14 x 8 is the prop this engine likes. Ifly with a XOAR 14 x 8 and have had an O.S. 91 in my Top Flite T-34, Direct Connection Sea Fury, KMPBlack Widow (2x) and all flew great with the 14 x 8. Turned just under 11k on the ground.

Flew at least 15 gallons of fuel through the O.S.'s in this config!</p>

Yeah, the XOAR props are great, very efficient.  About 10,500 on the ground is perfect.  Check out Bill Baxter's comments on this engine for some definitive input. 

Love these engines, typical OS, haven't even replaced the bearings (yet).   Florida weather/storage can be tough on bearings even using good fuel and after-run.   

Just for kicks, put two gaskets under your gloplug and see if you can tell any difference.  If you can, maybe it is overcompressed and a head gasket or two might be in order. 

After thinking about it, when I used to race MX way back in the 2 stroke days, my bikes would cackle too.  I betcha it a resonance point in the expansion chamber.  Different pipes made different sounds.

Problem solved.....

I no sooner got to the flying field and took the os#8 plug out and replaced it with a good ole "fox standard long glow plug". After a few minor adjustments on the groung with the low and high end needles, getting it tuned up, the crackling sound nolonger existed during my 1st flight.

Engine runs smoothe as ice now. New fox glow plug and some tweeking on the needle valves is all it took for my os engine to live up to its name once again.

Actually I blame it all on that new os#8 plug I had previously had in it. Now I am running a 14-8 prop at 8900 rpm (for now) and still get unlimited vertical, just takes a tad longer than the 13-8 took.

Thanks for all the help and advise.

My My.....look who got it right on the nose! Must have been a lucky guess....

After my engine continued to act up as it did I sent it back in. Hobby Services told me the front and rear bearings were rough and rusty! I told them I was using 18% oil. They told me I should have used after run oil at the end of a days flying.

thanks for all the replys

bad bearings usually sound like marbles rolling around in a tin can