What size main jet for CB250F?

[krat]

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.

[Thumper]

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.

[Fast_Freddy]

Originally Posted by krat

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.

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.