Did a warm compression check today (RevMaster R2300) and 3 cylinders have leaky valves. Compressions were not bad low 70’s on two cylinders and 65 one (over 80). Two cylinders hiss throuh the intake, one through the exhaust. My theory is a light coating of rust from not flying often enough. Oh, yes I did check the valve lash (still at .006”). Does my theory make sense and if so what can/should I do?
BTW - the compression check was for a pre-purchase/sale check. So we are at a decission point as to how to go forward.
Unless you have a borescope or pull the heads you really have no way of knowing what is causing the problem. Actually those numbers aren’t horrible. If it were me and the rest of the plane matched my expectations I’d discount my final offer by the price of a set of heads.
In reality you may only need to pull heads and lap the valves, which on a VW is pretty easy to do. Besides, if you’re gonna fly behind a VW engine you will get used to pulling the heads. VW engines in these planes work pretty hard and the valves take a beating. From my experience the heads will need to come off every 120-150 hours. Your mileage may vary …
If it’s just inactivity sickness, a lap around the pattern would clean things up. A borescope is a great idea. I helped out a forum member - he bought Darick’s airplane out in PA and was ferrying it back to California, and stopped here. The seller would not do a compression test, and insisted that it had good RPMs and was fine. We did one here and the numbers were poor, much worse than yours. Brought out the cheap chinesium borescope and found some clearly burned valves. He finished the journey in a UHaul. So it could just be rust on the seats, or something more. It ran great and he got up to nearly 10K feet on his trip to Illinois.
Dale’s advice is pretty sound. I’ve done better than 120-150 hours but it’s always something with a VW conversion. A new pair of heads from Sonex is something like $900 plus shipping etc. The worst part about lapping the valves is taking everything apart. It’s better to have new heads to put on, and tinker with the old heads if they just need a lap job. Then put them on the shelf for later.
One other thing to mention - I had a bad start on my original heads last year, and found low compression in #3. I had a loose valve seat. It fell out when I took the valves out.
Well, here is an update. The valves mostly seemed fine but cylinder #1 is rather unhealty. Cyl 2-3-4 all look about the same as each other. Does this look like preignition/detonation? Clearly not burning properly.
I’m not convinced #1 is unhealthy, but they all burn differently because of different mixtures and different CHTs. It’s hard to tell but the #1 intake seat does look a little hokey. Typically my heads and valves look pretty gunky, I think that is part of the problem of running 100LL but being limited to the temperatures that are OK for the VW. There is a nice uniform color on both exhaust valves, so no signs of burning. Nor do you have the big gap like in the picture I showed before. Going back to what Dale said, I bet if you pulled the heads and lapped the valves your numbers would improve.
