Trouble Reving

[Jaime]

Originally Posted by jonathan180iq

Afterfire, of course, would mean you are running a little rich and the unburned gas fumes are exploding in the muffler.

Actually, this occurs because there's gas and too much O2 in the muffler. There's always a small amount of gas going out unburned, and it fires in the muffler because of high temperatures and enough O2.
Some causes are a lean mixture or air entering a damaged muffler because of the vacuum created. If it afterfires with 0% throttle and there are no air leaks anywhere, you can reduce it by richening the pilot screw (counterclockwise).

Also, cold weather is the best for performance, as long as the mixture is rich enough to carry the O2 increase (cold air has more density than when it's hot = more air in each engine intake).
By the way, cleaning the jets with a wire it's not recommended, as you could enlarge its size...