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Raising Fuel Boiling Point

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  • alanahasan
    replied
    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
    Last edited by alanahasan; 05-29-2011, 10:11 AM.

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  • Hawklover
    replied
    Stock R-1 Carter pump, nothing else.
    Originally posted by Chucks Stude View Post
    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.

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  • Chucks Stude
    replied
    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.

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  • Hawklover
    replied
    But Jack, the car is a beech to start after a short shutdown...crank the seet out of it.......then hopefully she will fire:-(
    Originally posted by PackardV8 View Post
    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

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  • PackardV8
    replied
    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

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  • Hawklover
    replied
    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?
    Originally posted by PackardV8 View Post
    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

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  • PackardV8
    replied
    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

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  • JohnM15
    replied
    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?

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  • PackardV8
    replied
    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
    Last edited by PackardV8; 05-25-2011, 04:45 PM.

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  • gordr
    replied
    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.

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  • jnormanh
    replied
    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.

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

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  • JohnM15
    replied
    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.

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  • gordr
    replied
    John, actually, you'd want to raise the boiling point of the fuel, not lower it. The whole problem arose from the fact that the refiners have lowered the boiling point of fuel. They do it because they can. Modern cars with electronic fuel injection have closed fuel systems, and the fuel is maintained under pressure in the engine compartment, which keeps it from boiling, just as the pressure cap on your radiator keeps it from boiling.

    Adding Diesel fuel to gasoline may raise the boiling point, just as adding antifreeze to water raises the boiling point, but whether you can get enough of an increase before you lower the octane rating too far is open to question.

    The solution I outlined above will work, and it also addresses the problem of fuel evaporating from the carb when the vehicle stands unused for a few days, and the resulting excess cranking needed to pump fresh fuel up from the tank.

    Two other things that may help: 1. set the float level in the carb about 1/16" lower than spec. Modern fuel is less dense; the floats don't float as high, so the float valve doesn't close until the fuel level is higher than the jets like to see. 2. Obtain a small adjustable fuel pressure regulator that goes between the fuel pump and the carb, and set it at about 3.5 psi to start. Because the new fuel is less dense, the float has less buoyancy, and applies less pressure to seat the needle in the float valve, meaning that unregulated fuel pump pressure may be able to force the needle off its seat, causing flooding.

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  • JohnM15
    replied
    Looks like the real solution is to lower the boiling point of the fuel. When the engine is shut off the heat soaks the carb and the fuel boils. I have three 1/2" fiber glass carb spacers but they won't do much good if the entire engine compartment heat soaks.

    A return line from near the top of the carb fuel bowl would do the trick. If the return is at the float level then some of the fuel will flow back to the tank and the fuel pump will replenish. This "recirculating" will also help keep the fuel in the carb bowl cooler.

    Another good solution would be to add a vent pipe from a cool air location, maybe the grill area, feeding a fan that blows at the carb. There would be a timer to continue operating of the fan after the car is shut off for some length of time. The control circuit would be enabled by high ambient temperature (read: spring/summer months) or just engine compartment temperature.

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