1972 Chevy Truck Backfires through the carbureator

Tiny
BOBBY J. WILDER`
  • MEMBER
  • 1972 CHEVROLET TRUCK
  • V8
  • 2WD
  • MANUAL
  • 75,000 MILES
This engine is in a boat. In a 1972 firberform cabin cruiser. I don't know what vehicle might fit this engine. I assume all these engines could have this problem. The engine is a Chevy 350 225 hp. The problelm is that it backfires through the carburetor. But only when we throw the throttle quickly forward. This engine has a mechanical advance. Cintrifical. I am thinking that the mechanical advance may be faulty. However, When I move it manually by hand, the spring tensions seems OK but does not return the mechanism all the way back when released. It does not, however, feel sticky. Also, The manual says that the timing should be set at two different degrees. One place it says 10 Degrees, and another says 8 degrees. I am inclined to believe the 8 degrees. Could this be the problem also.
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Friday, December 4th, 2009 AT 5:08 PM

3 Replies

Tiny
MMPRINCE4000
  • EXPERT
I would reset base timing and start with 8 degrees, if problem persists, then go to 10 degrees.

Check timing chain by removing cap, (make sure ignition is OFF) rotate engine by hand with socket and ratchet on balancer bolt (clockwise) a few inches, then rotate counterclockwise while watching rotor. When you rotate counterclockwise, there should be no play in rotor.

This problem could also be excessive carbon buildup in combustion chamber, try a can of SeaFoam in tank.
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Sunday, December 6th, 2009 AT 7:47 AM
Tiny
BOBBY J. WILDER`
  • MEMBER
Dear sirs,

I appreciate your answers and opinions on the cause of the backfire through the carb. However, you didn't mention the mechanical advance, and whether or not that could cause the problem. Remember the mechanical advance did not return all the way when advanced by hand using the rotor to test the springs. We will try all of your suggestions however.

Thank you, Bobby J. Wilder
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Sunday, December 6th, 2009 AT 5:20 PM
Tiny
MMPRINCE4000
  • EXPERT
Possible the advance weights and springs are sticking, as a part of the tune, I would replace it.

Petronix sells a nice replacement dist. That uses an electronic sensor that replaces the springs and weights. And gives you a precise electonic pick up.
But it's your money not mine :).

You may also want to consider (especially if engine is original) that you have a weak lifter/spring.
I would probably perform a compresion test also, if an intake is sticking, it will cause a backfire.
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Monday, December 7th, 2009 AT 6:49 AM

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