Fuel Evaporating or Draining Back?
 

My 69 Lemans has a factory metal fuel line with no return. New rubber lines, new Spectra pick up in tank, new vented fuel cap, a rebuilt Carter 6907 fuel pump, late 70s Buick QJet built by Cliff that I used on bird years back - marine epoxy used at well plugs. When the car sits even for just an hour or two, I have to prime the carburetor to get it started. I don’t notice much of a pump shot and extended cranking doesn’t help. Once I manually prime it and get it started,! Runs well and will start up as long as it’s not left sitting. Fuel is 87 octane, 10% ethanol (MD). Any ideas on what the issue is?

What temperature does your engine typically run? Could be fuel percolating in the fuel lines. Try one of these one way valves placed at the fuel pump inlet hose. https://www.amazon.com/Engraved-Dire...43209887&psc=1

I have a similar issue I've been trying to get sorted. Mine takes a day or so before I have to give it a few cranks to get fuel to the carb. Some folks say it's the fuel evaporating, but I didn't have this problem prior to my rebuild (when the carb was also rebuilt, too).

I was debating installing an aftermarket check valve, but based on old threads I read most mechanical fuel pumps already have anti-drainback valves by design. The fuel pump in my car is at least 10-12 years old so I'm going to try two things - a fuel filter with an anti drainback valve and a fresh fuel pump.

A fuel pump cannot function without exactly the same kind of check valve(s) as has been suggested.

Remove the fuel pump inlet hose, connect a vacuum pump to the fuel pump. Find out if the check valve leaks, and if so, how much/how bad is it?

Would be worthwhile to determine if the intake manifold is being over-heated by an exhaust restriction on one side leading to too-much exhaust blowing across the intake manifold exhaust crossover. A seized-shut (or partially-shut) heat riser (EFE) valve, for example, or the inner wall of a double-wall exhaust pipe having collapsed. Crimped exhaust tubes, etc.

The engine runs at 180 exactly, which is the thermostat rating. It hasn’t been driven on road, just idle time.

This engine has RA manifolds and the correct intake gasket for a 1972 intake on a 1969 engine. When I rebuilt the fuel pump using a Then and Now Automotive kit, I performed the blowing and suction test on the diaphragms and they operated as designed. Won’t hurt to check again now that the fuel pump is on a running engine.

I have considered an in-line electric pusher pump to prime the fuel line but wanted to check here first for any testing I may have overlooked.

I'm far from an expert but, I do have a mechanical pump and a Quadrajet. No other external check valve other than what the pump (and qjet filter??) may have. After running the motor, it can sit for up to 72 hours before requiring cranking/priming the bowl. After that it does take a few seconds of cranking. If it was a daily driver, it would fire right up every time. So that tells me there is something up with your system. I would try to find and fix that before adding a pusher pump.

I dont think mine is quite as bad as an hour or two, but if the Q jet on my Olds 455 in my wagon sits overnight you have to crank it for a good 7-10 seconds to get fuel into the bowls and get it started. As soon as it gets fuel it fires right up.

I recently rebuilt the carb that is on there and it made a huge difference. But I didn't do anything to the well plugs. So Ive been wondering if maybe they are leaking. It also probably has a 40+ year old fuel pump so it wouldn't surprise me if it had seal issues.

That said, even my Firebird which has an electric pump and fuel injection, it has to crank for a second after it has sat. Not nearly as long as the wagon with the carb, but more than if it had been running an hour or two before. As has been mentioned here before the ethanol blend fuel does evaporate quickly.

To check for leaks, pull the carburetor, set it up on a raised stand. (I use four 2 inch long bolts, one in each corner with nuts, for legs.)

Place the raised carburetor on a clean paper towel. Fill the float bowl with fuel and let it sit. If it's leaking you should see it. I try to do this test with a hot or at least warm carburetor as heat can help expose cracks.

Good advice. 100% honestly, if I pull the thing off Im probably just going to do the little kit to it that Cliff sells. Might as try and generate some content out of it.

But yes Ive read that unless they are severe they often don't leak until good and heat soaked. Which makes sense.

Does not having a return line to the tank affect this at all? It’s as designed by PMD.

Check your fuel line clamps & accelerator pump shot

I have a big Holley and a Robb Mc fuel pump. If mine sits for a while, I have extended cranking time also.


This modern pump piss has a high evap rate.

Using the same carbs from the 70s and 80s. They didn't do this back then, but they do now. Its the junk gas.

Rubber lines get old and you can't always tell they leak. They may not drip, but under suction they may leak. If they're old, replace them. If you suspect leaking, retighten the hose clamps for starters.

I refuse to use mech pumps anymore, even the ones that were 'good' back then, aren't anymore.

I've argued with folks over the RobbMC ones too, even the 1100 ones, they just don't work as good as electric.

Most people don't even realize they are leaving power on the table until they switch.

You do a 'proper' bypass and electric pump setup and there will be no question. If you have evap or drainback issues, with an electric, gone.

If an EFi conversion tank is available for your model even better. And you don't have to run EFi.

I've seen people throw all kinds of time & money at a mech fuel system only later to finally cutover. Just cut to the chase, do it right, do it once.

Even back in the day folks knew the factory systems didn't cut it. They were borderline right off the dealer lots. At best.


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All the rubber lines are new and the fuel pump rebuilt with an American made kit from a reputable recommend company. I’m starting to think the carb plugs are leaking even if sealed with marine epoxy.

The new fuel evaporates like crazy. It's designed to. For spills on ground, evaporate before it soaks in. I put a Holley Blue on my 470cid. Turn key, hear pump fill carb. When pump sound changes a little, she full, ready to go

It's possible. Leaking fuel leaves a witness mark, should be able to tell.

Maybe the epoxy cracked, it happens. I know JB Weld doesn't do well with heat cycles and gas, not sure what was used.


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My issue may be two -fold, evaporating fuel is one thing but cranking excessively doesn't seem to restore prime. Only after manually adding fuel to the bowl will it start up. Although I checked and didn't see any evidence of a fuel leak in the long hard line, the car sat outside for about 20 years. It still had fuel in the line but smelled like varnish when I flushed it. Is it common for steel lines to develop pin holes? This line has what appears to be undercoating on it.

Do a pump test into a bucket and time it, might be a pump issue.

Or maybe a fuel sock, or crap in the tank.



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General replacement tanks for that car are only like $175.



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