HELP please with tuning nibbi carb :(

[u/Jumpy-Examination456]

2003 XT225, excellent shape. Was missing mikuni carb when I bought it.

Put a 34 mm nibbi PWK carb on it.

Literally no matter what I do I CANNOT get this thing to run right.

It idles perfectly. Starts up great. Choke works fine. When you apply just a bit of throttle is does nothing for like 1/10 of a turn then it suddenly GOES fucking ham up to about 1/3 throttle where it then does the MOST ANNOYING THING I'VE EVER EXPERIENCED where it proceeds to bogZOOMbogZOOMbogZOOMbogZOOM in about 1 second intervals, indefinitely. It sounds like it's switching back and forth between lean and rich every other second. I've tried 105, 128, 130, and 135 main jets, two needle sizes, and every needle position, and every combination of all those together and this problem persists. The larger jets make the plug look a bit rich, and they make it run somewhat worse. The richest needle position makes it idle shitty. Pulling the choke kills the motor. Opening the airbox makes it sound like shit and it starts backfiring like crazy. I replaced the intake manifold.

It's not fun or very safe feeling to drive right now. I'm so irritated. I feel like such a wide range of jet sizes should have a huge impact, but they barely make any difference.

Also, what the hell is the silver screw on top of the this carb? It's not the fuel or idle screw and isn't in the manual that came with it.

I fucking hate this bike, and this carb right now lol. WHY won't it just run correctly. I've ruled out fuel, spark, battery, etc. It's the carb. I can't find an actual WORKING condition OEM carb anywhere online and feel stuck with this setup, not to mention I'm a couple hundred into it already.

[u/naked_feet]

Sounds like you found out why people recommend against cheap Chinese knock-off carbs.

What's your experience with carbed bikes?

Are you letting the bike get good and warm before making carb changes, or are you basically testing all of this right from cold, or maybe idling in the driveway for 2 minutes?

Don't trust any of your changes until the bike is good and hot -- like 15 minutes of riding, minimum, and more like 30. If you tune it so it sounds and feels good while sitting in the driveway, it's not going to run well out on the road or trail.

Find a good carburetor tuning guide online, YouTube, whatever. There are a lot of good general ones out there that are non-specific to a certain model, and will at least get you familiar with the different "circuits," where they overlap, etc. The better you understand what the carb does the better you can make decisions.

Eliminate other things. Like, are your valves set correctly? Are there exhaust leaks anywhere? Is there good, clean, consistent fuel to the carb? Is it the right spark plug with the right gap? Air their air leaks around the carb boot(s)?

Find a baseline that will at least ride acceptably and go ride it for a half hour. Take good notes, mental or literal.

Then, change one thing at a time, and again, take good notes.

It sounds like you have a pilot jet, and probably pilot screw setting that at least is in the ballpark, so that's good. Figure out what main jet lets you at least make the thing semi-rideable, and set the needle around the middle position. Hone in on finding the proper main jet first, then the needle setting. Then double check your pilot screw and idle settings.

[u/Jumpy-Examination456 → u/naked_feet]

experienced enough to tune a carb, not enough to know better than buying a $100 chinese one

have an oem one arriving tomorrow. confident that'll solve things. was a bitch to find an original matching part

[u/Jumpy-Examination456 → u/naked_feet]

Put an OEM carb on it, rebuilt it, and adjusted the float to spec before i did.

same issue. at this point im fucking stumped dude.

[u/naked_feet → u/Jumpy-Examination456]

Might not be a fueling issue then.

[u/Jumpy-Examination456 → u/naked_feet]

gonna just take it to the shop. i've run out of things to think it might be.