I'm in the breakin stage of my new Magnum 120 Ringed 2-stroke, and I'm concerned about the compression. I only ran the engine for about 4-minutes at a rich half throttle setting, and then shut her down to cool. I noticed though after then run (engine was hot to the touch) I could easily rotate the prop over, and it felt as it actually had less compression than it had when it was new. I have not started it since.

My question is, the compression of "Ringed Piston" engines, does it get better, or worse after the breakin ? I'm afraid I ran the engine to lean during the 4-minutes of run time. It burned around 1/2 of a 12oz tank just in the 4-minutes, and the prop was a Zinger 16x5 wood. Engine was mounted on the plane, held on the ground etc....
Dennis

Breakin seems to be quite important to a ringed engine.

I ran a webra a few months ago, i pushed it a little after only two tanks, and it lost a lot of compression, it is slowly coming back after 20 flights but it doesn't have the compression of a couple OS 50's that i broke in more carefully. the most recent one i decided to treat like the saito folks recomend, ran it sopping rich, never let it get over 160 degrees, (measured with an IR) or over 4000 RPM ran almost a gallon thru it like that and now has fantastic compression.

hard to say if it will fully come back, but it is worth a try, just keep it cool and slow for a few tanks and see what happens.

Dm, I just lock the throttle on ringed engines at about 4,000 to 4,500 rpm and just let it run and in about 1/2 hour to 45 minutes it will start to speed up slowly. Then you'll know that the compression is improving. After one hours it can be flown, I just let flying take care of the heating and cooling cycles. It works every time.
Spiff, I pushed my Webra .50 too soon and had the same result as you, but it came back very quickly. It's hard to resist reving an engine that is so willing.

Self disipline is not something i have a lot of. :stupid: oh well.