94 sport jet 90 hp erratic, dies when running WOT.

[Golly wimped]

I have a sport jet running very erratic. If I run WOT for a minute or two it will die. If I back off throttle quickly it will stay running. Fuel pump is pulsing. Changed diaphram, still pulses. Only about 6 oz of fuel in 30 secs. What else could cause the pulsing. Fuel lines seem free from obstruction. Can blow through them freely. Thanks in advance.

 

[Texasmark]

I assume you have a squeeze bulb in your fuel line. When the engine starts acting up what is it's condition? If anything other than full diameter with a soft feeling when squeezed, engine running, you have a bad bulb or tank vent obstruction. Whatever the obstruction it is bad enough to inhibit fast fuel flow requirements, like at WOT, but not so bad as to prevent lower fuel usage requirements.

If it were the fuel pump, squeezing the bulb and forcing fuel into the engine would momentarily correct the problem. Also could be a cracked fuel line (inside or outside of the engine) allowing the pump to suck air and fuel.

Mark

 

[racerone]

The fuel pump is driven by crankcase pressure pulses.----So fuel pulses from the output of the pump.------Now if there is a problem in the crankcase and pressure pulses are weak then pump output may not keep up with demand for fuel.