Rough Engine

[jrwilson]

Had a rough running engine this weekend, M20C, 1963. Flew to Watsonville (on the coast) and spent the night. It was a pretty cold and clear night. When I first started the engine the next morning, there was some intermittent missing, not terrible, but noticable. OAT on start-up was almost 60 deg. Thought it might be a little carbon, so taxied out for a run-up, where it got a bit worse. The roughness and miss was evident on both mags during the mag check. Taxied back in to a maintenance place and shut down, they were closed, so I started back up and taxied to another one. This time it was very rough and I had to use about 1400 RPM to keep the engine running and I couldn't lean out on the ground. If I brought the power back to idle, then the engine sounded like it wanted to quit. JPI showed cylinder #1 had no bars on the graph, but I was busy trying to lean and taxi keep it from dying and such and didn't wait to see what the actual cylinder temp was. The other four cylinders had 2 bars on the graph. It is not unusual to have the #1 cylinder colder on the ground for my plane, though 2 bars versus 0 is unusual.

Found a mechanic, de-cowled it and he couldn't find anything obvious. He suggested re-start and check the JPI. And of course it ran perfect... And has continued to be fine in the following 3 flights. Mags are pretty new, right mag last year and left mag this year. Carb is freshly overhauled, ignition harness is about 2 years old. Intakes were inspected/new gaskets at annual. They seem tight. The #1 cylinder felt slightly cooler to the touch than #3.

The mechanic in Watsonville thought maybe a stuck valve, but would that explain the wanting to quit at idle? Any other ideas? My normal mechanic (where the plane is now for diagnosis and oil change) thinks maybe water/condensation in the carb, but how long would that last in the system before it went through the engine? That second taxi was over 4 minutes. Compression checks were 77, 77, 78,74 (#4 cylinder). Plugs look good. Induction boot looks good and is pretty new, oil is very clean, nothing in the filter. Air filter is bracket and had 15 hours on it.

Any other ideas for differential diagnosis? Mechanic going to try a wobble test tomorrow. Anything else they should look at?

Thanks,

John

[OR75]

water in the tank ? did you drain / sample ?

[jrwilson]

Sampled thoroughly before and after. Didn't find anything. The mechanic is going to drain the carb and check that, but I don't know how there would be anything after 3 subsequent flights.

[N33GG]

The no bars could be a clue. Water should have affected all cylinders... at least I think that would be the case. Same with carb ice. I would be very suspicious of a stuck valve, and yes, I imagine it would make idle very difficult. Wobble test should be interesting, and might indicate a problem. From what I hear, and I have talked to some of the best mechanics about this kind of thing recently, M20C's are the worst with stuck valves. Lucky us!

[Ellioth]

" JPI showed cylinder #1 had no bars on the graph..."

Isn't this a classic case of Lycoming "Morning Sickness?"

Have you ever had the exhaust guides reamed? If so, how recently?

[AmigOne]

Found this on a google search, very interesting and I believe related to your problem:

http://www.avweb.com/news/savvyaviator/savvy_aviator_57_making_metal_197700-1.html

[jrwilson]

I don't think the guides have ever been reamed. The wobble test should be done today, which should indicate if that is neccessary. I just thought the morning sickness was a little roughness, missing, which I had at first. I didn't know it was also associated with running very rough and wanting to quit.

[N33GG]

When you essentially lose one out of your four cylinders, it is going to be ROUGH!

[carusoam]

My m20C stuck a valve, then "cleared" itself. Look for bent valve stem while taking the offending cylinder apart.

Bent valves don't work very well. It might make sense to just replace the valve on the offending cylinder...

Best regards,

-a-

[N33GG]

Mine stuck a valve about a month ago, and cleared itself after a few seconds, but felt like minutes. It was dark and nowhere near any airports. I am going to have a top overhaul at the end of this year when my annual is due, just as a caution. My engine has relatively low time, around 1000 hrs, but was overhauled in the 1970s. When they pull the cylinders, if they find anything suspicious about the cam, it will be a total overhaul. Time to start saving pennies!

[danb35]

I am going to have a top overhaul at the end of this year when my annual is due, just as a caution.

Seems like an awfully expensive caution, with just as much chance of creating new problems as solving existing ones. Seems a bit of time inspecting the existing cylinders would be well-spent. Then, if any need repair/replacement, take care of the ones with problems.

[jetdriven]

If you ream the valve guides thats a lot easier solution than replacing 4 cylinders to fix valve problems.

[N33GG]

Already had Maxwell very recently perform a wobble test and inspect all of the cylinders. Will probably have all four cylinders overhauled, unless we see other issues when removed and opt for new ones. A bit expensive, maybe. Injecting new problems, also maybe (actually one of my biggest concerns). Always tough decisions. I will always err on the side of caution. It's only money.

[jetdriven]

You can get new cylinders for a parallel-valve Lycoming for 1000$ each. Installing OH unknown time cylinders for 500$ each, might not makes sense at that price point.

[N33GG]

I agree and will lean toward new cylinders.

[Skybrd]

Mine stuck a valve about a month ago, and cleared itself after a few seconds, but felt like minutes. It was dark and nowhere near any airports. I am going to have a top overhaul at the end of this year when my annual is due, just as a caution. My engine has relatively low time, around 1000 hrs, but was overhauled in the 1970s. When they pull the cylinders, if they find anything suspicious about the cam, it will be a total overhaul. Time to start saving pennies!

My deceased cousin taught me some tricks to help unstick engine parts. Marvel Mystery oil in the gasoline (4 ounces per 10 gals gas) and some in the crankcase will probably help unstick valves or at least help. I've used it in my experimental Tailwind many times to keep the valves from sticking.