Motorcycle Repair: 1984 1000 xlx backfiring and slowing down, intermediate jet, plug wires

Question
My father-in-law bought this Harley and it starts out well but once you get to a cruise it starts to backfire and slow down. you can down shift and it will start to speed back-up. we changed the plugs the other nite and the old plugs where both black. what can we do to solve this problem..

Answer
Since your plugs are fouling then it can be a couple of different things. I would OHMs test the coil and plug wires. If they pass the jets in the carb might be to large. to determine which jet it is take the bike and go for a ride. Keep the RPMs under 2500. Check the plugs. If they are foulded then change the intermediate jet. then go for a ride and ride over 2500 RPMs. If the foul then change the main jet
Good luck and happy riding
Mike