[snowman]

Back fires are from one of two places. one is out the exhaust the other is out the carbs, less likely. Which do you really have??

I would suspect you actually have backfires out the exhaust, much much more common. THis is caused by two things. One is an air leak into the exhaust. Without an air leak you cannot have back fires in the exhaust. You MUST have an approx 12:1 A/F ratio for an explosion. Without the air you CANNOT have an explosion. So this is the first place I would check. Leaking gaskets, holes in mufflers, pipes, heat exchangers. The second condition necessary for backfires in the exhaust is excess gas, ie unburnt fuel. Leaking floats, bad valves, whatever may be responsible. LEaking valves can be rulled out by a leakdown test. Carbs are the last thing to check.