Tim,
Who is it that builds these engines? I regularly get my gx 35 upto 11,000 rpms attached to my belt drive bicycle kit on the downhill side of a bridge that i ride every day. If I hold the trottle wide open with the rear wheel in the air I can get my tachometer/ hour meter to show over 10,000. The engine is stock. I removed the decompression system a while back due to starting issues and runs much better now. I also tapped the exhasust threads to a use 6 mm bolts instead of the the 5mm studs.