[jhein]

Below is my post from a different thread. No testing for evaporation but with a Qjet filter in place and a RobbMc pump, I have no drain back.

"So about 2 weeks ago I decided to remove a RobbMc in line fuel filter that I had right before my pump after I realized that it wasn't needed since I have a sock on my pickup and the Qjet filter in place. I had to move the car before I did the job so I knew the bowl was full. When I removed the line from the pump, not a single drop came out. The next day when I went to start it, it fired right up instantly like always. So I don't seem to have any drain back. I believe it's just good old fashioned evaporation which in my environment with my fuel takes about 3 days after a hot shutdown. It's also likely that most of it occurs while the engine is still hot. With the hood down and the air cleaner in place it probably stays warm for quite a while. Once it cools down the process must slow down."

[Shiny]

Thanks for the updates.

I also went searching for this thread after reading the new one and now remember why I talked myself out of adding a primer pump.

Many unanswered questions remain, but I still believe evaporation is the dominant mechanism for my carb emptying when hot.

But I also think the 2bbl pump I have might contribute to "how long" it takes to fill the carb once empty. Many suggest their cars start with much less crank time than mine when they are dry. There is nothing in this thread that says why some systems can fill an empty QJ in a reported 5 seconds, yet mine (measured) takes closer to 15.

If I get frustrated, the cheapest thing to try is a "tall can" 4bbl pump. Even though their max flow is not "rated" a lot higher, they may well flow a lot more... never found data to compare objectively.

Interesting topic, no easy diagnosis and very little real data (with exception of RobbMc).

Mike

[steve25]

Having a 2bb pump vs a pump rated for a 4 bbl volume needs should make no difference in hot restarting refill rates or cold condition refill rates .

Your fuel pump cam eccentric might be worn as might the pump arm.

[Shiny]

OK, thanks for that. The specs for "max" flow would suggest the same, but to me, those specs are about "zero" resistance to flow, or zero pressure at output. This is not the situation when pumping through the needle and seat, but as I posted, I cannot find any pump curve that actually compares flow vs pressure (with exception of RobbMc).

Cam is new, pump is new, so IMO is unlikely related to mechanical movement. Pickup is new, lines are clear, nothing else I can think of beyond a defective or otherwise low-flow new pump, which I suppose is possible.

All I know is mine takes almost 15 seconds of cranking to fill and others believe theirs takes much less. Would love to understand all this but I don't find the answers falling from the sky.

Mike

[SR-71]

I bought a pack of wide mouth 850ml (8 oz) HDPE lab wash bottles and gave them out to friends to use. Just squirt some gas into the fuel bowl vent and a little down the carb and it starts almost immediately.

[Shiny]

Yup...,that's my current solution... it's just a minor hassle to open the hood and remove the air cleaner cover. Somewhere between nothing and a nuisance right now. But if I ever drive the car, I'll want it to start like a vehicle and not like a test of manhood.

[Formulajones]

What's your fuel pressure say

[Shiny]

I haven't measured pressure since replacing my fuel tank, pump, and getting carb fixed by Jeff.

When the carb is filled the engine fires up quickly and idles fine.

My pump is a "short can" Delphi AMF0155. Maybe other pumps push more fuel at cranking speed ??? I tried to get answers in the form of actual pump curves but struck out.

It would be nice to know exactly which "stock replacement" mechanical pumps refill an empty carb in a few seconds. I'm pretty sure a performance aftermarket pump could do that....

My pump is spec'd @ 7psi max.

When I was chasing pump curves (no success), Carter provided the following:



This implies the tall can design may push more volume at low rpm, which is why I asked... If others use tall can pumps like this Carter M6122 and they fill an empty carb in 5 seconds, that would be good to know.

Mike

[Formulajones]

I have both tall and short cans in service. I don't have issue with either. The 69Z has a factory short can pump, single 3/8 feed and no return. I've let that car sit for 2-3 weeks and it starts on the 1st try after 2-3 revolutions. Maybe 3 or 4 seconds?? But I'm willing to bet it's firing off the squirter pump shot and what's left in the float bowls before the pump builds any real pressure.

My 70 Formula has the large can pump with return. It basically acts the same way.

That's the part that's puzzling to me. You guys shouldn't have to be waiting for a pump to fill your carb to get the engine started. There should be fuel in the squirter circuit, and fuel in the bowl. Unless you guys are just getting these carbs so hot you're percolating the fuel when you shut it off or something weird like that. It just doesn't evaporate on it's own even after 2-3 weeks. In fact I haven't started the Nomad in nearly a month and I'd bet money if I pull the side float bowl screw out and shake the car I could get fuel to trickle out of it.

If you guys really have a problem cranking the car for a few seconds after a period of hibernation, or you mysteriously have fuel that disappears and you can't figure out why, I think an in tank electric fuel pump is really your best and cleanest solution. It doesn't solve the disappearing act but at least it will build pressure and fill your carb before you even begin to try to start it.

[Shiny]

Thanks again.

You are living the dream it seems.

Are yours quadrajets? If not, Jon suggested in this or another thread that other brands/designs may be less prone to evaporative loss.

I put a new tank in about 2 years ago.. probably should have spent more for an in-tank pump then but that opportunity has now evaporated like the gas in my carb.

Good news for me is my car continues to hibernate in a garage and starting is just to say I can. I think I'd find some kind of electric pump quickly if I was actually driving the beater...with or without a mechanical pump. For now, just an exercise in what if... but a good challenge!

Mike

[Formulajones]

Yes the firebird is a qjet. Others are Holley.

[78w72]

Post #120

"Sorry to bump an older thread, took a minute to find this one, theres numerous pages when searching for "fuel starvation/evaporation" and they all explain the same thing that some cars lose fuel after sitting for X amount of days, usually a week or so, but I just wanted to update this after I did a quick test to see how fuel evaporates when its in a real world situation like in a carb on a warm engine.

Regarding a qjet fuel bowl going dry, the test above of putting a full cup or 2 of fuel in a cool container on the garage or house counter showing it didnt lose any fuel so we all must have something else going on...
Well just last week I remembered this thread and how the test wasnt what fuel does when on a hot engine/carb, so I filled a tablespoon or a little more of fuel in a small metal container, about the size & amount of fuel in a q-jet bowl, I set it right on top the carb after a short drive but fully warmed engine at ~180°. After letting it sit with hood closed in the garage for about 1.5 hours I came back to check & sure enough it was completely dry, not even a residue of wet fuel left. Granted inside the carb isnt exposed to open air but the carb is fully vented. This test is more of a real world scenario for fuel in a q-jet than a big cup of fuel at room temp, based on what I found after a short time fuel definitely can & does evaporate at a pretty fast rate when heated like an engine does. This was non ethanol fuel too, 10-15% eth will evap even faster.

I cant say for sure what else would cause the hard starting after sitting for X number of days but its clearly a real issue that many people have on q-jets, not just pontiacs, a quick google search turns up countless examples, maybe its also drain back related to certain fuel pumps, but fuel definitely does evaporate at a faster rate than some may be aware and Im sure it contributes to the problem.
"

The fuel in this test or on most the cars that have this issue is not getting so hot its percolating at shut off. The engine I did the test on is a E-head 72cc with no exhaust crossover & a 71 HO intake.. big air gap under the intake and the engine runs at 180° in 90f summer temps & barely gets to ~170-175 on cooler 75-80f days. Non ethanol 91. Overheated fuel is unlikely the issue here.

Fuel evaporates pretty fast when its a small amount and exposed to normal engine heat , even lawn mowers & small engine equipment have primer bulbs, because the fuel evaporates in the small carb bow during times of unuse, I have to prime my lawn mower every week I use it, the gas sits in the bowl with nowhere to leak down so it has to be evaporating.

Maybe some carbs/cars dont show this as bad, maybe the pump fills the bowl faster in some situations, but again, this is not new & there are all kinds of reports of it.

[78w72]

Test #2- just put the same amount of fuel in the same container, about 1tbs or a little under 1oz, size of a q-jet bowl... placed on the workbench in a 72f closed garage, completely evaporated after 1.5 hours. Non eth fuel...

Anyone else is welcome to do this test, temps & humidity will vary the results, but fuel definitely does evaporate pretty quick when its this small of an amount exposed to air or a vented carb.

[Shiny]

Originally Posted by Formulajones

Yes the firebird is a qjet. Others are Holley.

Thanks again.

I see you said tall can pump on the firebird! Specific brand and PN if you can remember?

[Formulajones]

It's a factory 26 year old AC Delco pump back when you could still buy them.

[jhein]

Originally Posted by 78w72

Test #2- just put the same amount of fuel in the same container, about 1tbs or a little under 1oz, size of a q-jet bowl... placed on the workbench in a 72f closed garage, completely evaporated after 1.5 hours. Non eth fuel...

Anyone else is welcome to do this test, temps & humidity will vary the results, but fuel definitely does evaporate pretty quick when its this small of an amount exposed to air or a vented carb.

Obviously I knew that Qjets have a comparatively small fuel bowl but 1 oz is really not much at all. Just for fun I asked AI how much fuel a Qjet bowl holds and it said 1.5 - 2 PINTS!!! LOL.

Anyway, based on your test result it's actually a miracle that I have enough fuel for a start after 3 days following a hot shutdown.

[78w72]

1.5-2 pints... LOL! Shows how dumb Ai is on so many things. most q-jet fuel bowls hold about 1oz or less is what I would guess based on seeing them but never cc'd the bowl.

I re did the test, test #2 was slightly flawed in that I think the small container wasnt fully level & may have let some fuel spill or wick out since I noticed a small stain on the cardboard it was on after I noticed it was dry.

So I did test #3 yesterday, filled 3 different containers, a soup can, a small plastic cap from brake cleaner that is impervious to fuel, & a glass shot glass ~1/2 full, was closer to 2oz in the soup can. I set the shot glass on the floor of the garage & the 2 other containers on top the sunroof on my daily driver for a low heat source, dark tinted glass in the sun that was 127F checked with a infra red temp gun, warm but way cooler than a average engine/carb.

The soup can & plastic cap lost ~99% of fuel in about 3-4 hours on a cool 74f low wind day, the shot glass took about 8 hours with no heat source. What was left was a small wet residue of fuel, about 95-99% gone from evap. 3 different containers, soup can with quite a bit more fuel than a q-jet holds, all mostly evap'd within 8 hours.

The final test #4 is what people having this issue can do... when the car sits for however many days it takes to have the hard start issue, simply remove the air cleaner and look down the primaries holding the choke flap open, then operate the throttle, on my car after ~5-7 days there is basically no fuel squirting out the acc pump, its barely a mist or small droplets for 1 or 2 throttle movements then nothing... indicating the bowl has gone dry, from mostly evaporation & probably some drain back. This is a 1978 q-jet full rebuild & calibrated by cliff R, I doubt its leaking, Pump is a Robbmc 1100.

A quick google search for "does fuel evaporate" or "how long" confirms from every source that it definitely does, the time is based on temps, humidity, fuel type & exposure to air. Q-jets are fully vented and sit on top engines from 180-195+ degrees and they absorb a lot of that heat by design. If some dont have this problem thats great, but many do based on searches here or the interwebs.

[jhein]

Just in case anyone wants to know, I just cc'd a 7040263 quadrajet bowl with the float in place and filled to the proper level, it's 78cc which is 2.63 ounces.

[69gtocv]

Originally Posted by Formulajones

What's your fuel pressure say

Sorry to hijack this thread, but I tried sending you a PM and it says you're all full! Can you send me one and I can respond to it? Thanks, Jeff

[jhein]

Originally Posted by jhein

Just in case anyone wants to know, I just cc'd a 7040263 quadrajet bowl with the float in place and filled to the proper level, it's 78cc which is 2.63 ounces.

OK I have a revised measurement. First one was flawed due to the fact that I did not have the needle and seat or the accelerator pump installed. The new measurement with those parts in is 63cc or 2.13 ounces.