1st time here, I bought an 89 four wins 18ft bowrider, omc with ford 302 great shape, 480hrs. ran great for 50hrs, started running rough, missing, tried some dry gas, worked for a while. Same thing the other day, missing again tried dry gas, could not get above 1500rpms or it would die. Engine was running at idle gave some gas and it backfired and would not turn over. Pulled #4 plug water, pulled rest of plugs some dry some wet on both side. Pulled starboard side exhuast manifold, dry, riser gasket, good. Need some help before I take anything else apart