Hello all, i'm new to Nitro planes. I just picked up a used OS 91 Surpass and I can turn the engine over with out a prop but with my fingers? I'm not sure how high the compression are for these motors? I haven't fired it up yet, but I was wondering how is the a compression gauge for these engines? What should the compression be? Thanks

It's not uncommon for four-stroke engines to lose their valve seal if they've been sitting around for a while. Assorted crud can get caught in the valve seat area and sometimes the valves can get a bit sticky.

Mount the engine in a test stand and put some fuel in and prime it, wind her over and start her up. 95% of the time, this should restore compression. If compression is still dodgy, check the valve clearances once everything's freed up again. They should be set to between .0015 - .004 inches.

If it is the Surpass I version, you must have the throttle open to feel any compression

Thanks for the tips everyone. I'm not sure what version this surpass is. I'll post a pic and hopefully someone can identify it.