Does any one out there work on and ride the old thumpers? I have a 79 xt 500 yamaha and just had the motor bored out new piston, set the valves and new flow exhaust, and k&n filter. Well when I was tuning it in all of the sudden I lost spark, so now I check every thing out and every thing appears to be good but the spark comes and goes, thus causing it not to start. Any ideas would be great. Thanks Cubby