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  • Fuel System: Raising Fuel Boiling Point

    I have a carb percolation problem with my M15A. I checked the fuel pressure, it is 4lbs. The float is also set correctly. I don't have a fuel pump shield yet, but I suspect that it may help some but the fuel is boiling out of the carb fuel bowl throw the anti-percolation valve (as it should when the fuel is too hot). It appears to only boil when the engine is at the operating temperature and the truck is shut off. It is less of a problem when the hood is raised after shutting the engine off.

    My question is:

    What is the best way to raise the boiling point of the fuel?

    I don't have access to non-Ethanol fuel. I have heard that adding Diesel to the fuel helps. Also MMO.

    Any suggestions?

    Thanks.
    1948 M15A-20 Flatbed Truck Rescue
    See rescue progress here on this blog:
    http://studem15a-20.blogspot.com/

  • #2
    An electric fuel pump, positioned near the tank will propel fuel into the carb at startup, while the fuel pump on the engine may be full of vapor and not be able to move fuel.

    The heat shield will help some, too, but won't prevent fuel pump vaporization.

    I tried the diesel fuel solution, it didn't seem to help at all. The electric pump and a fuel return line (from between fuel pump and the carb back to the tank) solved the problem.

    Comment


    • #3
      Tom B,

      I don't seem to have a fuel delivery problem at all. The problem I have is that the fuel bowl boils when shutting off a hot engine on a hot day. Engine starts and runs fine. The fuel boils out of the anti-percolate vent on top of the fuel bowl. I need old school fuel...
      1948 M15A-20 Flatbed Truck Rescue
      See rescue progress here on this blog:
      http://studem15a-20.blogspot.com/

      Comment


      • #4
        Years ago, I used to have trouble with fuel percolation after stopping for fuel or a short rest after sustained driving . For a long time I thought my Champion was just being balky, but I realize the problem was the fuel boiling out. An electric fuel pump would solve the hard restarting problem, but the fuel percolation with todays fuel may be hard to completely stop.
        When/if I get my 52 going again, I plan to install an electric pump and some sort of shield and/or insulator under the carb itself. Without fuel injection and a closed fuel system there's not a perfect solution that I know of.
        "In the heart of Arkansas."
        Searcy, Arkansas
        1952 Commander 2 door. Really fine 259.
        1952 2R pickup

        Comment


        • #5
          You can hang 2 fuel pump shields, one above the other. One attached to the fuel pump with extended studs, and the other attached to the front valve cover. I've thought about putting a scrap of insulation between the 2 shield but have not yet tried it.
          You could also add a bit of fresh air ducting directed right at the pump.

          Comment


          • #6
            with the electric fuel pump and a return fuel line, it will keep the "cooler" gas in the tank going to the carb and the "hotter" gas back to the tank. so upon shutdown, the gas in the carb is "cooler".

            then there's always the clothes-pins on the fuel line as insulator...
            saw a pic of that here last year - more solutions in "search" - vapor lock.
            Kerry. SDC Member #A012596W. ENCSDC member.

            '51 Champion Business Coupe - (Tom's Car). Purchased 11/2012.

            '40 Champion. sold 10/11. '63 Avanti R-1384. sold 12/10.

            Comment


            • #7
              A carb spacer would help with percolation of the fuel that is allready in the carb bowl. Don't know where you'd find one for a single barrel carb though.
              Frank van Doorn
              Omaha, Ne.
              1962 GT Hawk 289 4 speed
              1941 Champion streetrod, R-2 Powered, GM 200-4R trans.
              1952 V-8 232 Commander State "Starliner" hardtop OD

              Comment


              • #8
                Just think, a tunnel ram one barrel manifold.
                Jamie McLeod
                Hope Mills, NC

                1963 Lark "Ugly Betty"
                1958 Commander "Christine"
                1964 Wagonaire "Louise"
                1955 Commander Sedan
                1964 Champ
                1960 Lark

                Comment


                • #9
                  Chev used a plastic spacer on their 6 cyl in the early 50's, don't know if the base is the same tho.

                  Comment


                  • #10
                    JohnM15 has a very tenuous and problematic point regarding the carb boiling dry. The electric pump on my car only aids in countering vaporlock while on the run. Once you shut the car off after running at op temp, it will not start until cooled down. I've run the electric pump for up to 15 minutes and it doesn't do anything. You can't tell whether the engine is flooded from the percolation fumes or just bone dry, it just cranks forever.. The last episode, in 2010, cost me over 2 hours...
                    I have a 1" phenolic spacer under the Edelbrock, have removed the riser and blocked off the crossover to no avail. The problem takes the fun out of driving the car.. Recently there was a thread where someone managed to place some tubing on the carb vent and it wound up relieving some of this problem by preventing the fumes from going anywhere.
                    Wait until the alcohol content in gasoline gets up to 15% or more, that will really help things..
                    64 GT Hawk (K7)
                    1970 Avanti (R3)

                    Comment


                    • #11
                      On each of my Studebakers I have to floor the gas pedal to clear out the flooding from the manifold and cylinders after a hot soak. The only one that does not have this problem is the R-2 which has the fuel return to the tank.
                      Frank van Doorn
                      Omaha, Ne.
                      1962 GT Hawk 289 4 speed
                      1941 Champion streetrod, R-2 Powered, GM 200-4R trans.
                      1952 V-8 232 Commander State "Starliner" hardtop OD

                      Comment


                      • #12
                        Part of the answer is an electric fuel pump. The other part of the answer is to install a manual switch, and use it to cut off the fuel pump a few seconds before shutting the engine down. Then develop a new driving habit. When the car is warmed up, or downright hot, and you park somewhere, switch off the fuel pump as you are making your final parking maneuver, and let the engine idle until it stalls from lack of fuel, instead of promptly shutting off the ignition switch. A carburetor bowl that is empty cannot percolate fuel into the manifold and flood it.

                        For this to work, you will have to remove or bypass the mechanical fuel pump. You should also have a fuel pump relay controlled by oil oil pressure or ignition pulses as a safety measure to ensure that the fuel pump doesn't keep running in the event of a wreck.
                        Gord Richmond, within Weasel range of the Alberta Badlands

                        Comment


                        • #13
                          Gordr, to keep mine looking stock, I took all of the valves and mechanical parts out of the mechanical fuel pump, and left the diaphram. Also, I cut a thin metal plate, and put gasket material on both sides of it, and put it between the fuel pump and the block. That is to make sure that fuel does not get to the oil. The electric fuel pump pumps thru the mechanical pump, and it looks stock, but is not. Right under the ignition switch, is a rocker switch that turns off the pump. And in the trunk is a Ford inertial switch that will turn the pump off in the event of a wreck. I got the inertial switch from a Ford Bronco place in Ingram, Texas. They sell them used, with the pigtail, for about $20.00. They are like $80.00 new. It all seems to work well. And I promise it did not before I did all of this. I could only drive the car in the winter.

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                          • #14
                            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.
                            1948 M15A-20 Flatbed Truck Rescue
                            See rescue progress here on this blog:
                            http://studem15a-20.blogspot.com/

                            Comment


                            • #15
                              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.
                              Gord Richmond, within Weasel range of the Alberta Badlands

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