Backfire

[JOhnH]

We are due for our annual next month, which is only a few weeks away.
But the other day when doing a run-up on our O-360, we heard a small single backfire after pulling the throttle to Idle. Everything else seemed and sounded normal. We did another quick run-up/idle with no backfire. We flew a few hours with a coupe of landings and a few more run-ups and no more backfires.

Should I go ahead and get the annual done now and have this checked, or should I not worry about it and go ahead and put another half dozen hours on the engine before we get it in for the annual. the engine only has 92 hours on it.

 

[RichNY]

How quickly did you go to idle speed? Might be pulling air in from an induction or exhaust leak. I'd have those connections checked at the annual.

 

[tonycondon]

We are due for our annual next month, which is only a few weeks away.
But the other day when doing a run-up on our O-360, we heard a small single backfire after pulling the throttle to Idle. Everything else seemed and sounded normal. We did another quick run-up/idle with no backfire. We flew a few hours with a coupe of landings and a few more run-ups and no more backfires.

Should I go ahead and get the annual done now and have this checked, or should I not worry about it and go ahead and put another half dozen hours on the engine before we get it in for the annual. the engine only has 92 hours on it.

were you running at full rich before and during the runup?

 

[JOhnH]

were you running at full rich before and during the runup?

Yes (full rich), but here is a little additional background.

As I said, this engine now has 92 hours on it. Since this engine was installed a few months ago the engine monitor has been showing rather high CHT values. (~39-410 level flight/ up to 455 during climb). Those values have been coming down slowly. When I put the plane in the avionics shop last month for a panel upgrade, I asked their A&P to check out the temps. He thought that it was adjusted too lean even when set to full rich so he adjusted it up a little (to make it more rich at full rich). What he did seemed to help and the CHT temps are now pretty reasonable (around 380).

So, it was at even more full rich when it backfired than it used to be.

 

[tonycondon]

you should probably start leaning aggressively when you taxi. not sure if that over rich condition is what caused your backfire but it certainly isn't helping your spark plugs

 

[gismo]

Yes (full rich), but here is a little additional background.

As I said, this engine now has 92 hours on it. Since this engine was installed a few months ago the engine monitor has been showing rather high CHT values. (~39-410 level flight/ up to 455 during climb). Those values have been coming down slowly. When I put the plane in the avionics shop last month for a panel upgrade, I asked their A&P to check out the temps. He thought that it was adjusted too lean even when set to full rich so he adjusted it up a little (to make it more rich at full rich). What he did seemed to help and the CHT temps are now pretty reasonable (around 380).

So, it was at even more full rich when it backfired than it used to be.

The idle mixture is a separate adjustment although it may be affected by the high speed mixture setting.

 

[Dan Thomas]

Pulling the throttle rapidly to idle puts a really lean mixture into the cylinders. Too lean and they won't fire but the unburnt fuel goes on through and accumulates in the muffler, and either hot spots in the muffler will set it off or a bit of flame from a lean and late firing in a cylinder will do it. The only harm might be to the exhaust system itself; you can blow gaskets or crack things.

Dan

 

[kgruber]

Make slower throttle reductions.

 

[Skip Miller]

Pulling the throttle rapidly to idle puts a really lean mixture into the cylinders. Too lean and they won't fire but the unburnt fuel goes on through and accumulates in the muffler, and either hot spots in the muffler will set it off or a bit of flame from a lean and late firing in a cylinder will do it. The only harm might be to the exhaust system itself; you can blow gaskets or crack things.

Dan, what you are describing is technically called afterfire, not backfire as described by the OP. Does that make a difference in this case? I know backfire is a commonly used term that the lay person will use for either back- or afterfire.

-Skip