Not biplane related, but you all have an amazing store of knowledge, so I'm hopeful someone can get me pointed in the right direction. My Champ, with a C-90-14F, has recently developed some roughness. Here's what I know:
* The feeling of roughness is very similar to the feeling of the engine when leaning a little bit too far: RPM-related in terms of its frequency, but that kind of random rumbling feeling that's not like it's happening with every power pulse. I mostly feel it through the seat and pedals. It reminds me of a cartoonish Dickensian old man grumbling to himself.
* Mixture doesn't seem to affect the roughness. I can run at full rich or my leanest cruise setting, it doesn't change. I can definitely make things rougher by going too lean, as normal. Carb heat doesn't seem to affect it either.
* I first noticed it a few days ago on the return flight to Paine, after flying ~20 minutes from Paine to Arlington. The only thing unusual about the flight to Arlington was going through a few minutes of light rain, during which time I used carb heat to avoid ice.
* At Arlington, I added a quart of oil, taking it from 4.1 qt to 5.1. This shouldn't make a difference, but it is a thing that happened. The engine requires 4-6 quarts, and I tend to run it with 4-5 most of the time. It blows out anything much over 5.
* In a couple hours of flying time since then, I've determined that the roughness is most obvious at my usual 2200 RPM cruise speed, at cruise power. At lower power it becomes less apparent, or seems to go away completely. At full power, climbing out after take-off, I can't feel it at all. At full-throttle level flight, I can still feel it, but it's not as obvious as it is at 2200.
* Engine power doesn't seem to be down, but I don't have any way to know for sure. It wasn't climbing at its fastest today, but it was also heavy, with full tanks, so that means nothing.
* The engine monitor doesn't show anything weird: temperatures are as expected, and move like they should with either mag switched off -- no hot or cold cylinders. The roughness is present whether running on both mags, or either mag by itself.
* The lower plugs were noticeably lead-fouled on cylinders 2 and 4, though cleaning out the lead didn't make a difference. Plugs were replaced mid-2020, and have about 100 hours on them now. I normally see exhaust tracking from the #2 and #4 (left) side of the engine, though today I noticed significant exhaust soot coming from the #1/3 side as well, which is new. Might have been my experimentation on today's flight.
* Pulling the prop through after today's flight, it seemed like there was a bit more resistance than normal, particularly rocking back and forth between compressions, like something wasn't getting lubricated properly. I am, at this point, hyper-sensitive to everything, so it's probable this one is just in my head. I also hear what seems to be new, extra clanking that feels like play in the engine. Again, it's probably in my head, the engine has always made clanking noises when pulling it through.
* Oil pressure has been normal, it seems to hit 40 PSI and stick there no matter what. Oil temperatures have been below 150 F, but getting the oil temp gauge up to 200 F takes a 100+ F day and a steady climb. The engine is way over-cooled. Ambient temperature has been 65 F or below for the times I've flown.
I took an oil sample after today's flight, and I'm sending it in for analysis. What I'm afraid of is that some major component has started to make metal, so I'm going to park it until I get the analysis back. If anyone has thoughts of what this might be, or things I could check, I'm interested to hear them.
* The feeling of roughness is very similar to the feeling of the engine when leaning a little bit too far: RPM-related in terms of its frequency, but that kind of random rumbling feeling that's not like it's happening with every power pulse. I mostly feel it through the seat and pedals. It reminds me of a cartoonish Dickensian old man grumbling to himself.
* Mixture doesn't seem to affect the roughness. I can run at full rich or my leanest cruise setting, it doesn't change. I can definitely make things rougher by going too lean, as normal. Carb heat doesn't seem to affect it either.
* I first noticed it a few days ago on the return flight to Paine, after flying ~20 minutes from Paine to Arlington. The only thing unusual about the flight to Arlington was going through a few minutes of light rain, during which time I used carb heat to avoid ice.
* At Arlington, I added a quart of oil, taking it from 4.1 qt to 5.1. This shouldn't make a difference, but it is a thing that happened. The engine requires 4-6 quarts, and I tend to run it with 4-5 most of the time. It blows out anything much over 5.
* In a couple hours of flying time since then, I've determined that the roughness is most obvious at my usual 2200 RPM cruise speed, at cruise power. At lower power it becomes less apparent, or seems to go away completely. At full power, climbing out after take-off, I can't feel it at all. At full-throttle level flight, I can still feel it, but it's not as obvious as it is at 2200.
* Engine power doesn't seem to be down, but I don't have any way to know for sure. It wasn't climbing at its fastest today, but it was also heavy, with full tanks, so that means nothing.
* The engine monitor doesn't show anything weird: temperatures are as expected, and move like they should with either mag switched off -- no hot or cold cylinders. The roughness is present whether running on both mags, or either mag by itself.
* The lower plugs were noticeably lead-fouled on cylinders 2 and 4, though cleaning out the lead didn't make a difference. Plugs were replaced mid-2020, and have about 100 hours on them now. I normally see exhaust tracking from the #2 and #4 (left) side of the engine, though today I noticed significant exhaust soot coming from the #1/3 side as well, which is new. Might have been my experimentation on today's flight.
* Pulling the prop through after today's flight, it seemed like there was a bit more resistance than normal, particularly rocking back and forth between compressions, like something wasn't getting lubricated properly. I am, at this point, hyper-sensitive to everything, so it's probable this one is just in my head. I also hear what seems to be new, extra clanking that feels like play in the engine. Again, it's probably in my head, the engine has always made clanking noises when pulling it through.
* Oil pressure has been normal, it seems to hit 40 PSI and stick there no matter what. Oil temperatures have been below 150 F, but getting the oil temp gauge up to 200 F takes a 100+ F day and a steady climb. The engine is way over-cooled. Ambient temperature has been 65 F or below for the times I've flown.
I took an oil sample after today's flight, and I'm sending it in for analysis. What I'm afraid of is that some major component has started to make metal, so I'm going to park it until I get the analysis back. If anyone has thoughts of what this might be, or things I could check, I'm interested to hear them.
