Sudden loss of compression

I'm intrigued, can I have good compression while having a 'bad feel'?

And when would you go ahead with a start and when would you take off the cylinder head? Trying to avoid that as much as possible, obviously.

According to Mike Busch one needs to perform a compression test on a warm engine. Thus, you will need to start your engine to achieve something like operational temp.

I do understand your caution. On the other hand it does not seem very likely that the reduced compression you feel developed right between your last flight and your manual compression test. So who knows how many times you started your engine with a reduced compression on a cold cyl.#1 without knowing (and with no bad effects)?

Besides, I agree with you to leave the cyl.heads on until absolutely necessary to pull them. I suggest to borescope all cylinders to get some good feeling back and then give it a try.

"I'm intrigued, can I have good compression while having a 'bad feel'?"

Absolutely

Leave the heads on, but you can check under the valve covers without causing any issues IFFF you have a bad compression test. If you have a cylinder with a low compression test then this may be the issue. What may be the issue is the rings on the piston have a gap in them and the rings turn when in operation. Once in a great while these gaps line up and compression drops. I've seen this numerous times. So if this happens to me I will start and run the engine for a several minutes to to get the ring gaps to change position and except for a time or two this has always cured the lower compression on that cylinder.

Hi Peter,

I know it's tradition and old school teaching to always check compression on a warm engine. One of my research projects years ago was testing compression on  Rotax 912 series engines that has much tighter tolerances vs a looser tolerance Continental or Lycoming that do need warming up.

I tested 12 engines during annuals. When they came into my hangar hot I did a compression test. Then I did it again when the engine was cold the next day. The difference in pressure was anywhere from zero to one psi difference. So because the Rotax engine has nice tight tolerances it really won't make a difference hot or cold, but that's your choice. I'd want to know if there was an issue before a start.

Gents,

thanks again for your advice.

So this morning I did identify that the hissing air was coming from the carb, not the oil tank. As such, my assumption was that the inlet valve was not closing correctly. What I could see under the valve cover did, not reveal anything unusual (to me). Also, a borescope exam of both valves didn't show anything major but then again, I'm not an expert. But a missing chunk would have stood out 😅.

I did notice quite some (carbon?) deposit around the inlet valve, so I crosschecked both adjacent cylinders and they were similar.

So I went forward and did an engine run. She struggled a bit but then started. Egt on the affected cylinder was kinda low but, since the cowling was off and the relevant cylinder is in the front, I didn't make much of it.

After warming up the oil a bit, I shut off engine, and voila, the compression was back to normal on all 4 cylinders (through 'prop feel'). We did do a quick compression check and the values were similar on all 4 cylinders.

So no idea what exactly was going on, but at this point I would rule out piston rings (since the air hiss came from the carb). Maybe truly just some carbon deposit?!

Anyway, I'm a relieved happy camper 😊👍

Mark

Good out comes always feel good