Why is my engine backfiring?

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Q: I have a Lycoming O-320 (150 hp) on a homebuilt Super Cub.  I am using 91 octane auto fuel (no alcohol). This engine has developed a backfire problem after starting.

At idle (700-750 rpm) there are some pops and then several louder pops for several minutes and then it seems to quit. My hearing isn’t wonderful, but I think it is coming from the left side of the engine.

I checked the points and condenser and timed the left mag (it has the impulse coupler) to the engine. Right side mag was right on the 25° mark.  

I can’t find anything loose, it doesn’t seem to lack power, and backfiring quits after warming up and at higher rpm.

I am stumped. Your thoughts would be appreciated.

Tim McDermott, Newell, SD 

A Lycoming O-320 engine.

A: Tim, the condition you mentioned could be something simple or something more difficult and costly.

My first guess is that it’s something simple like an induction leak that is leaning out the fuel mixture.

I’d suggest you do a very close inspection of the complete induction system, beginning with the gaskets at each intake pipe where it attaches to the cylinder.

You may also remove the two bolts that hold that pipe in place, looking for any possible cracks at the intake pipe flange. This is typically a result of tightening down the attaching bolts unevenly, which results in the pipe flange cracking.

Inspect the rubber hose where the intake pipe attaches to the sump for deterioration and clamps for tightness.

Also check where the intake pipe attaches to the oil sump, making certain the intake pipe tube installed in the sump is secure. That tube is swaged into the sump and on occasion— especially on older engines — they have been known to become loose, resulting in an induction leak. Lycoming has a special tool for reswaging those tubes, which is available through any Lycoming distributor.

I believe the reason you are hearing the pops is because something has

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