Debra
12-21-2007, 04:16 PM
Hello, Mr. Lee.
Thanks for your great contribution to our hobby for these forty plus years! I've always enjoyed your articles. Perhaps you can help me with a 12 to 15 year-old O.S. FS-90 four stroke. It is well worn, and I put it away in the box for a couple seasons, and made sure it was well lubricated. I pulled it out this week for installation in a new plane, and I noticed that the compression was initially good, but after a few days on the bench the compression dropped to nothing. I disassembled the engine, cleaned the parts, re-lubricated it, and the compression returned to normal......for only a few minutes. Then it dropped to nothing again. All parts, including piston, rod, gaskets, valves, rocker arms, bearings, gears, and glow plug appear to be intact. The valves appear to move in proper sequence. I recall that a couple years ago the engine's compression was "soft", being well worn, but it seemed to run quite well. Yet the next-to-nothing compression now makes me wonder. I have not attempted to start the engine as I am still getting it installed in the plane. Do you have a ny thoughts on this apparent loss of compression in an older four-stroke?
Thanks.
Richard B.
Dear Mr. B.,
Either the ring is sticking or one or both valves are sticking or not seating. Load the engine with oil and turn it over while listening at the exhaust and intake ports. A loud hiss means the valve is leaking and needs to be lapped. A slight hiss is normal. If the valves are okay and you hear an internal leakage, it is the ring that is sticking or needs to be replaced. If replacing ring, it is a good idea to also replace the piston as the ring groove in the piston may also be worn so that the new ring will not seat properly. Let us know what you find.
Regards, Clarence Lee
Thanks for your great contribution to our hobby for these forty plus years! I've always enjoyed your articles. Perhaps you can help me with a 12 to 15 year-old O.S. FS-90 four stroke. It is well worn, and I put it away in the box for a couple seasons, and made sure it was well lubricated. I pulled it out this week for installation in a new plane, and I noticed that the compression was initially good, but after a few days on the bench the compression dropped to nothing. I disassembled the engine, cleaned the parts, re-lubricated it, and the compression returned to normal......for only a few minutes. Then it dropped to nothing again. All parts, including piston, rod, gaskets, valves, rocker arms, bearings, gears, and glow plug appear to be intact. The valves appear to move in proper sequence. I recall that a couple years ago the engine's compression was "soft", being well worn, but it seemed to run quite well. Yet the next-to-nothing compression now makes me wonder. I have not attempted to start the engine as I am still getting it installed in the plane. Do you have a ny thoughts on this apparent loss of compression in an older four-stroke?
Thanks.
Richard B.
Dear Mr. B.,
Either the ring is sticking or one or both valves are sticking or not seating. Load the engine with oil and turn it over while listening at the exhaust and intake ports. A loud hiss means the valve is leaking and needs to be lapped. A slight hiss is normal. If the valves are okay and you hear an internal leakage, it is the ring that is sticking or needs to be replaced. If replacing ring, it is a good idea to also replace the piston as the ring groove in the piston may also be worn so that the new ring will not seat properly. Let us know what you find.
Regards, Clarence Lee