Have 2003 650 Rincon idles perfect but wont accel.When you press throttle to incease rpms engine coughs,backfires spit but wont go,I have changed carb from another running atv still same.adjust values[ was good] changed ECU from an running atv still same, inspect ignition timing at flywheel and it does advance when accel,new NGK spark plug,Engine cranks over like it out of time drags and backfire during starting but when atv starts idles great no misses just purrrrs.I havnt tore down to camshaft or cam chain yet because engine is quite no chain noise and values clearance is always same when inspect just wondering if anyone has had any issues like this before on the 650 rincon?Ever had the crankshaft chain gear spin on crankshaft?Thanks in advance.Engine has 2400 miles

 

Had an Eiger with similar problems after a lay up. I also did all the checks that you have done. Turned out to be the exhaust blocked, no idea why, rodents or a baffle moved I guess.

 

Forgot to mention I removed exhaust no change