boiling fuel

[Michael C Henry]

I went another direction. I installed a BG diaphram adjustable fuel log with return to fuel tank. I have fresh cool fuel circulating just a couple of inches from the carb fuel bowls. I went a little too nuts with the electric fuel pumps. Two Carter 100 gal per hour at 15 psi, seperately switched with check valves , Y , and 5/8" supply fuel line, 1/2' return line, fuel pre-strainers, filter, and fuel log pressure set at 6 psi to two Holley 600 CFM carbs.

[double ugly]

I mix 100 LL aviation gas at 25-33 percent. I don't need to since I'm at 10:1 CR and have access to only 91 octane but I like the smell. Reminds me of the old days.
John

[My427stang]

I dont think most of the nation can understand the heat off of the pavement, especially from the desert.

Mine ran great everywhere, until I got stationed at Nellis AFB. I then fought percolation like you, pavement temps of 150+ are something east coasters (like I was) never even considered.

High fuel pressure from pump to as close to the carb helps vapor lock, but you have to keep heat away from the carb because it operates at zero pressure. Ultimately, even a carb heat shield will allow the heat to saturate if you dont have airflow to take the heat away.

Look to have a return line to keep fuel as cool as it can be, at the starting point, insulate lines from ground heat where you can as well, and make sure the carb isnt buried in goodies preventing airflow around the body of the carb. Anything you can do to get airflow AROUND the carb, not just INTO it will help dramatically

In addition, drop base air cleaners act as heat umbrellas, so any way you can run a flat base will help as well to let heat evacuate

Finally, west coast fuel, and 50 state winter fuel will flash off much easier at low PSI and zero psi internal to the fuel bowls, I wouldnt expect AVGAS to have that issue though, so you could always increase your percentage

Ultimately, the real "perfect fix" is EFI, but I bet you can get a lot closer with some heat management

[Michael C Henry]

My dad had a large 1 ton and loaded it up and towed a trailer NEVADA ,Arizona, and New Mexico . He delt with vapor lock and all. His final cure was to add a splash of ATF to raise the flash point.

[MrMagoo]

ATF, hmmm. How about good old MARVEL MYSTERY OIL That is an interesting idea, and one of the many cure all benefits of the MMO explained.

After reading everyone's ideas on heat, I think I may replace a stainless braided hose that goes TO the fuel pump with rubber. Do you think that ambient heat from the road at 150F makes a difference when you have 800F coming from the headers, and an oil temp in the motor of 220F? I'm sure it doesn't help, but I wonder how significant that would be.

[Bxx1]

Facinating link, you guys give me soooo much to think about. The knowledge here is terrific!

[SLHetal]

John,
It might be a stupid question but how are your spark plugs reading? Fuel loaded? Maybe ask your mechanic about plug heat range for your current high ambient temperatures.
Sam

[decooney]

I have a return line, and a Turkey Pan to help cool the float bowls, and when I'd get to a very specific stretch of road between Fernley and Sparks Nevada (every year) mine does the same. Loads up, and runs like the fuel is just boiling in the bowls. Take it out at night and run it around, when it's cooler, and see if it loads up in the same way. Hmmm, vapor lock. Dry / Hot-Heat / Hot pavement / Thin air... same scenario. Over seven years with my car (rarely driven though) this situation only occurred for me in the desert in the exact same spot and stretch of road, 4,300 foot elevation, same time of year and temp, two years in a row and nowhere else near my home - knock on wood. Well, we have a 105 degree day today coming... we'll see.

Good tips from many folks here on this topic... I may end up replacing my braided line from the fuel pump to the carb fuel log next.

[double ugly]

The car goes to Modern Auto next week. I'll ask Troy about the needle and seats. The fuel filter is in the stock location on the left front of the engine. I'll also ask about the return line.
John

[Mike Bartlett]

DU I had the same problem in Reno, what decooney was referring to, with my car. Hot as hell 105+ and high altitiude. My car stalled while in slow moving traffic through Viginia City. Acted as if too much fuel getting into the carb. Got back to The Nugget in Reno and lowered my float bowl level a bit. I went through even hotter temperatures the next day driving home. No problem at all. A quick fix that just took a few minutes.

Another thing a buddy of mine and I checked was front bowl temp after that hot drive. We got back to Reno and with almost identical motors my firends Cobra front bowl was considerably cooler (with Turkey Pan) than mine with no Turkey pan. I think that aluminum really helps keep the bowl cooler. I do have the Phenolic spacer and my fuel log is isolated but obviously it didn't help much. I believe the problem is the fuel gets real hot at times, then with thermal expansion, it forces itself through the needle/seat causing the motor to richen up then flood itself?

The only other observation I made was my fuel pressure at the time was about 8-9 psi. I am not sure what the "norm" or max should be?

Mike

[MrMagoo]

The float bowl pressure should be equalized, that's what those two pipes sticking up out of the air horn are for, they vent to the inside of the air filter, thus keeping the pressure inside the air filter and inside the float bowls equal.

A fuel pressure of 8-9 is WAY too high for a holley. Max should be 6.5. You need a regulator. Are you running a mechanical fuel pump or electrical?

[Mike Bartlett]

My pump is mechanical. Normally it runs around 6-psi but that hot weather it jumped to 8+. I do not have a regulator...yet!

Mike

[jhv48]

My BG does the same thing now that the weather has turned hot.

This is definitely a heat soak problem. I have the stainless wrapped lines, metal fuel filter sitting atop one of the valve covers and a drop base 14 inch air cleaner.

As soon as I get a couple of miles on after hot restart, things cool down and it runs normally. But after sitting in the sun, restart acts like it is loading up.

I adjusted the floats down a bit and wrapped the fuel filter to insulate it. Problem got better. Now I will try another air cleaner and see if that helps. Spacer is next along with rubber fuel lines.

[Power Surge]

Originally Posted by MrMagoo

The float bowl pressure should be equalized, that's what those two pipes sticking up out of the air horn are for, they vent to the inside of the air filter, thus keeping the pressure inside the air filter and inside the float bowls equal.

Really? How's that work?

[MrMagoo]

Sorry Sal, I may have stepped beyond my pay grade on this one. I read it in a Holley tuning book, and I will get the quote directly for you and maybe you can straighten me out on this. I try not to play pretend expert but sometimes the desire to be helpful gets you into dangerous territory

[Power Surge]

Originally Posted by MrMagoo

Sorry Sal, I may have stepped beyond my pay grade on this one. I read it in a Holley tuning book, and I will get the quote directly for you and maybe you can straighten me out on this. I try not to play pretend expert but sometimes the desire to be helpful gets you into dangerous territory

I thought maybe you had a brain fart .

The tubes are strictly vents. You can't have pressure equalization between them because the air cleaner is obviously open to atmosphere (otherwise the car wouldn't run). You'd need some kind of sealed enviornment for pressure equalization to occur.

Also, the higher end Holley based carbs with the upgraded needle/seats can handle 8-9 psi.

[undy]

Originally Posted by Power Surge

I thought maybe you had a brain fart .

The tubes are strictly vents. You can't have pressure equalization between them because the air cleaner is obviously open to atmosphere (otherwise the car wouldn't run). You'd need some kind of sealed enviornment for pressure equalization to occur.

Also, the higher end Holley based carbs with the upgraded needle/seats can handle 8-9 psi.

Not true..

Actually you do have a pressure equalization between the two. The vents are common to the inside of the air cleaner which is under atmospheric pressure or a little less depending on rpm, throttle position and the negative pressure within the air cleaner. That all being said, they will equallize due to both bowls venting to a common area. If there wasn't a vent then under the right conditions the bowls wouldn't fill due to being "air locked".

Remember... Atmosphere (@ sea level) is around 14 PSIA, which is still a pressure

Dave

[Rick Parker]

If you are running meatal lines from the mechanical pump to the carb, the heat soak problem will become greater as the line conducts heat from the mechanical pump. It is best to change to a rubber lined hose.

[MrMagoo]

If heat is causing your fuel pressure to rise to 8-9, then that could be a huge part of your problem, since Holley's are very particular about excess fuel pressure above 6.5. Before you get a regulator, do all of the cooling suggestions first, and see if you can prevent that from happening. That way, you will know if the cooling you are doing is effective.

[Gaz64]

The fuel in the log is "boiling" because of the combination of three things:
high ambient temp, engine bay temp, and slow fuel flow because there is less fuel flow through the secondary fuel bowl than the primary.
The secondary idle circuit is not supplying the motor as much as the primary.
So, how about feeding the fuel log from the rear, or fit an electric pump with a bypass regulator and you'll always have cool fresh fuel at the inlets.