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.
run it on each mag separately, If you note a difference run it on the mag that has worst performance. using an inexpensive temp gun, find the cold cylinder. Look at the plug that mag is running.
Move it to a different position. If the problem follows the plug, you found the problem
Instead of wasting your time solicitating guesswork, simply contact an engine shop and let them check over the engine. The problem would be anywhere, though 25deg timing does seem too much. I thought it was more like 17deg, but then again, I’m not an engine tech. Simply put, go to a real expert for help of this nature.
Tim , check if the primer is fully closed .
Also check to see if fuel injectors are clean.
This is a 150 hp O-320.
I agree with the comments given. But one thing to try is after starting, when this “backfiring” starts, try switching to the left mag only, then the right mag only. If the backfiring stops on one mag or the other, the cause may be flash over in the distributor block of one mag. Something to try, anyway!
To help find an induction leak use a flammable aerosol spray like carb cleaner in suspected area and note rpm change if there is a leak.
If this is a used engine that ran on 100LL, it sounds more like the dreaded Lycoming ‘morning sickness’ , where a valve is not closing fully due to lead deposits.
I’d remove the top spark plugs and use a compression tester and find if any valves are leaking. It should be obvious which valve is not closing fully.