What causes this 1/4 throttle stumble: Vid

[NacsMXer]

Hey guys I have this weird stumble/sputter on 1/4 throttle tip-in after I decelerate. Say I ramp up the RPM's in 3rd gear, decelerate, and then get back in the throttle 1/4, it sputters a little.

Do you think this a pilot jet or needle issue? Do you think this is from being too lean or rich?

Right now the needle is on the 4th clip/middle which COULD be one clip too rich. It runs ok other than the stumble in the needle's throttle range.

The pilot jet is a 55 with the fuel screw only 1.25 turns out. It starts, idles, runs ok other than the stumble.

In this vid, you can hear it at 1:12-1:13, 1:14-1:15, and 1:16-1:18. Thanks a lot for your help!
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[CarGuy7a]

I would say play with the needle setting. Usually from 1/4 to 1/2 throttle is where the transition between pilot jet and your main jet takes place, and the needle adjusts that transition.

Try going one notch down with the clip to richen up the transition a bit and see if it helps. It sounds to me like a little bit of a lean popping when you go from about 1/4 to 1/2 throttle.

The rest of the jetting sounds good though.

[NacsMXer]

Hey thanks for your input. I actually thought the needle was on the rich side all this time.

Here's my reasoning. This symptom doesn't seem to be present as the motor is still warming up. But in the vid, as the motor was up to full operating temp for the WOT runs, the sputter appears as i'm cruising around.

The motor tends to run richer the hotter it gets due to less air density. So if the sputter gets worse the richer it runs, then it was rich to begin with?

I suppose I could try it one way or the other, but anyone agree with this? Thanks

[CarGuy7a]

Usually when mine has sputtered and popped a little like that the needle was lean. If I had it too far on the rich setting it would get a second of bogg then go away.

But yeah I'd definitely play with the needle setting. Take it out and take note of what notch the clip is in, then move the clip one notch down and ride it a few times. Usually if it runs better I won't mess with it anymore but if it runs worse, then go one notch up from where it was originally and then test it. It's all pretty much trial and error and can take a few times of taking the carb apart and doing small changes like that to get it setup right.

[steve181]

mine does the same thing and u have the same carb as me... hummmm :P

[slightlybent47]

I’m no expert but from what I have read, the needle should be in the center passion and you should re jet the main and pilot first and get it as close as you can with the needle in the middle. If you still cant get it right then adjust the needle if nothing else works.

Also the stock carb is in my opinion is not very responsive compared to the 450 carb.