We have about six hours on the OH cylinders and after a stout ground run/leak check and two flights, some swapping out spark plugs etc. it seems to be getting worse.
#3 cylinder is cooler than the others, both EGT and CHT. We checked the intake tube, everything is OK there. It has glazed over one bottom spark plug so far so we replaced the bottom plugs with new.
The real issue though, is that the engine starts easy, and runs well with just an occasional very small stutter, until you get it fully warmed up after a long taxi. When it's good and hot it has a pretty substantial miss, completely at random and on either mag. It is less substantial running on both, but still there. This is the second time we've grounded the airplane for it. After the last time, the mechanic came out and we drained the oil and cut the filter to check for metal (none) and checked the compressions - #1 and 2 are 78, #3 was only 62-64 and #4 is 70. My mechanic said the rear two always lag behind the others a little bit and #3 is the the wayward child. He pulled off the rocker box cover to check the rockers, valves, springs, retainers, etc. and everything was OK there. The bottom plug on #3 was oily/glazy again even though it's new, so we put a used and tested good plug in, and it started and ran fine with the same very slight occasional burble which he said wasn't a real issue. Strangely enough, the EGT on #3 also kept pace with the other three for once.
But once again, after it has sat for a bit and we go to fly it - EGTs on #3 are 50 degrees cooler, it runs OK until it gets hot and we do the runup and the miss comes back.
Today we first had an issue with fouled plugs, when I rolled it up to 2000 rpm, switched to the right mag it was perfect, to the left mag it dropped 200rpm and ran pretty bad. We did a full throttle run up on both mags and leaned it out, then tried at 2000 rpm again. This time is was only a 50rpm drop on either mag.....but by this point the engine was up to full temperature, and the hot miss on either mag had materialized and we can't get rid of it. Again, totally at random, and feels like my instructor is kicking at the rudder pedals. On both mags it is there but not as bad, on L or R it is very noticeable.
It still makes 2275rpm with a 60 pitch prop at WOT on both mags, but it buffets the airplane so much I can't tell if its missing much or at all. It used 1/2 qt. of oil during the leak check, full throttle ground test, and 2.5 hour test flight, and used a negligible amount on the next 1.4 hour flight after that.
My mechanic is out of town until September, so I might have another guy look at it, if anything just to get a fresh set of eyes on it. I'm thinking start with the high voltage leads, then check mags. Mags are Slick, according to the logs the "500 hr. OH" was done 200 hours ago in 2010.
Any thoughts on what to look for? I have a plain steel cylinder on hand that is a good low time core, I'm thinking of having it serviced in case we need to switch out #3.
ETA: It's been VERY humid here in AZ the last few weeks, and I also had a issue with my primary radio. The Controller couldn't hear us on it. Could hear the second comm radio just fine, but not the primary. Never had a radio problem before either.
#3 cylinder is cooler than the others, both EGT and CHT. We checked the intake tube, everything is OK there. It has glazed over one bottom spark plug so far so we replaced the bottom plugs with new.
The real issue though, is that the engine starts easy, and runs well with just an occasional very small stutter, until you get it fully warmed up after a long taxi. When it's good and hot it has a pretty substantial miss, completely at random and on either mag. It is less substantial running on both, but still there. This is the second time we've grounded the airplane for it. After the last time, the mechanic came out and we drained the oil and cut the filter to check for metal (none) and checked the compressions - #1 and 2 are 78, #3 was only 62-64 and #4 is 70. My mechanic said the rear two always lag behind the others a little bit and #3 is the the wayward child. He pulled off the rocker box cover to check the rockers, valves, springs, retainers, etc. and everything was OK there. The bottom plug on #3 was oily/glazy again even though it's new, so we put a used and tested good plug in, and it started and ran fine with the same very slight occasional burble which he said wasn't a real issue. Strangely enough, the EGT on #3 also kept pace with the other three for once.
But once again, after it has sat for a bit and we go to fly it - EGTs on #3 are 50 degrees cooler, it runs OK until it gets hot and we do the runup and the miss comes back.
Today we first had an issue with fouled plugs, when I rolled it up to 2000 rpm, switched to the right mag it was perfect, to the left mag it dropped 200rpm and ran pretty bad. We did a full throttle run up on both mags and leaned it out, then tried at 2000 rpm again. This time is was only a 50rpm drop on either mag.....but by this point the engine was up to full temperature, and the hot miss on either mag had materialized and we can't get rid of it. Again, totally at random, and feels like my instructor is kicking at the rudder pedals. On both mags it is there but not as bad, on L or R it is very noticeable.
It still makes 2275rpm with a 60 pitch prop at WOT on both mags, but it buffets the airplane so much I can't tell if its missing much or at all. It used 1/2 qt. of oil during the leak check, full throttle ground test, and 2.5 hour test flight, and used a negligible amount on the next 1.4 hour flight after that.
My mechanic is out of town until September, so I might have another guy look at it, if anything just to get a fresh set of eyes on it. I'm thinking start with the high voltage leads, then check mags. Mags are Slick, according to the logs the "500 hr. OH" was done 200 hours ago in 2010.
Any thoughts on what to look for? I have a plain steel cylinder on hand that is a good low time core, I'm thinking of having it serviced in case we need to switch out #3.
ETA: It's been VERY humid here in AZ the last few weeks, and I also had a issue with my primary radio. The Controller couldn't hear us on it. Could hear the second comm radio just fine, but not the primary. Never had a radio problem before either.
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