I'm going to put in some leaner main jets for my local altitude (6,000-8,000 feet). It currently runs fine, but the plugs are looking a little nasty (all black), and it loads up and backfires when coasting down from high speed. I'm not talking a little popping, but a hugh KABOOM!.

Will it be necessary to adjust the jet needle? Most people talk about shimming their needles (to make a richer condition), but how do you go the other way? Or is it even necessary?

 

I wonder if your too rich at full throttle (mains) and too lean at idle (pilot) I still think my pilot jet is lean because of backfiring and backing off when letting off throttle--I have adjusted my fuel screw out 3 1/2 turns to try and reduce backfiring but out that far seems to hurt acceleration. I have screw somewhere around 2 1/2 turns (cant adjust on a 360 while running because screw is impossible to get to) and looking forward to increasing pilot jet size

 

WileyCoyote......its only natural to go KABOOM with a name like that. [img]i/expressions/face-icon-small-smile.gif[/img]

 

Initially, I thought a lean condition could exist also. But the pop-pop-pop you get when lean isn't the same as the cannon fire I'm getting out the tail pipe. According to the Prairie service manual, backfiring can occur because of a rich condition, and the appearance of the plugs would seem to corroborate that. Furthermore, I almost never need to use the choke, so apparently the mixture is rich enough as is, and would only get richer as the engine warms up.