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Rough running engine

kaweeka

Well Known Member
I'm reaching out to the brain trust on this. A friend recently purchased a Mooney M20J with an IO 360 A3B6D with approximately 600 hours on the engine since major overhaul through a respected engine shop. Logs are pretty complete all done through a Mooney service center in Florida since new. The only thing I don't see anywhere is the 500 hour service to the mags. Anyway, he purchased it about 2 weeks ago and flew it to Northern California form the Phoenix area feeling it ran well. He wanted me to fly it last weekend. It felt rather rough on start up but run-up felt smoother with mag drop WNL. Applied power and I thought this was accelerating kind of slow. I passed this off as being used to my -9A. Climb out was dogged to say the least climbing at 400 fpm at 85 kts from an airport 160 feet MSL. I immediately returned to the airport thinking this wasn't right. My friend is an A&P and as a favor, started to look things over with me. We checked plug wires then plugs. All good except for #1 which had a pretty good soaking of oil top and bottom. Compression was 0 on #1 but 78 on the others. Staked the valve with no improvement. Definitely heard the hiss of air through the exhaust so, naturally, we thought the exhaust is valve stuck open. Borescope showed good movement and minimal crud on the seat. Cylinder walls looks perfect. We lapped the valve and checked valve guides. Compression came right up to 78/80 so we thought we're home free. Nope! started her up and still runs really rough. #1 is notably much cooler to the touch than all the others. (The insight CHT/EGT gauge is inop). Checked fuel flow at the injectors and is even all the way around. Restrictors are clean. Starts well but runs very rough. Swapped plugs around but the problem stayed with #1 cylinder.
So, I've got compression, fuel and air and the only thing I can think of that's left is spark. It's not the plugs since the rotation changed nothing. This has the dual (2in 1) magneto we all love so much but the mag drop on a run up is about 75 RPM. My A&P friend only works on jets and he's stumped as well. I'm at the limit of my skillset and this is a certified plane so I can't really go any further, not would I. He's going to try to find one of the local A&Ps that work on the field. I would really like to understand what could be the problem. I don't know how much resistance the plug wires should have but measures at 70.5 ohms. Any ideas?
Thanks,
David
 
Spark issue sounds likely. I'd also use the borescope to check how much each of the valves open. A cam lobe that's worn would reduce power.
 
Mag service needs to be done anyway - and I agree with the others that it is the high percentage player on this particular problem.
 
If the rpm drop on each mag is only 75 RPM with no roughness, I doubt it is ignition. Seems unlikely that BOTH mags would be dead on only #1. A four cyl engine running on 3 cylinders is pretty noticeable. 0 on a leakdown does not mean the cylinder won't fire when running. Only a traditional compression test will show dynamic compression. When leakage is found in the valves, a leakdown test can be very poor, yet there is still enough dynamic compression to light the charge and the only symptom is low power, not a dead cyl.

I suggest another compression test on number 1. Possible / likely that the valve lapping effort did nothing, but the disassembly / reassembly of the #1 exh valve cleared the issue long enough for the compression test, but went back to leaking once running again. Issues in the guide can show this behavior. Did you do a wobble test while in there to rule out a sticking valve?

Larry
 
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Thanks but we re-checked multiple times before and after for just to be sure.

Still think something else is going on. I don't buy that a simple valve lapping exercise will take compression from 0 to 78, unless you had a chunk of carbon stuck to the valve or seat.

A spark event is pretty binary. It either fires or it doesn't. One plug not firing should give a noticeable variation in roughness and RPM drop during the runup. Two plugs not firing and the cylinder is dead and things get quite rough. consistently a bit rough with a good mag check makes ignition not so likely. Intermittent roughness points to ignition or only roughness under high MAP.

Still think compression is the culprit here.
 
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Yep, I agree. This is what I want, some fresh thought. I will re-check compression anyway. It surely cannot make it worse!
 
Yep, I agree. This is what I want, some fresh thought. I will re-check compression anyway. It surely cannot make it worse!

If you get a good leakdown and still rough and thinking compression, I suggest going to the parts store and get a free rental of a traditional compression tester. There is a place for leak down testers, but a good old fashioned auto compression tester is invaluable if you know how to interpret the readings and it more reliably indicates what is happening when the engine is running. You can have 78 on a leakdown and still have low dynamic compression, due mostly to valve train issues. Compare #1 readings to another cyl, as dynamic compression is all relative, not static numbers, like leak down tests.

Larry
 
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Have you pulled the valve covers & dialled the valve lift? Could be a worn cam lobe.
Have you checked intake runners & flanges for leaks? This would give roughness at low rpm, but reasonable power at higher rpm.

Have a friend that had a super smooth 360, both in idle and higher rpm, but didn't climb at all. Good compressions, good spark, etc. Eventually thru extended investigation it was determined that all the cam lobes were severely worn down. Couldn't produce power if air & fuel can't get in...
Maybe your engine has one cylinder worse than the rest.
 
Have you pulled the valve covers & dialled the valve lift? Could be a worn cam lobe.
Have you checked intake runners & flanges for leaks? This would give roughness at low rpm, but reasonable power at higher rpm.

Have a friend that had a super smooth 360, both in idle and higher rpm, but didn't climb at all. Good compressions, good spark, etc. Eventually thru extended investigation it was determined that all the cam lobes were severely worn down. Couldn't produce power if air & fuel can't get in...
Maybe your engine has one cylinder worse than the rest.

Thanks Ralph. That's also a good line of thought that I didn't follow. The valves "appeared" to move adequately but I didn't measure the movement. I'll do that and compare with the other cylinders. Just as a filler, the new owner just noted that there was an oil analysis through Blackstone at the pre-buy that gave excellent marks.
 
Thanks Ralph. That's also a good line of thought that I didn't follow. The valves "appeared" to move adequately but I didn't measure the movement. I'll do that and compare with the other cylinders. Just as a filler, the new owner just noted that there was an oil analysis through Blackstone at the pre-buy that gave excellent marks.

Oil analysis is for trend monitoring as the baseline will be unique for each engine. A single analysis is useless.
 
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