I´m experiencing intermetent backfire problems at deceleration on my 1987 Suzuki Intruder. The mecanic has checked exhaust gaskets, carburator, and has had it in the shop four times now without solving the problem. He keeps thinking it has to be air fuel mixture because it is intermitent. The mecanic takes it for a test run and it doesn´t backfire. I go pick it up and it backfires. Maybe not on the ride home but shortly there after. Any suggestions???