I've been asked to look at my father in-law's Rincon 680, it starts and idles normal, but when you give it throttle, it bogs down and is basically undriveable over 15km/h. Sounds like its shooting air back through the throttle body/airbox when you give it gas instead of sucking the air in. I was thinking it might be a dirty injector or a plugged fuel filter, so I took the fuel pump apart/drained it and blew the filter with the air gun and soaked the injector in injector cleaner, then sprayed it out with air, but it didn't help at all (there is also injector cleaner in the gas). Then I grabbed an injector from one of my 420's and put It in the Rincon, and I know they are different sizes or whatever, but it worked for a few hours, the bike drove fine, but just wouldn't idle smoothly and now its doing the same thing again, sooo I switched back to the original injector. Also the spark plug turns black after only a minute or 2 of running, I took it out 3 or 4 times and cleaned it off while I was trying different things with the injector. Any ideas on what is happening? I don't mind ordering a new injector but no sense spending 80 bucks if I don't have to, right?