fuel drain back

[big dan]

I have an 85 gmc w/ 350, q-jet and stock mechanical fuel pump. when my truck sits for several days all the fuel drains back into the tank and makes starting my truck a pain. I have to crank it till the pump pumps fuel back to the carb.

what, if anything, is suppose to keep the fuel from draining back? my dad told me that the pump should keep the fuel from draining back to the tank but I have never heard that before.

I know I could add an electric pump that would pump the fuel up when the key is in the on position but would prefer to keep a mechanical pump. thanks in advance for any help.

 

[toomany]

I don't know if the pump is supposed to keep fuel from draining back, but how about installing a one-way filter between the pump and carb? I've got one on all of my carb'd motors and they always start without much of a problem. Usually have to crank it a few times if they have been sitting awhile and the gas has evaporated from the carb.

 

[dyeager535]

From what I've heard, yes the pump is supposed to act as a one way valve.

However, if you look at the design of the q-jet and it's float bowl, you'll see that the inlet for the fuel into the carb is higher than the bottom of the float bowl...which means you CAN'T suck the fuel out of it via siphon.

Any fuel not sucked out of the bowl via siphoning (fuel level could be high enough in there to suck some back out I suppose if the level is above the fuel inlet into the bowl, but I doubt it) should be more than enough to run the engine at idle for a few seconds, enough to get the pump primed, if the lines indeed go dry.

Your problem is almost certainly due to leaking well plugs, which is a pretty common problem, and can be cured with epoxy. (lame pun intended)

Should be able to find a writeup of some sort on here I would imagine.

 

[big dan]

Ive never heard of a one way filter. where to you get such a thing?

I rebuilt the q-jet that is on the truck. the well plugs are sealed with epoxy along with several other leak prone areas. I understand what you are saying about the fuel not being able to drain back out of the float bowl though so Ill check to see if its leaking off somewhere.

 

[73 K5 Blazer]

I am having the same problem 82 GMC with a 454 and fairly new edelbrock carb on it. You can look at the fuel filter and there is no fuel in the filter at all.

 

[dyeager535]

Do you have a return line? I'm not sure how (or if) that helps the fuel "boil off", but that was another issue GM had problems with. At least the fuel reaching the carb would be cooler, so if you were borderline without the return line, it might be just enough to resolve the issue.

Now that I think about it, I believe that was bad on aluminum intaked 305's. GM ended up installing a small electric fan that blew on the carb for a certain amount of time after the engine shut off.]

Remember, this issue is not universal/inherent in these designs, so something is not right with *your* setup, and anything other than finding the root cause is a band aid to the real problem. Not pointing fingers, just making sure effort is focused on something productive.

 

[big dan]

no, I dont have a return line. and I dont want to put a bandaid on it, I would really like to solve the problem. My last truck I owned had a similar setup and it could sit for weeks at a time without the fuel bleeding off so I know there is something wrong, just trying to find where to look.

Im gonna pull the carb off when I get home and make sure the well plugs (or something else for that matter) are not still leaking.

 

[dyeager535]

IIRC there are at least two sets of well plugs, but I am unsure if the large ones that are easily visible are the only ones that seal the float bowl, or if the others also seal it.

Another idea is your float...how is it set? If it's too low, I'd expect fuel boil off to be more likely.