Mikuni pilot jet tuning

[BrandonR]

Hey guys,
I've recently picked up a 82 honda Z50 with a Lifan 125 swap on it. It has the 22mm Mikuni slider on it, plunger choke. Now, it idles wonderfully. smooth as butter. The top end (high rpm) wonderful. However, the just off idle or just cruising at a somewhat low rpm, it tends to spudder and "miss".
sitting in neutral if you try to blip the throttle it will bog. you have to ease it on till it starts to rev, then you can blip it fine. The current jetting is 17.5 pilot, 105 main. Needle clip is one notch down to richen. I see black soot on the mufflers baffle, so I wanna say its rich. However, ive read that most pitbike guys run 30 to 40 pilot jets. Does it sound like its lean? or Rich? The pilot screw is 2 turns out. Havent turned it in yet. When you blip it and it bogs, if you try to blip it a fews times itll eventually rev, but i do see a hint of smoke when it bogs then returns to idle or you ease the gas. never smokes otherwise. its a whitish color. Not blue. I think its unburnt fuel from bogging. I can get a video if I need too. Any help is much appricated!

[Weldangrind]

A rich condition would produce black smoke, in addition to a sooty muffler. I'd be inclined to leave the jets alone for now and make some more adjustments. First, turn the pilot jet screw in until it is gently seated and back it out 1.5 turns; go for a ride and see if it is better or worse. The pilot mixture screw is responsible for the idle to main transition, and that's where you're having trouble. You should be able to optimize the transition to the point where a rapid opening of the throttle doesn't produce a bog. If you find that you need to open the screw more than two turns, your slow jet is too small.

[BrandonR]

Well after extensive research apparently I have the clone mikuni even though its stamped. The pilot screw was originally 1.5 turns out. Going to 2 turns made no difference to me. From what I've read it's not uncommon for this carb to use wild jet combinations. You wouldn't think it's the needle position would you? Riding it it stumbles and pops somewhat cruising at light throttle. But going from almost idle to rolling the throttle it'll bog if you don't get the rpms up first. Under heavy load (like up a steep hill) it seems to act better on the low end than flat surface

[BrandonR]

Here's a video.
http://m.youtube.com/watch?v=zVE1nwD75SU

[Weldangrind]

Sure, it could be the clip position. Move it one notch and go for a ride. Does that make it better or worse?

[BrandonR]

Ok so after having the time to tune it, I tried a 20 pilot set one and half turns out with the needle clip in the middle. Ran worse. Had real low idle would barely rev. Went back to a 17.5 pilot two turns out. Idles great again. Tried needle clip one up to lean it. Ran horrible. Then tried the needle clip all the way down. That seems to be the best so far. My main jet is 105. I have a 100,102.5,110,120,125, and 130 main jets on hand that came with the bike. It has an exhaust and uni cone filter. So I would assume she's gonna need more fuel than factory. My question. Is it still had a small hesitation off idle, I'm assuming because the needle is all the way up. Should I step up a jet and leave the needle alone and see if it gets better? What jet size? I'm about 500' above sea level, temps currently 50-70* F. Thanks for the help weldangrind

[Weldangrind]

You're lucky with those temps in February! :D

I'd make one change only, and see what happens. My first step would be to change to the main jet to 110 and go for a rip.

[BrandonR]

Ok. I haven't had the opportunity to ride test it yet, still waiting on my tube to come. Popped it Thursday. But I'll let you know. I've heard you shouldn't leave it all the way down or up as far as the needle clip goes. That true? It had a 110 in it when I got it and the 105 felt like it revved better top end wise

[Weldangrind]

The clip should be wherever it runs the best. Those grooves are there for tuning.

[BrandonR]

So after installing my new tube, I got the chance to really ride test it. It runs beautiful now, no issues. Sitting still if you chop it WOT it'll still bog a bit but I guess that's to be expected with a slide carb. Thanks for the help good sir!

[Weldangrind]

Glad to hear it's running better for you. I agree that a round slide carb has its limitations, but you might be able to tune it a bit more. Where is the pilot mixture screw now? Is it backed out more than 2 turns?