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Engine runs rough at higher RPM's under load
Greetings A-4 sages:
My A-4 has been running great all season, until now. After over 150 hours on it this year, it has started running unevenly above 1200 rpm under load. In neutral, and at low rpm under load, it runs fine. I have to pull the throttle out more than usual to get it to go to 1500 rpm, and then it's not happy. The vacuum gauge used to read about 7 under load at cruising speed, around 1800 rpm. Now it reads around 3 at 1500, and won't reach 1800.
I figured it was junk getting into the carb, so I replaced the Racor filter, and the two inline filters, one before the mechanical fuel pump and one after. There was some particulate in the gas drained from the Racor, but no water. The in-line filters weren't very obstructed. Fuel pressure was and is about 3.5 on the gauge. I also disassembled and cleaned the carb and jets. New gasket between carb halves. Didn't see any junk in the carb or jets. I turned the idle mixture screw 1 turn out from seating, and the main jet screw 1.5 turns out. Usually when I replace filters and clean the carb, it solves the problem. Not this time. Actually the problem is now worse. Engine hesitates when accelerating above 1200 rpm, then runs rough. In reverse it seems to do better, but that's probably because reverse puts less load on it. The plugs are a little sooty around the rims but the contacts are clean. I have not tried turning the distributor to change timing, or replacing the cap and rotor, or plugs. I have electronic ignition. The spark seems good from the center wire on the coil. I have not looked at my 2-blade urethane prop from CDI, maybe a piece has broken off it. I use non-ethanol fuel.
Any thoughts? I await your collective wisdom.
My A-4 has been running great all season, until now. After over 150 hours on it this year, it has started running unevenly above 1200 rpm under load. In neutral, and at low rpm under load, it runs fine. I have to pull the throttle out more than usual to get it to go to 1500 rpm, and then it's not happy. The vacuum gauge used to read about 7 under load at cruising speed, around 1800 rpm. Now it reads around 3 at 1500, and won't reach 1800.
I figured it was junk getting into the carb, so I replaced the Racor filter, and the two inline filters, one before the mechanical fuel pump and one after. There was some particulate in the gas drained from the Racor, but no water. The in-line filters weren't very obstructed. Fuel pressure was and is about 3.5 on the gauge. I also disassembled and cleaned the carb and jets. New gasket between carb halves. Didn't see any junk in the carb or jets. I turned the idle mixture screw 1 turn out from seating, and the main jet screw 1.5 turns out. Usually when I replace filters and clean the carb, it solves the problem. Not this time. Actually the problem is now worse. Engine hesitates when accelerating above 1200 rpm, then runs rough. In reverse it seems to do better, but that's probably because reverse puts less load on it. The plugs are a little sooty around the rims but the contacts are clean. I have not tried turning the distributor to change timing, or replacing the cap and rotor, or plugs. I have electronic ignition. The spark seems good from the center wire on the coil. I have not looked at my 2-blade urethane prop from CDI, maybe a piece has broken off it. I use non-ethanol fuel.
Any thoughts? I await your collective wisdom.
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