Mechanics

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

1967 Chevrolet Malibu • 200,000 miles

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.
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John Ro
October 6, 2012.




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

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

Roy
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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
John Ro
Oct 6, 2012.
Remove the valve cover and you need to check the amount of movement on each valve to confirm.

Roy


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