Cessna 120-140 Association

My O-200 suddenly started running rough, literally a day after being fine. First noticed a bit of roughness and surging in cruise, with an associated +/-25 RPM change that I have never seen it do before. This was followed by some light popping backfires when throttling back to near idle in the pattern (also new). Flew it again the next day and it was still the same. Mag checks are fine, take-off and climb out seem fine. I was past due for new plugs and Slick mag overhauls so we did that today as a starting point but it didn't help. Still roughness at higher RPMs when doing a run-up, and the backfire after slowly coming down to idle. Wondering if anyone has had similar issues or suggestions.

Do a magneto check to see if is isolated to one set of plugs.

Check the timing.

If nothing there do a compression check and look for a stuck valve.

Rick

The backfires are indicative of an induction leak.

2356 - Thank you. My mechanic was leaning that way too and is planning on checking that first.

Rick - Thank you too. The roughness was there before and after the new plugs were installed and the mags were overhauled so it's not that. The timing was redone with the mag overhaul. Compression check was fine. Checking for a stuck valve is #2 on my mechanic's list.

Take a look at the gaskets on the intake elbows and also the rubber sleeves and spacers on the intake tubes.

My guess is those rubber boots, replace all four they are easy and cheap, of course your doing the gaskets too. There are two of those boots on ea cylinder if memory serves, it’s not been long I should remember

Stuck valve should show up in a compression check and be worst on a cold motor and go away by cruise, besides it’s not a Lycoming

Thank you John and a64 for the recommendations. Will do!

I would sugest going to "Breeze Liner Clamps" to secure the hoses of the intake manifolds.
It has a liner that protects the hose from the screw of the clamp as it is tightened.
Aircraft Spruce has a large selection.
Make sure you have a good idea of the size you need when you order as they have a limited amount of clamping range.
I switched to these on my Cherokee and really like them.
They cost a little more but I feel they are worth the expense.

Rick

Thought I would post an update just in case anyone searches on this subject in the future. My mechanic also thought it was an induction leak, but while looking for that he found a large fracture in the right exhaust stack at the rear flange. It was so bad that when he unbolted the front flange, the stack came off and the rear flange stayed on the engine. This is on a `46 with straight pipe exhaust. I always wiggle the exhaust when I do my preflight and didn't feel any "give", so I was surprised that it was such a bad break. There was a smaller fracture at the front flange as well. Replacing the stack fixed the problem.

Thank you for the update, so often you never hear what the fix was