Another thing you may want to check. After about 150 to 200 flights, sometime a layer of oil residue/ film can developed between the disc valve & the backplate, & the engine rpm could drop. Disassemble the disc valve from the backplate, thoroughly clean both parts with denatured alcohol. Reassemble by tightening both screws until both parts are snug with some friction - not free spinning - & it should regain some rpm.

I usually get about 6,900 to 7,100 with my 175 on 30% heli & 20 x 10.5. However, I don't think by cleaning the disc valve/ backplate will increase the engine by 500 rpm. Usually, about 200 to 300 more. If you only get 6,400, it could be cam timing or need new piston ring.

Hope this helps.

Adrian