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Didn't think I would need to post so soon but.....

My 4648 was running so smooth. Suddenly now it runs well until I get the RPMs up and it stutters and backfires. Thought it might be running rich since I could smell gas and occasional grey smoke. Have a Edelbrock AVS2 650 carb and leaned it out. Didn't help. Could it be an electrical issue?

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...My First Guesses would be...LOW fuel-pump Pressure. Also, Check the Distributor for Cap and Rotor, 'Carbon Tracking'. And Spark Plug WIRES for Arcing to Ground...You Test for this, in a Darkened Garage, watch the Wires while a helper gives the Engine a couple of Revs. You will see every arc-over, Instantly.  There are 50+ causes for what You describe! From 'Water in the Fuel', to a 'Bad Coil'!

Good-Luck

MJ

...a side note. You smell Gas. How Old is that (Mechanical) Fuel Pump Diaphragm?? If the Diaphragm should be rotten/torn, it may work, barely...and will Pump Fuel into the Crankcase. Gray Smoke is Burning Oil. Running Rich is Black Smoke. White Smoke is Water.

Last edited by marlinjack

Since it idles & runs well at lower RPM, I would be thinking possible fuel filter / plugged fuel delivery system (pick-up, line, carb needle valve, etc.), but certainly agree Marlin has a pretty comprehensive list.

The good thing about my diagnosis is that it’s probably the cheapest items on the list of possibilities to address…

Rocky

Last edited by rocky

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