i just got an 87 mighty max with a few problems. i replaced the spark plugs and drove it maybe 2 miles and it started missing so i rebuilt the carb cleaned the already blackened plugs and again ran it about 2 miles and the plugs started missing again. when i pulled the plugs they are smut black.i think i may have an intake valve leaking do u think this could be the problem?i dont think there is anything wrong with the carb.