I have a '96 6x6, it runs great, but I could hear a rattley sound when driving around 20 mph, I adjusted the valves to .006" as is in the service manual, also I cleaned the carb, and adjusted the choke.

Now it is running great, idles great, but when I drive over 20mph or so, when I slow down it will Backfire.

Also, before adjusting the valves and choke, the plug was black, only 6 months old, after the adjustments, the same plug that was black, is now light brown.

Looking for suggestions...