SPONSORED LINKS

Why does the engine backfire, thru the carb, at cruising speed.

Tiny
JOHN RO
  • MEMBER
  • 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.
Saturday, October 6th, 2012 AT 3:46 PM

3 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Check the dist cap for any carbon tracks. It may be crossfiring.

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

Roy
SPONSORED LINKS
Was this
answer
helpful?
Yes
No
Saturday, October 6th, 2012 AT 4:40 PM
Tiny
JOHN RO
  • MEMBER
  • 2 POSTS
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?
Was this
answer
helpful?
Yes
No
Saturday, October 6th, 2012 AT 9:31 PM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Remove the valve cover and you need to check the amount of movement on each valve to confirm.

Roy
Was this
answer
helpful?
Yes
No
Saturday, October 6th, 2012 AT 10:15 PM

Please login or register to post a reply.

Related Engine Backfire Content

How To Fix an Engine Backfire
VIDEO
How To Fix an Engine Backfire
Instructional repair video
Backfire Repair
GUIDE
Backfire Repair
When your engine backfires it can do so in one of two ways. First and the most common way is when the engine is running a small explosion of unburnt fuel is ignited inside...
VIEW MORE

Sponsored links