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I have a 72 torino w/ 351c , C4 trans, running a stock dist. w/ msd ignition, recent change to 3.50 gears. Engine timed to 10 deg btc

I was driving the car on sunday. i was driving at 75 mph. I noticed when my rpms were between 3500 to 4000 I would get a small constant back fire out of my tailpipes. the back fire would only occur between 3500 and 4000 rpm. if i slowed the engine to 3400 or went faster then 4000, the back fire would stop. The backfire didn't seem to affect the way the car ran, i.e. acceleration felt normal.

My question is what would cause this?

I was wondering if it might be the msd ignition. I believe the msd is a multispark below 3000 rpm and above it becomes a single spark. I was thinking mabe if the msd is getting worn out it may possible the msd is doing a multi spark when it should be only doing a single spark.
 

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Disconnect the vac advance and go for a drive. It sounds like you have too mich timing, and disconnecting the vac advance is the easiest way to see if retarding the timing would help.

Greg
 

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Check your spark plug gaps. They say .040 to .050 from MSD. If they are in this range, then try .035. At that RPM, if you have backfire, this is from gas not igniting at all while in the cylinder. So raw compressed gas is hitting raw air on the way out and causing your kaaa-booom Any skipping at Idle?

The MSD will not wear out. It is electronic. Are using a good coil or stock?

Your dizy maybe worn. Meaning it is not centered and you have a bad bearing in it. You can tell this by examining the cap on the underside.
 
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