Boiling Fuel!

drolston

drolston

Well-known member
Put Offy intake with three 97's on my 59A/B, more for looks than speed. Rigged banjo fittings to the carbs connected with clear fuel line. Pressure regulator set at 3.5 Electric fuel pump back at the tank pumps through the stock mechanical pump. Cold, it runs fine with the electric pump off. Hot, (normal driving on a 75 degree day, at sea level) I can see much boiling and bubbling in the fuel lines, and the engine starves for fuel unless I turn the electric pump on.

Can anything be done to remedy this, or is this normal with alcohol in the gas?
 
Turn the mechanical pump into a pass through and use the electric pump as the primary, in most cases (as far as I can attest) that will eliminate the boiling fuel and the resulting Vapor Lock, all the while giving the appearance of a stock mechanical fuel pump on the intake, if that's what you are going for.;):)
 
There is a correct way to set up a Mechanical fuel pump and one of the tests is that it must hold vacuum on the inlet side ,I suspect you have leaks it only takes the tiniest leak to reduce the efficiency of a mechanical pump, The electric pump masks this .There is a Vacuum tester gauge you can get ,set that up at the tank side of electric pump or tank out let on the pipe. start the motor run it for 5 Min to 10 min ,you will start to get a reading at the vacuum gauge shut of and observe the gauge . Ted
 
Haven't tested the vacuum side, but the pump will sure hold pressure on the pressure side. I was befuddled by zero pressure reading on the fuel pressure gauge with both electric and mechanical pumps running and fuel flowing. So I searched on fuel pressure gauges and found numerous similar complaints for liquid filled gauges. Some recommended taking the fluid out. I simply popped the little rubber plug out of the top of the gauge and the pressure indication popped right up to 2 PSI, three hours after I had shut down the engine and electric pump. So, no need to drain the silicone liquid out, just vent the gauge, and it will read true, regardless of the temperature in the engine compartment. Now the mechanical pump puts out about 1.5 PSI regardless of RPM (which is pretty lame, and explains why the car was starving on mechanical only), and the electric pump takes it up to 3 PSI, which is where the pressure regulator is set.

I am thinking, rebuild the mechanical pump and get a higher pressure electric pump to keep the fuel from boiling out of the mechanical fuel pump (something retained from Freshman Chemistry), and let the regulator lower the pressure into the cooler situation in the fuel manifold and carbs. The cabs are sitting on 2" risers, which are sitting on phenolic insulators.
 
Stock look is not an issue with three chrome deuces. I don't trust either the mechanical or electrical pumps on their own. Both fail with some regularity; so I keep both for the "get home" redundancy.
 
The problem (besides the ethanol in the fuel) is more than likely the heat coming from the fuel pump. The electric pump with a pass thru stock fuel pump is a great idea as Vic Piano said. Or just electric if you don't care.
You can also go online to see if there are any gas stations in you're area that have gas without ethanol.

Sal
 
At 75F I don't see this as vapour lock ,but doing up your mechanical pumps a good idea .1/12 1b is the Minimum for a Stromberg and 31b is the Maximum ,I think the bubbling is Air being introduced into the system .Ted
 
Query: If you run an electric pump through a standard mechanical pump, is the fuel pressure going to the carburetor governed by the standard stock pump, or the electric pump?? Inquiring minds want to know.
 
Lawson, Think of the two pumps as two of your womens... Which one carries your luggage, the prettiest one, or the strongest one? :D
 
Lawson, pressure at the carb will be controlled by which ever pump is the higher pressure one.
On having an electric pump pushing through the stock pump, If the diaphragm in the stock pump develops a pinhole, gas will be pumped into the crankcase, diluting the oil. Of course, this can happen with just the stock pump too.
 
40 Deluxe said:
Lawson, pressure at the carb will be controlled by which ever pump is the higher pressure one.
On having an electric pump pushing through the stock pump, If the diaphragm in the stock pump develops a pinhole, gas will be pumped into the crankcase, diluting the oil. Of course, this can happen with just the stock pump too.
Click to expand...

That is just one reason I convert all the OEM mechanical fuel pumps to 'pass through' and run electric fuel pumps. Among other benefits, it eliminates the possibility of fuel getting into the crankcase.:)
 
Pretty sure it is not an air leak on the suction side (at the electric fuel pump intake, back at the tank), as there would be a fuel leak when not running.

Of course, at ambient temp all was well; the boiling started after running around town and getting the engine and engine compartment hot.

So, I will pursue this mystery in two steps; First rebuild the mechanical pump to put out normal pressure, to see if the added pressure within the pump raises the boiling point enough to stop the boiling; Second, fill up next time with non-ethanol fuel to see if that solves the problem. The boiling point of ethanol is 173 degrees at sea level. The various petroleum fractions have boiling points between 230 and 300 degrees.

I will report my findings to this thread.
 
drolston said:
Pretty sure it is not an air leak on the suction side (at the electric fuel pump intake, back at the tank), as there would be a fuel leak when not running....
Click to expand...

You're pretty sure? Based on assumptions? Air will enter where liquid will not exit.
 
drolston said:
Haven't tested the vacuum side, but the pump will sure hold pressure on the pressure side. I was befuddled by zero pressure reading on the fuel pressure gauge with both electric and mechanical pumps running and fuel flowing. So I searched on fuel pressure gauges and found numerous similar complaints for liquid filled gauges. Some recommended taking the fluid out. I simply popped the little rubber plug out of the top of the gauge and the pressure indication popped right up to 2 PSI, three hours after I had shut down the engine and electric pump. So, no need to drain the silicone liquid out, just vent the gauge, and it will read true, regardless of the temperature in the engine compartment. Now the mechanical pump puts out about 1.5 PSI regardless of RPM (which is pretty lame, and explains why the car was starving on mechanical only), and the electric pump takes it up to 3 PSI, which is where the pressure regulator is set.

I am thinking, rebuild the mechanical pump and get a higher pressure electric pump to keep the fuel from boiling out of the mechanical fuel pump (something retained from Freshman Chemistry), and let the regulator lower the pressure into the cooler situation in the fuel manifold and carbs. The cabs are sitting on 2" risers, which are sitting on phenolic insulators.
Click to expand...

Check the temperature of the mechanical pump and the running fuel pressure. You will find the pressure drops as the pump gets hotter. I like to see a little over 4 lbs pressure when the pump is cold at startup and the temperature of the pump 80 or 90 degrees depending on the ambient temperature. As the pump gets hotter the fuel pressure will drop. Running at 2 lbs hot gets into the area if it gets down in the 1 1/2 lb or less range your getting into the start of VL. As this starts you will see the pressure start to fluxuate. The fuel pump on a 39 or 40 will get up to 300 degrees on these engines in 90 degree weather, use a laser pointed thermometer so you know what's going on. The other thing is driving down the road at 50 MPH there is a good amount of fuel running through the pump and the pump is cooled by a good flow of fuel from the cooler tank fuel. At low speed and idle not much fuel is flowing through the .098 hole in the needle valve to the carb so the fuel gets hot. I installed a fuel return line after the pump with a .100 or slightly larger orfice hole so the engine wouldn't run out of fuel at high speeds. With fuel flowing through the pump at all times I haven't had any problem on that car since. I never used an electric pump so I don't have an opinion on them. G.M.
 
Sorry coop man but what you are saying is the pump is somehow hotter than the rest of the motor .Extreme under hood temp should equal first a boiling Radiator 212 F .then possible Vapour lock .He has six to seven joins in the main pipe before it reaches the pump, Air inclusion . Also the pump is feeding three jugs .and sucking through the electric pump and maybe the pressure valve ?? Ted

V8COOPMAN said:
I will reiterate.............AMBIENT temp (75F outside air temp) DOES NOT cause vapour lock. EXTREME under-hood engine heat at fuel pump causes vapour lock. DD
Click to expand...
 
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Just to add the fuel pump is possible the coolest component under the hood next to the carburettor ,because the fuel is replaced approximately every five minutes .The pump will refill maybe two to three times to the carburettors once .Someone might want to do a calculation given MPH verses consumption ,drolston should make sure his push rod is the correct length. Ted
 
FlatheadTed said:
Sorry coop man but what you are saying is the pump is somehow hotter than the rest of the motor .Extreme under hood temp should equal first a boiling Radiator 212 F .then possible Vapour lock .He has six to seven joins in the main pipe before it reaches the pump, Air inclusion . Also the pump is feeding three jugs .and sucking through the electric pump and maybe the pressure valve ?? Ted
Click to expand...

Ted maybe you should get a laser pointed thermometer and take some temperatures of the fuel pump when it gets hot so you know what your talking about. G.M.
 
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