4x4 problem

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I have a used quad 04 Honda, plowed snow this year, and did well. I decided to put some seafoam in tank. it was running, and while in the garage, I notice the idle increased. so I backed it off a bit. it ran good all over the yard, put it in reverse, nothing but chug chug chug, backfire, would hardly pull itself put it in drive runs like it is new. reverse, chug chug, etc. what in heck would do this?

4x4 problem