Nibbi PEZ28 on 2024 CSC TT250

[Sdmfjon]

Hello

I can now confirm the Nibbi PEZ28 is a direct fit for the 2024 TT250. I did not have any issues with fitment. This carburetor sits almost exactly where the stock Keima sits. The throttle cable is just long enough to work. I did not have to modify anything to make it fit.

Tuning on the other hand, this is where I'm at.

Fully stock bike with emissions removed and a Nibbi PEZ28 carburetor fitted. I am in Maine and live at the beach. My elevation is less than 50' and it is summertime. Sunny day and about 85 degrees air temp.

Idle jet - 38 (factory was a 40 and too much for my stock setup)
Air screw 1 and 1/4 turn out ( factory is 1 and 1/2 )
Needle - position 3
Maine jet - 110

Idle is beautiful and strong. WOT is ok. Midrange needs adjustment.

I still can't quick throttle without it dying. I can roll on and it is fine. Only had like 45 mins to tune before having to go to work.

The max jet the carb came with is a 110. I think I either need to mess with the needle or change to a 112.

The air screw on the side is awesome.

[ProDigit]

First try to lean out the mixture with the AF screw. 1/2 to 1.5 turns.
If that works, and you're happy with wot performance, then lean out the mixture by lowering the jet needle (raising the clip), and resetting the AF screw back to before.
It'll reduce low rpm torque, but if it gets rid of the stalling, you'll have to find that perfect acceleration setting.

If it got worse by leaning out the mixture with the idle screw, try richen the mixture by 1/2-1.5 turns, and see if that improves throttle response.
If it does, raise the needle instead.

[red2003]

If the carb is starting well with the choke, and has a good clean idle, leave the pilot jet and air screw alone. If briskly opening the throttle causes the bike to cut out, the needle needs to be raised. One notch can change a lot. Try position 4 and test. You should notice a difference. If it still cuts out even after it's warmed up and under load, you could try position 5, but I doubt it will need that much more fuel.

[bigdano711]

Take picture of needle. I will do the same since I yanked it out of my Nibbi PE28FL. I will lay it next to a Keihin needle and you will see obvious differences. The best solution for me in the mid was a Keihin JJH. Didn't I JUST type this recently......?


I got this needle/jet kit off Amazon. I can find the link if anyone is interested. I had no idea they were Keihin clones. You can see in the macro pic that the entire adjustment range is lowered, yet there is significant overlap. I tried every position on the Nibbi needle, with no jet changes, and could not discern any kind of appreciable difference from one setting to the other. Maybe yours will be different as there is a significant altitude drop from me to you. I read on some other forums about guys having really good luck with the JJH and, since I had one to try, I figured what the heck. I got immediate feedback, letting me know my pilot jet was too large. Adjusted that back down to 38 and slowly worked my main jet down as it was too large. I am now at 38 pilot, middle position (I think) on the JJH needle and a 105 main. The main is still slightly rich, but I think it works. That Nibbi needle is just too fat.

[Sdmfjon]

Day 2 this time I have about an hour to tinker.

Moved the needle from position 3 to position 4. Bike is not happy. It's too fat, sounds like a tractor, and now started stumbling in that range.

I tried moving the air screw from 1 1/4 to 2 turns out. Tried moving the air screw in from 1 1/2 to 1. Anything below 1 turn makes the idle speed drop.

Using a diagnostic inductive tachometer, my highest idle is between 1 1/2 - 2 turns out. Beyond 2 turns the idle starts to become unstable. At 3 turns it is not happy.

Still can't crack throttle fast.

Thought process currently. Maybe try moving needle to position 2. Or going up to a 112 main jet and resetting needle to position 3.

I took a picture of the needle in my PEZ28 for other to compare to.

It says J6Z

[Thumper]

Originally Posted by red2003

If the carb is starting well with the choke, and has a good clean idle, leave the pilot jet and air screw alone. If briskly opening the throttle causes the bike to cut out, the needle needs to be raised. One notch can change a lot. Try position 4 and test. You should notice a difference. If it still cuts out even after it's warmed up and under load, you could try position 5, but I doubt it will need that much more fuel.

Nice carb, nice install. It looks great


Yeah... Don't lean it out with the air screw! That'll screw up your idle and possible also the transition to midrange. And if you lean out that idle/pilot range, your engine will overheat.

A single clip on the needle to lift the needle is the first step. It will fix midrange if it is still too lean (sounds like). You might have to bump the main one step if WOT still isn't right.

There is a chance that needle is too high already (too rich, too early), but your initial settings seem fine. It would have to be way too rich too choke it out. People run on the rich side to keep the head cooler.

Lean running heats up the cylinder and cat too (for some reason), and exhaust can make a crack instead of a thump. Lean sounds different.

[bigdano711]

Originally Posted by Sdmfjon

Day 2 this time I have about an hour to tinker.

Moved the needle from position 3 to position 4. Bike is not happy. It's too fat, sounds like a tractor, and now started stumbling in that range.

I tried moving the air screw from 1 1/4 to 2 turns out. Tried moving the air screw in from 1 1/2 to 1. Anything below 1 turn makes the idle speed drop.

Using a diagnostic inductive tachometer, my highest idle is between 1 1/2 - 2 turns out. Beyond 2 turns the idle starts to become unstable. At 3 turns it is not happy.

Still can't crack throttle fast.

Thought process currently. Maybe try moving needle to position 2. Or going up to a 112 main jet and resetting needle to position 3.

I took a picture of the needle in my PEZ28 for other to compare to.

It says J6Z

Did you make one change at a time? You said you moved the needle from 3 to 4. Is that the ONLY change you made, or did you mess with the air/fuel mixture screw as well?


Here's how I tuned mine. I put that air/fuel mixture screw 1.5 turns out, where it wants to be, then I left it there and changed everything else. If it's idling, don't mess with the pilot. One change at a time.


If the bike is able to be ridden, get on it and bring it to WOT in 5th gear. If you back off hard from WOT, do you get a weird bog, or does it sound fine? For figuring out main jet, I went too fat and forced the fat bog, then slowly came down until that bog goes away.


At this point, pilot and main are SET. Time to mess with the needle, which should be in the middle position.


The other difference with the needles is finish. The Nibbi has what seems to be a porous finish, not good for letting a fine mist of atomized fuel pass by. And it's FAT.


Bottom line: I found the Nibbi needle to be doing a not so good job. Try every position on that needle and report back your findings. Good luck!

[bigdano711]

This guy seems to know precisely what he's talking about. While explaining the accelerator pump, he also explains why the bike dies when you crack the throttle vs. roll on throttle. I'm honestly trying to save the OP some hair pulling, but pull that hair, bro!


https://www.thumpertalk.com/articles...tor-pump-r173/

[Sdmfjon]

1 change at a time.

When I moved the needle from position 3 to 4 it started stumbling in low end range so I tried to lean out the air screw. Didn't make any progress.

So I reset my air screw back to highest idle. I had to go to work so tomorrow I'm going back to the drawling board.

I have been taking a very close look at the needles and the posts regarding the jjh. I think I'm going to try to exhaust my options with jets. I still have a 112 and a 115 main jet.

[bigdano711]

Originally Posted by Sdmfjon

Idle jet - 38 (factory was a 40 and too much for my stock setup)
Air screw 1 and 1/4 turn out ( factory is 1 and 1/2 )
Needle - position 3
Maine jet - 110

Idle is beautiful and strong. WOT is ok. Midrange needs adjustment.

Because you said that, do NOT do ANY of this:

Originally Posted by ProDigit

First try to lean out the mixture with the AF screw. 1/2 to 1.5 turns.
If that works, and you're happy with wot performance, then lean out the mixture by lowering the jet needle (raising the clip), and resetting the AF screw back to before.
It'll reduce low rpm torque, but if it gets rid of the stalling, you'll have to find that perfect acceleration setting.

If it got worse by leaning out the mixture with the idle screw, try richen the mixture by 1/2-1.5 turns, and see if that improves throttle response.
If it does, raise the needle instead.

[bigdano711]

Originally Posted by Sdmfjon

1 change at a time.

When I moved the needle from position 3 to 4 it started stumbling in low end range so I tried to lean out the air screw. Didn't make any progress.

So I reset my air screw back to highest idle. I had to go to work so tomorrow I'm going back to the drawling board.

I have been taking a very close look at the needles and the posts regarding the jjh. I think I'm going to try to exhaust my options with jets. I still have a 112 and a 115 main jet.

Excellent! Yes, if you have fatter main jets to try, no problems with that. Sounds like you got your idle dialed, and that's how you do it, tune the a/f mixture screw for highest idle while keeping in mind you don't want to go more than 1 turn in or 2 turns out. If it feels like you have to go further one way or the other, that's telling you to change the pilot. When the pilot is too big, you can't get the RPM's to come down to 1500rpm idle no matter how far you turn the a/f mixture screw AND idle adjustment; means come down a size on the pilot jet.


When you say stumbling in low end, was it stumbling while idling or only when you gave a little throttle? Idle was still rock solid?

[Sdmfjon]

Didn't have much time today because of thunderstorms and work.

I decided to give it the sauce. I skipped the 112 and went for the 115 main. What a difference.

Very clean WOT and better throttle response.

Insterestingly, the low end stumble moved from the low end to right at about 1/2 throttle and it was steady, definitely rich it acted and sounded just like a Rev limiter. I could make it stumble everytime. I could roll off or above the range and the stumble went away. So I thought. Let's try going to position 5 on the needle........ Bad idea. Stumbled way worse.

So, back to position 3 and had to go to work. Have no further data right now, just wanted to update the forum.

Stock bike with emissions removed and a Nibbi PEZ28 carburetor fitted.

Idle jet - 38
Air screw 1 1/4
Maine jet - 115
Needle position 3

[bigdano711]

Originally Posted by Sdmfjon

Didn't have much time today because of thunderstorms and work.

I decided to give it the sauce. I skipped the 112 and went for the 115 main. What a difference.

Very clean WOT and better throttle response.

Insterestingly, the low end stumble moved from the low end to right at about 1/2 throttle and it was steady, definitely rich it acted and sounded just like a Rev limiter. I could make it stumble everytime. I could roll off or above the range and the stumble went away. So I thought. Let's try going to position 5 on the needle........ Bad idea. Stumbled way worse.

So, back to position 3 and had to go to work. Have no further data right now, just wanted to update the forum.

Stock bike with emissions removed and a Nibbi PEZ28 carburetor fitted.

Idle jet - 38
Air screw 1 1/4
Maine jet - 115
Needle position 3

Sounds like you're really close to dialed. Excellent! My guess is position 3 on the needle is going to work.

[Sdmfjon]

I made progress today. Id say I'm nearly there.

There remain some minor inconsistencies. At this point I have a setting that I know works.

Looking through the Nibbi manual I realized that I'm at the stock setting for a PE28, with the exception of a 38 slow instead of 40.

I noticed the PE28FL stock setting is a little different with a 112 main, and needle at position 4.

I have some time this weekend to experiment.

[Sdmfjon]

Took a video. Feedback appreciated. Idle is 1550ish (indicated by inductive digital.)