Rough running engine after backfire

[svanarts]

I'm not much of an engine guy so any advice would be helpful.

A little background first: this is an O320-D2A from Aerosport Power and has always run great. Has a standard MS carb and Slick Mags.

Today I started my engine after doing my annual condition inspection. The engine took quite a few cranks to get going. Then it backfired then started but ran pretty rough. Sounded like it was missing. I ran it for a little while but the roughness never left. I tried a run up and it still sounded rough. I did a mag check; ran REALLY rough like almost wanting to quit when I switched to the right mag. The engine didn't sound any different on the left mag than it did with both mags. Any ideas of what I can check? Did I break something?

[robertahegy]

You say you just did your condition inspection. What did you all do? Something you worked on or adjusted may have been the cause. If you adjusted the timing, did you make sure you didn't time a mag before the impulse coupling triggered? You need to turn the prop beyond TDC, then back to 25 deg BTDC to properly adjust the mags. Did you get all the plug wires back to the correct cylinders? Are the plugs gapped correctly? Go back and recheck what you did to see if something is amiss.

Good luck,

Roberta

[Mel]

Make sure you have the plug wires correct. Sounds like you could have cross wired plugs.

[Mark Burns]

Originally Posted by Mel

Make sure you have the plug wires correct. Sounds like you could have cross wired plugs.

I was thinking maybe the backfire caused an intake leak but Mel's idea is WAY more likely.

And I see Roberta mentioned the same possibility.

Mark

[svanarts]

Good replies all. Everyone that I've talked to keeps pointing me back to the ignition system so that's where I'll start. This was a plain vanilla inspection, no adjustments were necessary, just cleaned the plugs and put them back in. I'll checked for crossed wires tomorrow when I go back. I'm really hoping it's something stupid like that.

[az_gila]

Originally Posted by svanarts

Good replies all. Everyone that I've talked to keeps pointing me back to the ignition system so that's where I'll start. This was a plain vanilla inspection, no adjustments were necessary, just cleaned the plugs and put them back in. I'll checked for crossed wires tomorrow when I go back. I'm really hoping it's something stupid like that.

Do you have 4 cylinder EGT/CHT monitoring? If you do, you could probably isolate a problem to a specific plug...

gil in Tucson

[Stephen Lindberg]

Check the plugs with a bright light and a magnifying glass. A tiny sliver of metal shorted out a new plug I installed. The sliver was about the size of a hair and very hard to see. It shorted the center electrode to a side electrode. Came that way on a new plug, seemed to be welded on.

[kevinsky18]

Not sure if you have a baffel in your muffler but if you do always check it after a backfire. It can come loose and block your exhaust.

[Mark Burns]

A "backfire" is when the fire goes backward. It's a fuel/air mixture being ignited in the intake somewhere.

An "afterfire" is when an fuel/air mixture is ignited in the exhaust.

The "afterfire" is normally louder than a backfire and sounds more like a shotgun.

I know this is getting picky. Most everyone calls an afterfire a backfire anyway. But for troubleshooting you sometimes need to know which one it is.

Someone correct me if I'm wrong here.

One more thing to mention, on the ignition leads, the caps that screw onto the spark plugs are marked T1, or T2 or B3 etc. It tells you what cylinder they go on! It doesn't really stand out and if no one has told you before you may not realize they are marked.

Mark

[RV505]

If it has a carb check the venturi, I had a flight school plane that ilded terible . It had a backfire /carb fire and melted the aluminum venturi.. it idled as you described.. I canged it and it was AOK