Okay… so let’s get started…..
Lightman has an FM100 installed and the OEM fuel filter has been removed. Therefore, the fuel valve (otherwise known as the thermostat valve) has also been removed from the system.
Interesting tidbit of info: The feedpump spec. is 80 Cubic Centimeters of fuel per second at 2,000 RPM (pump RPM). That’s 1.27 Gallons Per Minute. That’s brisk. At about 2,000 RPM (engine) the pump is turning 1,000 RPM, and the feedpump is pumping about 38 gallons per hour. The engine is consuming about 1 gallon per hour. Huge difference.
Take a look at this sketch of Lightman’s fuel system:
It is a simple loop. Fuel is drawn from the tank through the pump and back to the tank. This is the configuration that Lightman is having high RPM problems with. Remember, the fuel return flow rate is easily 30x the fuel consumption rate.
Now let’s take a look at the OEM loops:
NOTE: The X in the fuel valve is the thermo control device. When cold, it restricts flow, when warm it doesn’t restrict flow much more than a regular tube. The triangle in the circle is the check valve. It is very important here.
When it is very cold, the thermo device closes off the fuel flow back to the tank almost completely. BTW, I bought a fuel valve just to play with and figure out how it works. So where’s the fuel gonna go? It’s gonna open the check valve and go right back into the filter and back to the pump. Round & round it goes… Any fuel that is burned gets drawn from the tank: this is a very slow draw rate. The fuel in that little pump-fuel valve-filter-pump loop moves pretty quick though. The feedpump is a positive displacement pump.
Now let’s assume the check valve is seated (sealed) and the thermo valve is open. We are at low RPM. As RPMs increase, the feedpump fuel flow increases. The pressure at the pump inlet will go down and the pressure at the pump outlet will go up. The pump outlet pressure goes up much faster since the return line is a smaller diameter than the supply line. The pressure across the check valve will increase as the difference between the inlet and outlet pressures increases. Regardless of the temperature, the check valve will blow off and short circuit the return-feedpath across the pump inlet and outlet and cap the minimum pressure that can be drawn and max pressure that can pump out. It will make a little loop that allows the pump to pump right back to itself, drop the draw rate from the tank, and eliminate the pressure drop of the high flow through the long tank feed & return lines.
My gut tells me that to
draw 1.27 GPM from the tank is a bit ambitious.
Lightman, the configuration you have might very well be drawing in air. Air in the fuel would screw up timing control. A la the timing DTC. The filter is not the problem, the configuration is.
The problems you are experiencing here are the very reasons I decided to add the FM100 in series with the OEM filter. I determined that the fuel valve must not be deleted for this very reason.
But hey, I’ve been right before!
