WHY DOES THE ENGINE BACKFIRE, THRU THE CARB, AT CRUISING SPEED.

1967 Chevrolet Malibu

Tiny

John Ro

October, 6, 2012 AT 3:46 PM

It's actually my Boat Motor. It's a Mercruiser using a Chevrolet, 250 Cubic Inch, straight six. When cruising at a constant speed with the throttle set anywhere from 2000 to 3800 RPM, after several minutes, it will backfire thru the Carb.

FOUND 426 RELATED QUESTIONS

3 Answers

Tiny

ASEMaster6371

October, 6, 2012 AT 4:40 PM

Check the dist cap for any carbon tracks. It may be crossfiring.

Either that, or you may have a bad camshaft lobe.

Roy

Tiny

John Ro

October, 6, 2012 AT 9:31 PM

Ok, then it could be the Cam.
Before this last trip it was starting hard with a rough idle, it's been using a bit of oil but didn't backfire. I had the carb boiled out, did a compression test, was OK (#1=142psi, #2=127, #3=120.5, #4=135, #5=119 & #6=133), . I installed a new Distributor cap, Rotor & Spark plugs. The coil & plug wires were changed recently.
Starting was Ok & idle improved a bit. During this trip, about 60 miles from Long Beach, CA to Catalina Island & back, the backfire started, rough idle & hard starting returned.
Can you tell me how to verify it's the Cam before I pull the motor?

Tiny

ASEMaster6371

October, 6, 2012 AT 10:15 PM

Remove the valve cover and you need to check the amount of movement on each valve to confirm.

Roy

Please login or register to post a reply.

Fuel Pump Replacement Done Right
Fuel Pump Replacement
Fuel Pump Replacement Ford Mustang