Low RPM

[driesbabe]

Hi Guys,
lets see if one of you can help me.
I have just rebuild and completely overhauled a RCGF 100cc bottom induction twin. The reason for this was that the owner thought that if he cut any openings in the cowl, the plane would not look as nice as it did coming out the box. Fine! this guy did not have a single and I mean not the smallest opening in his cowl or any other part of the plane. So the engine sized up due to overheating, destroying the cylinders, pistons and bearings.
I have exchanged everything inside this engine except the crankcase. As i did the initial test run on this engine I know that it turned the 26X10 Menz S at 6700RPM before it was destroyed, but now after the rebuild it will run at 5900-6000 RPM and no more.
Timing was checked and advancing properly, (I even went and got a timing light) and everything else is new. Reed valve, spark plugs, bearings, crank, cylinders and pistons.
I just don't get. It runs well, throttles very well, dose not overheat and idles well, but will just not reach the RPM that it did.

All parts replaced, are identical to the parts that where in the engine and the prop I am using is the exact same prop that was used the first time the engine was run.
Now some people might say, give it some time to run in, but I must be honest, I have never seen any engine pick up 600+ RPM with a run in. 200-300 maybe, but not 600 contrary to some peoples believe.
What am I missing? I am at a loss, as this is not the first rebuild that I have done, and defiantly not the first engine I have run???

Maybe some of the more experienced engine gurus have some comments.

Thanks you
Andreas