jetting help please

[SET THE STAGE]

on my 300ex (running a 400ex carb) it had a 150 in it, and it seemed to be running lean because when i would let off the gas it would backfire and pop. so i went out and bought a 155, 160, and 165. they all make it bog, though?

so i'm thinking, maybe the main jet was fine and i need to change something else on it? i heard something about changing the "slow jet" for mid-throttle (which is where it seems like it's backfiring and popping now that i think about it)

my question, what's the slow jet? i also keep hearing people changing the clip position on the needle jet for their 400ex carbs. what does this do?

[F-16Guy]

richen up the pilot jet. Probably a 40 or 42 would do it. If you have backfiring on deceleration, it's almost always the idle circuit. There's a How-To in the "articles" section; click the link at the top of the page. I think what people don't realize when they install a larger carb is that the velocity of the air going through the carb will be reduced because of the bigger opening. The lower velocity will lessen the venturi effect that causes fuel to be drawn out of the float bowl and into the carb, causing a lean condition. Richening the jetting will compensate for that.

As for the main jet, you could probably try going a little leaner. If it's bogging by going up one or two sizes, then you're probably on the verge of being too rich.

[SET THE STAGE]

yeah i don't think it's like TOO rich yet with the 155 in it even, though. it only seems to bog when i go from almost nothing-1/4 throttle to close to full. but it didn't do that when it was 150. but when it was at 150 it seemed very lean.

so just get a 40 or 42 pilot jet? what about the needle jet? should i maybe drop it down a notch? i heard the 3rd notch is pretty common for the 400ex.