ORIGINAL: mitchmcf
I too have a SK 90 and it runs so nice but did have one problem it would slowly lean out when in the air. No mater what we did it still would lean out and heat up but never dead-stick so I called the Tech. support and asked them and this is what they said. The first thing he asked was what prop I had on it and I told him it was a 14x8 mas and was told that it was to small to go to a 15x6 or 15x8 . what was happing was that in the air it would unload and get over 11000 rpm and it is not ported for RPM over 11500 so it would lean out once I put on the bigger prop no more lean condition and runs like a champ.
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What a crock!
Overloading an engine heats it up too much and advances the timing too far. The symptoms of having the timing advanced too far is an apparent leaning out of the engine and overheating. The correction to retard the timing can be accomplished in several ways. One can reduce the prop load, one can reduce the compression, one can drop to a cooler heat range glow plug, or one can reduce the nitro content in the fuel. All of this is prefaced upon the engine having adequate cooling air.
A 13x8 or a 14x6 is more appropriate for a .91 two-stroke. Also, being a ringed engine, it is going to need some break-in time running rich to seat the ring and to keep it cooled off during break-in.
If an engine runs past its porting/timing, nothing is harmed unless it is ridiculously high. Breaking-in ringed engines is done with a smaller prop than the normal flying prop, not a larger prop.
There have been no changes in technology that would warrant this advice. Someone doesn't know what they are talking about.