Odd engine behavior after first shutdown of the day

[Tom Wells]

Sledge,

I fought this on the Cobra and a subsequent car for years - literally. Their symptoms were essentially identical to yours except occasionally after a good high speed run the idle would go away and the engine would quit suddenly.

The solution was to switch both cars to fuel injection.

The reason for the problem as far as I have been able to determine is the low vapor pressure of the substance we now laughingly call gasoline.

When the lead was removed and alcohol introduced, the boiling point of "gasoline" went way down - at least for the alcohol portion.

I ran one of the carbed cars with the hood and air cleaner removed and observed "Old Faithful" after the engine was fully warm - the bowl vents would spew fuel several inches into the air, and that gasoline would fall back into the carb and literally make the engine run so rich that at idle it would choke down and quit...

I tried heat resistant gaskets, heat shield and everything else I and my group of carb-lovers could think of without any respite. The only one we didn't try was to direct the output of the car's air conditioning system at the carb.

Nothing got rid of the problem.

I'm convinced it is not the carb's fault! It is the gasoline which has changed that makes this happen.

That's why I went to EFI - and no more problem. At least not of that kind ;-)

My theory is that the float bowl temps become higher than the "gas" can withstand without vaporizing, the fuel boils in the float bowls, and "thar she blows!"

Good luck!

Tom

PS: One thing that I didn't get to try was the no-alcohol gas. Maybe you could try a tankful or two (if it wouldn't hurt your engine!) and let us know if it changed any of the symptoms?

[jhv48]

I also live in California and every carbureted engine I’ve had in my cobras over the last ten years have exhibited similar symptoms.
And I came to the conclusion that it was the low boiling point of our blended crappy California gas. Without a return line to the fuel tank, after shutdown, all that fuel in the pump and lines wants to boil and vaporize.
Tried spacers and different carbs. No change. Just learned to live with it.
After hot restart, it takes a minute or two to flush out the hot fuel with fresh cool fuel from the tank then all is well.
Winter California gas formulas have a higher RVP index which means it evaporates at a lower temperature than our summer gas formula. So, the problem is worse in the winter. Supposed to help with air pollution. Summer gas formulas have a lower RVP index which raises the boiling temp slightly.
And, yes, EFI should solve the problem.

[Sledge]

I wish I could afford fuel injection but at the moment I can't.

Is it the recirculation of the fuel that does the trick for EFI systems? If so I could install a recirc fuel system and keep the carb...wishful thinking on my part I suppose...

Thank you for confirming the symptoms are fuel quality related vs something I did wrong to the carb.

Cheers

[Tom Wells]

Bill,

Yep - been there - tried a return style regulator. This would help if the heat came in with the fuel from the tank. It didn't help. The problem seemed to be that the fuel already in the two float bowls got heated up after a while. Since that fuel didn't recirculate, the return didn't help.

The no-alcohol fuel is sold here in Florida all over the place but it was a rarity when I was having my carb misadventures. It is a couple of octane points lower (I think 89 vs 91 for our alcohol-laced premium I usually run); if that wouldn't cause your engine to knock it would seem to be worth a try. My engines are around 10:1 compression so I think it may have been OK.

The advantage to EFI is that it runs at 43.5psig all the way to the injector. The higher pressure prevents boiling. The pressure in a carb float bowl is 1 atmosphere, or 43.5 psi lower than the EFI.

Tom

[eschaider]

Originally Posted by Sledge

I wish I could afford fuel injection but at the moment I can't.

Is it the recirculation of the fuel that does the trick for EFI systems? If so I could install a recirc fuel system and keep the carb...wishful thinking on my part I suppose...

Thank you for confirming the symptoms are fuel quality related vs something I did wrong to the carb.

Cheers

Bill, check out the MS3Pro Plug-N-Play units at DIYAutotune. Use one of these systems with an OEM Ford wiring harnessing and you are off to the the proverbial races with a very easily tunable high quality EFI at a bargain basement price, click here > PNP System. This is the Mach I version system.

If you get a Mach 1 wiring harness from a salvage yard and set your engine displacement in the engine setup window (see below) of the included tuning s/w,



you can have a very capable system at a bargain basement price.

The reason EFI does not have the same fuel vaporization problems that carbs experience is because the base system fuel pressure is higher, typically 3 BAR (43.5 psi). This has the same effect as putting a radiator cap on with a higher pressure (temperature) rating. The higher pressure raises the boiling point of the coolant, just like the higher base system fuel pressure raises the vaporization point (vapor locking point) for gasoline.

Beyond the simple pleasures of being able to enjoy driving your car again you will get tuning capabilities that can only be dreamed about with carbs.


Ed


<Follow on Thought>

Did you ever notice how hot the under hood environment in your daily driver is? When was the last time you experienced the kind of problems the Cobra is having with your daily driver. My bet is never! There is a reason and it has nothing to do with good or bad karma. It is all about the fueling model you are using. Carburetors were made for cars up through the late 70's some early 80's. Fuels have changed significantly since then and most notably they have added at least 10% ethanol universally across the country. There is a website where you can look up "pure" gas, gas stations but they are almost always the better part of a tankful (sometimes more) from wherever you are.

If you want to purposefully end the headaches and aggravation you are going to need to do an EFI conversion. The longer you wait the higher the ante at the poker table. Your car, your engine, your dollars most importantly your decision.