Motorcycle Repair: 1992 RM 80 starting/running, reed valve, rm 80

Question
Hi,
I just bought an old RM 80 from a guy that seems to really now his stuff. He said it will need a top end soon, but want to check if that's my problem before stripping the thing down. The bike starts on the choke when cold and will run at medium/ high revs for a couple of minutes, but won't tick over and loses power then stops. Will re start 5 minutes later but same happens again! I have spark, fuel, 20-1 oil/gas mix and the plug is clean but black/ wet which suggests flodding I think? Any ideas before I go for a top end switch?
Thanks
Jim

Answer
It could be low compression needing a top end refreshed but some of the RM80's had exhaust power valves that got gummed up and stickey causind a runability problem also check the reed valve assembly in the intake side of cylinder as once in a great while if the bike had after market reeds it would suck one of the petals in and it would roun like a car with a valve out. Check these two areas before you invest in a top end. The factory says to refrosh the top end every 20 hrs of riding. Unless you are heavy into racing that simply doesnt happen.