I have a 1979 XL1000 sportster and having problems with the 1980 keihin carburator that was installed on it. There is a 165 jet in it and my plugs are fouled and backfiring. I tied my choke open , it´s running a little better but still showing problems when riding it. What size gap should my spark plugs be, i have it at 37 now. Should I replace the jet?