[FED 387]

looking at a used onan model 4BGEFA 26100J ---says he has been having problems starting the unit also when he had it last running the engine would rev up then go down then rev up again---- not much to go on but any ideas on a fix & is this a good unit??? Price is almost a steal but do not want to buy a money pit either-- whats a fair price on a used one--- Thanx & Regards FED 387