Raising Fuel Boiling Point

[JohnM15]

gordr,

Oops got the raising lowering backwards... I don't seem to have a vapor lock problem just the carb float bowl boiling. It doesn't boil dry and other than the outside of the carb covered in fuel, there isn't a problem. The anit-percolate valve is doing it's job by opening at throttle plates closed and venting the carb fuel bowl to the outside top of the carb. An electric fuel pump won't fix the fuel float bowl boiling. It wouldn't hurt and may even lessen the fuel float bowl from boiling if I also used a return line to help cool the fuel entering the carb fuel bowl.

[Lothar]

Gord
Following the plan that you described, I wouldn't need to run a return line to the fuel tank?

[jnormanh]

You can't raise the initial boiling point by adding anything to the fuel. The most volatile components will always vaporize first. Gasoline formulators increase boiling points of the gasoline they produce in hot climates in the summer, so if you have a tankful of cold weather gasoline in hot weather it can cause a problem.

Some cars just naturally get their carbs too hot. Anything to reduce heating of the fuel helps. Route fuel lines away from high heat locations, supply cool air to the carb, use insulating spacers and heat shields, and consider using a lower temp (165-170) thermostat in summer.

[gordr]

There seems to be enough anecdotal evidence to support the idea that adding Diesel to gasoline raises the boiling point. It would be easy to test, with a few tins of mixed fuels, a hot plate, and a thermometer. I see the problem being that enough Diesel to appreciably raise the boiling point might also make the gasoline unfit to use as fuel.

The fuel return line can help some. They were used on Avantis as stock. Once the engine is shut down, though, they can't do much. When the fuel boils in the carb bowl, some of it vents off through the anti-percolation valve, and some will still percolate through the jet standpipes and into the venturi, flooding the engine. Problem is, those anti-percolation valves were designed in an era when gasoline had a much higher boiling point. They were adequate then, but are marginal now.

The refiners have modified the fuel; to get best results from it, we are going to have to modify our fuel systems. If you burn off the fuel in the bowl upon shutting down, so that it's flat-out empty, it cannot percolate, period. The pulse-type electric fuel pumps are demand devices, and they only pump a stroke as needed. If the carb is supplied with fuel only as it burns some off, there is less likelihood of the needle and seat being overpowered.

One thing that might helps is to enclose the carb in a box, along with an insulating spacer, and duct air into it using a 12 volt computer case fan, drawing cool air in from outside. In hot climates, and if the car is air-conditioned, draw cool air from the passenger compartment.

It seems like a lot of fuss and bother, but we are trying to feed these cars a grade of fuel they were never intended to burn.

[PackardV8]

Gord is always so 100% right about most stuff, I hesitate to disagree on this:

There seems to be enough anecdotal evidence to support the idea that adding Diesel to gasoline raises the boiling point.

However, I've gotten it not from some shadetree ancedotes, but from a couple of OEM automotive engineers. They gave me all sorts of info, most of which I didn't understand, but the bottom line is "Adding diesel fuel to pump gasoline is one of the worst things you can do to the combustion process. Don't do it under any circumstances."

What we need to do is not futz around like alchemists trying to turn the unlead into gold, but here's where Gord is 100% correct.

we are going to have to modify our fuel systems.

Yes to: insulator under the carburetor, heat shields around the carburetor, fuel line and fuel pump, ascertaining the heat riser valve is fully opening (I remove the unreliable piece of junk!), electric fuel pump and shut off switch, return line to the fuel tank, correct float setting, all will help.

On V8s, I block the exhaust crossover ports at the intake manifold gasket surface. If I were having problems with a 6-cyl, I'd look into cutting the intake manifold free of the exhaust manifold, milling both to get some clearance and brazing a cover plate on the top of the exhaust manifold. Taking the conduction heat of the exhaust manifold away from the intake manifold should lower the carburetor temperature significantly.

jack vines

[JohnM15]

Jack,

I thought that decoupling the intake manifold from the exhaust manifold is a bad for fuel atomization? Is it possible that with today's fuels this coupling is not necessary? Wouldn't slow warmup in winter be a problem?

[PackardV8]

Wouldn't slow warmup in winter be a problem?

Yes, definitely. However, very few Studes get much real winter driving these days. The exhaust heat to the carb is for the upper midwest and northeast which can be both frigid and humid. Carburetor icing could occur.

In your Virginia weather, you'd only have a problem in the rarest of circumstance, when it was below freezing and damp.

jack vines

[Hawklover]

My R-1 has a new e carb, no heat riser, heat spacers under the carb. When shut down fuel is seen in the glass filter bowl bubling..or peculating, so i assume the fuel is doing same in the carb bowls also?

Yes, definitely. However, very few Studes get much real winter driving these days. The exhaust heat to the carb is for the upper midwest and northeast which can be both frigid and humid. Carburetor icing could occur.

In your Virginia weather, you'd only have a problem in the rarest of circumstance, when it was below freezing and damp.

jack vines

[PackardV8]

My R-1 has a new e carb, no heat riser, heat spacers under the carb. When shut down fuel is seen in the glass filter bowl bubling..or peculating, so i assume the fuel is doing same in the carb bowls also?

Not exactly. The fuel in your filter, while the engine is running, is under approximately 5 pounds of pressure from the fuel pump. When the engine is shut down, the pressure drops to zero and any air bubbles in the fuel expand, like uncapping a bottle of soda water. The fuel in the carb bowls is not under pressure.

Heat from the heads, transferred to the intake and the carb can cause fuel to boil, but that's not necessarily what is causing the bubbles in your fuel filter.

jack vines

[Hawklover]

But Jack, the car is a beech to start after a short shutdown...crank the seet out of it.......then hopefully she will fire:-(

Not exactly. The fuel in your filter, while the engine is running, is under approximately 5 pounds of pressure from the fuel pump. When the engine is shut down, the pressure drops to zero and any air bubbles in the fuel expand, like uncapping a bottle of soda water. The fuel in the carb bowls is not under pressure.

Heat from the heads, transferred to the intake and the carb can cause fuel to boil, but that's not necessarily what is causing the bubbles in your fuel filter.

jack vines

[Chucks Stude]

Hawklover, do you have an electric fuel pump? If not, your tune up perameters need to be right on the money, too much advance, and it is harder to hot start. From the school of hard knocks.

[Hawklover]

Stock R-1 Carter pump, nothing else.

Hawklover, do you have an electric fuel pump? If not, your tune up perameters need to be right on the money, too much advance, and it is harder to hot start. From the school of hard knocks.

[alanahasan]

My 1950, 50 years ago. had problems with vapor lock and percolation with the old formula fuels. i installed a shield over the exhauist manifold and a asbestos gasket under the carburetor and solved the problem. these two serve to retard heat transfer,not eliminate it.

maybe your solution is to install an electric fan, meither as a booster or replacement for your reguloar fan, and use a timer to make it run for ten-fifteen minutes after your engine has shut off to cool down your radiator, engine and exhaust pipes. the combination may be enough to prevent your percolation.

alan