Yes, that is a symptom of low compression.
At some point the actual flight prop has to be bolted to the engine and fired up. This is what the compression needs to be set for.
The "bench prop" is just to get a few minutes of heat controlled parts conditioning. If the engine feels good, it's time to tune for the flight prop now. Whatever size prop is considered small for a .15 speed engine. I think a 6 x 4 that's rated for the rpm would be a safe bet to launch a small delta for the first flights.
Last edited by combatpigg; 06-07-2015 at 10:56 PM.