pull plugs check compression, even check with your finger over the hole to start, see if there is a difference, mine did this, and it was a ring failure on right side. if they feel the same get yourself a compression guage cheap and check compression, 100 or below top end time, if compression is good same ol check and clean carbs sync carbs, rebuild carbs and set floats. and so on check for air leaks nasty little critters happen in the air inake boots right at the crossover tube etc. just some thing to start with. ................