fuel supply shortage

[resto75]

My 75 doesn't seem to keep the fuel up to the carb when it has been sitting for a few days. It seems I have to pump it up from the fuel tank each time before it starts. I can pump the gas but it won't start until I have cranked it for a minute or two.

Is it possible that the gas is draining back to the fuel tank and has to be cranked back up the line each time? Would the fuel pump be the problem causing this?

 

[brusso]

I believe that it could be something as simple as the fuel filter is in backwards. I'm not sure, but it is simple enough to check. The rubber end faces the fuel line.

 

[vettenick]

There is a valve in the fuel pump, but even if you were draining back to the tank, there still should be enough fuel in the carb bowl to get the car started. Next time, before you try to start, remove the air cleaner, and move the throttle to see if you're at least getting a fuel shot from the accelerator pump. If not, maybe the accelerator pump in the carb is bad. Just a thought.

 

[JohnZ]

It won't drain back to the tank, since that's uphill; if you don't see any evidence of external leakage, remove the air cleaner in the morning and shine a flashlight down the venturi to see if there's any raw fuel pooled in the bottom of the manifold from an internal carburetor leak. If you pull the dipstick in the morning, do you smell gas in the oil? If you see fuel in the manifold or smell gas in the oil, get the carb checked out and rebuilt and change your oil.

 

[Roy]

Fuel Feed Problem

The C3 Vette uses a fuel pump that feeds the carb and pumps the extra fuel back to the fuel tank, recirculating it to keep it cool.

Fuel can "drain down" between the carb and the fuel pump if there are loose connections or old ratty fuel hoses that allow fuel to permiate the hose. If this happened, however, you would smell gas fumes. Check your hoses and conections starting at the carberator and moving rearward.

Change your fuel filter which is probably located between the carb and fuel pump. On Q-Jets there is also a filter inside the carb body where the fuel line connects.

Test your fuel pump to see if you are getting sufficient flow. Ab easy way to do this is to remove the feeder hose from your carb intake and have someone crank the engine over while you hold the hose in a container. You should get plenty of gas coming through in the first 2-3 seconds. If not, then the suspects become: 1. Fuel pump 2. Plugged pick-up in fuel tank. If it were me, I would tackle the fuel pump first.

Note: Last year when I disassembled the engine block on my 78 Vette with 90,000+ miles on it, I removed the fuel pump for inspection. Guess what? The actuator arms inside the fuel pump that is actuated by a push rod ring on the camshaft was almost impossible to move up and down. That meant that the fuel pump was almost froozen solid and was not pumping the volume and pressure of fuel needed by the engine. Seriously, I have no idea how the engine ran with the fuel pump being that bad.

Remove the fuel pump that is held on with 2 bolts and check its operation. When in doubt, replace it as they are relatively inexpensive.

If the fuel pump is fine, you may have a restriction in the fuel line of at the pick up point inside the fuel tank.

And one final possibility is ... make sure that you do not have a vacuum in your tank or fuel line. This can happen when the gas cap or the charcoal cannister do not allow air into the system. If this happens, the more the fuel pump draws fuel from the tank, the more vacuum builds within the system. This would explain why your engine is hard to start after sitting for awhile. If vacuum built up while you ran the car, then vacuum in the tank could "suck" the fuel out of the fuel lines as it attempts to equalize internal pressure in the tank.

Try these ideas out and let me know what happens.

 

[KenSny]

I would be looking at the carb. The float bowl(s) should still have fuel there even after sitting a couple of days. It is not all going to drain back to the fuel pump. It was just stated that a stock carb filter should be installed with the rubber end towards the fuel line then fuel CANNOT flow towards the pump. That's why GM installed this type of filter - in case of a rollover the gas in the carb would not drain out and cause a fire. Altho other things will start fires......

An easy test would be to temp install a small plastic see-thru filter between the carb and the fuel line. Next time you go out to start the car check and see if there is gas in the filter.

 

[Vettehead Mikey]

I assume you already made sure the choke is closing, right?

 

[resto75]

Maybe I should of gave you more info on this. The engine has a new Edelbrock 600 cfm performer carb and a new Russell in line filter. The fuel line from the pump to the carb is new also.
I replaced my Holley 750 vac with this new setup a few months ago but I haven't really been happy with it. The only problem I had with the Holley was a rough idle but that probably could of been dealt with. The engine ran smooth thru out the rest of the rpm range. As I didn't know the age of the Holley I just replaced it with the new performer.

I also noticed that it is very touchy as far as timing is concerned. A tad to much either way and you either end up with pinging, engine run on when turned off, or a sluggish cranking condition. To be honest I haven't really played with it too much but will be looking into it in the next week or so. One other thing I noticed now with the timing set at its present setting is that the engine seems a little sluggish ( kind of like driving with you brakes dragging).

I think I just need to retune from square one and get this properly set up but would like any suggestions.

 

[vettenick]

Do you still have the EGR valve and catalytic converter on the car?
What kind of mileage do you have on the motor? Have you done anything mechanically on the motor?
I'm just trying to get a feel for your car.

 

[resto75]

No EGR and the engine from what I have been told has forged pistons with appox 10.1 with double hump heads. It also has a Crane 284 cam and roller rockers, headers and dual exhaust. It has two of the performance type cats as they were doing emissions checks a few years ago and would not of passed without them.

 

[vettenick]

Try an engine vacuum test. See what it is at idle. Then bring the RPM's up to about 2500. If the vacuum starts to drop, one or both of those cats could be beginning to block up. If they are, that could explain why you're experiencing sluggishness. This test should take less than 10 minutes.

 

[JohnZ]

The fuel in the float bowl isn't evaporating - it's draining - either externally or internally. Once it's in the float bowl, it can't get back out through the inlet, which is above normal fuel level. Its only route out of the float bowl is through the main jets into the intake, or through a crack in the carb casting.