Electronic Control of Electric Fuel Pump

[Gary1911]

Mechanical fuel pumps usually work well when new . I am referring to those on Kohler , Briggs , and Kawasaki engines , including the pulse vacuum type. However all my Kohler engines are old : 1971 K321 (14Hp) ; and 1995 CV15.5Hp . I am not a fan of the new plastic fuel pumps , which do not last as long as the originals. A leaking diaphragm puts fuel in the oil sump . Which brings me to electric fuel pumps . I have been using them on engines since 1970. I have one on my 1.9L Bobcat that has been running strong since before 2005. The problem comes when trying to use one on the CV15.5Hp (single cylinder) . The pressure is too high even at 4psi ; the flow is also too high at 28 to 30 gal per hour. That is enough fuel o run a 300Hp V8 at wide open throttle ! Various techniques have been used in the past : split the fuel pump output with a T-fitting , run a return line to the gas tank , add a fuel pressure regulator to the carb at .5psi . The regulator alone is $55. The technique I am showing today is a Spice model of an electronic control. The On time is controlled by R1xC1 ; Off time by R2xC1; nominally set to .5sec On , 10sec Off. Tolerance of parts must be considered: 20% on C1 can be offset in the opposite direction with each resistor . Best to measure the C1 first to be within 5% overall timing. All components are available at Digikey. C1= Rubycon 100uF , 35V , 20%. U1 = Ti LMC555 CMOS Timer. M1 Mosfet = Fairchild /Onsemi FDP8896 (obsolete but in stock). V1 at 5.3V is the output of LM317 regulator in a TO220 package. D2 is a 3A 200V Fast recovery diode across the fuel pump leads. L1 and R5 represent typical values for a 1.2A pump.
V2 at 13.3V is the switched battery voltage with charging. Estimated flow is 2.8 to 3.0 gal per hour; more like 30Hp.
Build notes : add a .1uF capacitor right at pin 8 of U1. U1 and the gate of M1 are static sensitive. Wear a grounded wrist strap and use a grounded tip iron. Follow the layout notes in the datasheet . On time can be reduced to .25sec by reducing R1. Off time can be extended to 20 sec by increasing R2.
Pumps available on Amazon : HEP02A and Megaflint are available at reasonable cost . However the reviews indicate quality problems on some pumps. To give the pumps the best chance at survival , mount them to the tractor frame away from engine vibration , with the output aimed upward at 30 to 90 degrees (prevents air bubbles). The pump inlet should be no higher than 12" above the fuel level , and still be able to prime without burning out the pump. I will also be adding a momentary grounding switch in parallel with M1 , to pump the fuel bowl full right before starting .

 

[Mule Meat]

Cheap 1 to 2.5 psi electric fuel pumps can be found for sale all over the web.

You should have no problems using a 4 psi pump on a Kohler 15 HP engine. If it is flooding out. You have a carb problem that no fuel pump is going to fix.

Flow rate does not matter. GPH (gallons per hour) or GPM (gallons per minute) is only a measure given to the maximum amount of fuel the pump can supply in a given time. For the only amount of fuel that will enter the carb bowl will be delivered while the needle valve is open. When the needle closes the flow of fuel to the carb stops regardless if the pump has a flow rate of 5 ounces or 100 gallon per hour. Or at least it should if the carb is working correctly.

 

[T.R.K.]

On those Command 15.5s I usually just block off the old fuel pump port in the block, drill and tap the block off plate for an elbow, run upward to a pulse type fuel pump and it works fine. One reason the new OEM pumps don't work much better than the old ones is the lobe on the camshaft gets worn and won't push the diaphragm as far.