17.5"-18.5" from the glow plug to the high point on the hatori 650 pipe. If it goes a little lean on the tops of verticals then lengthen the pipe 1/4" and try it again. This is the best way to tune the setup.

Another note is the nitro content could go up. I ran about 25% in mine years ago. I would try the 30% Heli fuels that are around now. The heli motors love them and so do the YS 4 strokes and my OS 140 RX likes it too. I found RPM to be about the 10,500 for best performance.

These are long stroke motors they are basically setup timing wise to a certain RPM. Going with a lower loaded prop is not going to really increase RPM too much. But overloading it will definately drop the RPMs. The 12-11 and 12-12 was a good choice for me. Also the 13-10 was excellent. OS #8 plugs is also a key. This was on 7.5-8lb models. If it got windy or the model was heavy 8+lbs then the 13-10 was a pig.

You ran it open exhaust with only the header this is a really misleading set of numbers

#1 it was not a tuned setup, and #2 the back pressure was not right. Chances are any setting you make are now going to be really lean. I found the motor went rich when the exhaust coupler would burn through or come off. So beware of this. I would open the high speed needle a least before running it on the pipe. I bet you will really have to richen the low end a bunch. I lost a pipe one time and the motor went so rich that it just snuffed out I bet it was like a full turn on the needle to get it to run properly without the pipe.