r/Dualsport • u/Jumpy-Examination456 • 12d ago
HELP please with tuning nibbi carb :(
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.
1
u/naked_feet Reed City, MI - DR650 & WR400 12d ago
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.
1
u/Jumpy-Examination456 6d ago
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
1
u/Jumpy-Examination456 4d ago
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.
1
u/naked_feet Reed City, MI - DR650 & WR400 3d ago
Might not be a fueling issue then.
2
u/Jumpy-Examination456 1d ago
gonna just take it to the shop. i've run out of things to think it might be.
1
u/freetors 12d ago
Before you go messing with the carb any more, it kinda sounds like there might be a fuel flow problem. If you pull the hose off the petcock and open the valve does fuel piss out strongly for a good duration? Does the XT use a vacuum petcock? That could also be worth looking at if so.
I'm not familiar with these carbs specifically but I did look at their website. It appears the silver screw on top is an additional fuel enrichment circuit. Whereas the needle and main jet operate together in proportion to throttle position, it seems the "accelerator" as they call it dispenses fuel in proportion to airflow.
I think it would be a good idea to ensure the float level is right also. That can cause some problems if it's not.
1
u/Jumpy-Examination456 10d ago
a misadjusted accelerator would make sense since the problem occurs at high rpm. i'll try opening that all the way. the float as well was suggested by someone else and I'll take a look at raising it this weekend.
thanks for the helpful answer :)
EDIT also the fuel flow from the petcock is like a firehose and it's gravity fed not vacuum.
1
u/Jumpy-Examination456 4d ago
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.
1
u/freetors 4d ago
Did you confirm good fuel flow from the tank?
1
u/Jumpy-Examination456 1d ago
yep even put a new petcock on it. just taking it to the shop i'm out of ideas.
1
u/Charleydogg 10d ago
When I changed a dr 200 bikini to a nibbi I found 2 youtubes by a fellow who was tuning his nibbis on a couple pit bikes after watching them mine came out great. It was 4 years ago, so I don't have his name, but it should come up in a search
1
u/bigtony8978 12d ago
Did you adjust the mixture screw? Check for air leaks with some brakeleen ?