WHY DOES THE ENGINE BACKFIRE, THRU THE CARB, AT CRUISING SPEED.
1967 Chevrolet Malibu
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.
Check the dist cap for any carbon tracks. It may be crossfiring.
Either that, or you may have a bad camshaft lobe.
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?
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.