New Bike! Looking for some tuning advice...

Heyall, I'm super stoked to say I bought myself a new bikes yesterday, and naturally had to join the forum!
I'm a farm mechanic and have restored and repaired every kind of machine there is, almost. But other than some CT70s, I have never done any mods or real tuning on "lawn mower engines".
I picked up a 2021 Coleman CT200U-EX. The previous owner seemed to take good care of it, but also had no idea what he was doing with regards to tuning and mods.
If I had to guess, he bought an Amazon kit to replace the carb/intake and exhaust. I was able to start the bike and drive it, but it is running super lean, and needs full choke to keep running, and is a PITA to start. The spark plug is nice and white after a short rip around the barnyard.

I was hoping someone might be able to provide me with some tips on where to start with tuning. Other than the carb and exhaust I mentioned, the governor has also been removed. Otherwise the engine is stock. I also ordered up a torque converter for it, but I don't have plans for any further mods at this time.

The carb is labeled CX25, which I assume is a Chinese Mikuni for the predator engine. This I think:
Based on these mods, is there a good starting point as far as what jets to run? I removed my fuel bowl, and the primary jet is labeled as 125. There is nothing stamped on the pilot jet. I really just want to make sure i'm in the ballpark for jetting... no point in getting into fine tuning if it's all out to lunch.

Appreciate any advice, thanks!


Well-Known Member
A 125 jet should be more than adequate for a stock engine. You may want to pull the carb and intake off, then align and bolt the intake to the carb first, then put the intake and carb assembly on. It may have an air leak in between the carb and intake.

A torque converter is a good idea and will keep engine RPM safe without the governor.


Well-Known Member
Post pics. We like pics and you can tell a lot from pics.

You're not the first person to have trouble with the Chinese "motorcycle style" carburetor upgrade. Seems like there's one every week...:)
Thanks everyone for the replies. I bought the bike Thursday, and spent a few hours on Friday tearing it down. It used to be one of the red and white bikes, but I found it too flashy. So attached is a pic of my near final product.
I actually found a jet kit in my shop, so if I have to change them I can. I did remove the carb from the intake and made sure everything was aligned. The intake to engine gasket looks good.
The idle circuit seems to be the main issue with starting and idling, it really wants choke. I think the way the PO had routed the fuel line was limiting fuel to the carb. I have resolved that now, so hoping the engine will be more responsive to tuning. I'll keep at it today.

As an aside, my torque converter arrived, but the backing plate interferes with the cross bar of the frame. I had raised the engine by an inch already. Do I need to raise and move the engine forward to make it fit? I noticed some of the commercially available mounts appear to do that.

Thanks again for the words of wisdom.



Well-Known Member
Yes up and forward if you want to keep the bar. I guess it's possible to trim away at the backing plate to get it to fit. I've never tried.

Some of us chop the bar off, and some don't. You decide. Both of my 200u's are chopped off and ran without a chain guard. Straight up risers.

Up and forward can be beneficial with more weight toward the front of the bike.
Checking back in: Can I tune the carb without the pilot jet? Just kidding, but I tried to remove the pilot jet during cleaning and it broke off. Wonderful. I understand these carbs use Mikuni main jets, but not I guess I'm cooked. Anyone know what the pilot is, or if I can get one?
Assuming I need to buy a new carb now, is there a generally good go-to carb? I'd like to stick with the same general setup if possible so I can keep my cables, filter, etc. Given what I paid for the bike, also not looking to spend big bucks.
Thanks guys. This morning I ordered a second Fakuni in case I can't remove my broken jet. I'll start by swapping the new carb on, then I can deal with the broken one later. I ordered a set of pilots (both Keihin and Mikuni) so i can start swapping with whatever might happen to be there. I already have spare main jets, so hopefully I'm good to go. Should have the parts by end of week, so I'll let all ya know on the weekend how well it runs. Cheers.
Well I'm happy to say I got the old pilot jet out of the carb. Slightly oversized the hole and it came out with an Easy-out. Removing the main jet, I found the tube to be pretty cruddy, so gave that a good cleaning. I attached a couple pics of the pilot jet... I'm thinking it's a Mikuni style, but I did order Keihin jets as well just in case. I have a good feeling about it now, once I get the new jets. They are scheduled for delivery tomorrow. New carb coming Friday, just in case.
Only problem is, if I have an extra carb lying around, I may need to build a bike around it! :D


Looks like a mikuni, the thread diameter is different between a mikuni and a kehin. does it have a number on it? they used to came with a 15 pilot which works for most combinations.
Unfortunately it's not labeled. I read somewhere that a 17 or 17.5 is about right for a starting point, not far off a 15. I have a full assortment coming, so I can get her dialed right in. I'll start on the small side and work up depending on where the mixture screw wants to be. I'm in a cold climate, so typically I find they prefer to be a little extra rich on the pilot circuit.
Do you understand how the pilot circuit works on the chikuni design? Its not like a mikuni where the adjuster controls the air bleed and pilot controls the fuel.
I'll let you correct me if I'm wrong, but my process is always to get the bike idling, and adjust mixture to achieve highest RPM. Then set idle. If the mixture screw needs to be opened more than 2-1/4 turns or so, then I'll bump up the pilot jet by a size, and start over. Likewise if it needs to go leaner. I do the same on the main circuit, but adjustment with the needle instead of a screw.

I know some Keihin/Mikuni carbs have an air screw rather than a fuel screw where adjustments are made inversely. My understanding is that these carbs are a fuel screw. Hopefully I'm at least partly right... ?
The whole first part is correct the last part is wrong and in the past is what I believed as well. Havasau Dave corrected me and we carefully looked at one while conversing and he was correct the air comes from the air horn thru a brass jet, the fuel comes from the pilot and that mixture (both fuel and air and unadjustable) then goes to the adjustment screw which varies the amount of fuel and air that comes out of the little hole behing the slide. This makes the slide height adjuster sort of a mixture adjuster. That is why what works for one person as far as pilot jet may not work for another as if his slide is adjusted higher then the mixture will be slightly leaner. Hope this makes sense. No idea why it was designed this way as the air bleed adjustment which is in front by the air channel for the pilot circuit has worked well for 50 years.
Interesting... thank you for confirming that. I agree, if it works, why change it? Maybe part of the design was still under patent? But that never stopped them before!