Trouble with gas

THE place to start is the ignition system and that's where your mechanics have started, but they perhaps haven't gone far enough. Backfiring is common and experienced mechanics replace the spark plugs, spark plug leads and even ignition coil packs if they are fitted. If the problem is with the coil pack, replacing the leads will improve the problem, but won't solve it. The coil will simply kill the new leads within days and the problem will come back. Replace all three components in the ignition system. If that doesn't fix it, look at the mixer because backfire can damage it.