Skip to main content

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?

Original Post

Replies sorted oldest to newest

...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'!



...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…


Last edited by rocky

Add Reply

Link copied to your clipboard.