hooked up too, and it fixed the problem. I think it must have been pulling air through somewhere, although i replaced everything. If it were vaporlock, it would do it at other times than full throttle.

Problem now is, if i could pull air in, can't it push the gas out now? I let the elec. pump run for a while but couldn't see any leaking out.