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: 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 a crack in it and as the engine temperature increases, the metal expands and closes the crack.
That being said, you may have a more serious problem, which could be a cracked cylinder in the intake port area.
Not knowing the history of your engine, I’m curious to know if any of the cylinders have been weld repaired in the past? Sometimes older cylinders that have been weld repaired will develop cracks adjacent to the weld repair. If this happens to be in an intake port, you may be seeing the results.
I would suspect that you do not have a manifold pressure gauge installed in this aircraft? If you were to hook up a manifold pressure gauge and check it with the engine in the idle range you mentioned, I’d expect, assuming we have an induction leak, a manifold pressure reading of 11-12 inches plus MAP. If there is no leak in the induction system, I’d expect to see a manifold pressure reading of 10 inches MAP.
I hope these suggestions will give you a good starting point as you continue to troubleshoot your problem.