What size main jet for CB250F?
 

I'm running a 125 but suspect it may be a little lean. Just curious what others are running. TIA
BTW I realize the importance of elevation but unless noted otherwise I assume that most are below 2000' ASL as I am and therefore should be jetted pretty close.

Hello I'm running a 135 on my TBR7,runs alot better now.

You are probably both a bit over the upper limits of a 250cc. My Honda Shadow 750 only uses 120 mains. My 600 uses 114 mains.

You don't just keep shoving in fuel and going faster and faster. After you pass the air/fuel top limit you start pushing fuel into the exhaust and burning it when it hits the oxygen.

That limit is a 115 jet on my TBR7. Past that it resembles riding a roman candle down the road as the gas dumps and explodes. I had to back off of a 117.

Main jet, OEM YouAll carb
 

I increased my main jet from 120 to 125 on my Templar X bikes and pilot jet from 40 to 42 (45 was too rich)... This includes the original 5 speed with the 172FMM-3A (which I still own), and my counterbalanced 6-speed 172FMM-5

In hot Summer weather, the 5 speed might need transient choke but usually starts without. In the Fall, it needs choke. I don't think I could get it closer to perfect.

Midrange responds flawlessly, but needs to warm up a bit when it is cold to run right. There is some back firing when I let off the throttle in gear, which might indicate slightly lean, but when I check the spark plug, it has no indication of lean condition. It looks perfect.

I am at about 700 feet.

Jetting depends on a multitude of factors including but not limited to displacement. The Templar PE28 carb ships with a 120 main jet and runs too lean below 1000' ASL. IMO 125 is near the bottom of the acceptable mixture range in this application. The range that people are running ranges from 120-140. My goal here was to gather some more info to help me decide whether to test a 130 or 135 MJ next. My goal is to run around 12.5:1 AFR @ WOT which is slightly rich but very safe and a little cooler. I'm gonna try a 130.

Thanks Thumper, I'm running the same 42/125 based on your recommendation. Mine runs great, like yours! Needs just a little choke when cold and warms up quickly. I'm confident about the 42 pilot and 1mm needle shim and just a little curious if the 125 is as rich as I want. It runs great but possibly a little hotter than necessary. I think I'll try a 130 MJ and see what happens.

Oh, I forgot that I shimmed by ~1mm as well.

Yes, this set up has been good for my altitude. It has full power with wide open throttle as well. No hiccups at all. Quick rollon is equally responsive.

I had to take my shim back out. At cruise, 50 mph in 6th, it was "fish biting". Like a rolling intermittent hesitation. Mine's happier with the stock needle position apparently.

So you leaned out the midrange. Hmm. Richer is usually OK, just blackens the plug. Not sure what could cause what you are describing. I have no mods, but a 1mm shim can rescue a lean situation in midrange performance..

What size shim? I noticed that the needle sits in a recess milled into the slide that is just slightly larger in diameter than a M3 washer. If a larger SAE #4 washer were used it would not fit into the recess and therefore shim the needle by about 1.8mm. Two 0.8mm thick #4 washers would raise the needle by 2.6mm.

Perhaps a single M3 (0.5mm) shim might work out.

I machined the shim to 0.039" if I recall. "Machining" being me modifying a washer out of my stash to the proper diameter and thickness with emery paper on a piece of glass.

I changed nothing else at that time and it runs great through the rpm range now. Only issue is I have to enrich it a bit on cool mornings a little longer than I would like. Also there's some lean pop at low RPM before it fully warms. Thinking about bumping up the pilot jet one size and seeing if it cold starts more like what you guys say yours does. I think it's at 42, I didn't take notes. Which, yes, I should have. I set the idle air screw when fully warmed. Until I did I was getting a touch of hesitation jumping on the throttle. A pet peeve. On a recent all day trail ride it performed great. My commute in the mornings is where I wish it to warm up a little quicker. More my fault for wanting to rush than anything.

Maybe you can fine tune with the airscrew (gas screw in our case). It sounds like you are close.

I tweaked the air screw when fully warmed up. Fully warm performance is about dead on. If I get around to it I might put a scope on it and eye the waveforms. Since the shop I used to work for sold their water dyno tuning isn't as easy.

Well im a believer now, i used 42/125 jet that ive seen so many talk about Works Great, a big difference. I bought a nibbi 28mm after watching some youtube reviews before i learned on here about drilling the plug out and cutting slots in the bowl screws, i now have a back up. I really appreciate this forum ( everyones input)...
Im around 1300 +/- ASL

I keep what I’m currently running updated in my signature. I’ve been running this for so long now I think I’m settled on it for good. I also follow the start and stop procedures as force if habit now as written in the actual manual. Keeping in mind no two carbs come off the assembly line exactly identical, my plug chop readings reworked my intial trials to what I run today.