Hi Johnkom.
Candoarms is right about the unspent fuel entering the muffler and causing a backfire. I have seen this condition so bad that the backfire caused the nylon teeth to sheer off the cam gear resulting in a complete camshaft replacement. The problem is usually caused by the nylon float in the carburetor rubbing on the end of the aluminum fuel inlet needle valve. The float is actually abrasive enough to wear the end of the needle valve where it contacts the float.(When you compare a new needle to the old one the difference in lenght is quite obvious.) This results in less float pressure force to hold the needle on it's seat and allows excess fuel to flow through the engine and into the exhaust system. One of your spark plugs is probably running richer than the other as well - usually the front. I spent considerable time on this problem after the first 6x4's had been in service for a few years and this situation arose. Replacing the fuel inlet needle should take care of the problem. Be sure to check your pushrods and valve clearances as well. I have seen some bent push rods due to the backfiring. Hope this helps.
Deano

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