TY175 spark plug

[Takhli44]

Looking for your thoughts on this. B7ES (stock) plug occasionally fowls or starts acting like a rich mixture. A plug change fixes it. Running a BP5ES (2 ranges hotter and extended tip) works well with no fouling. When I check it after a run it appears about right, not black and not white like too hot. I live at 4000 ft elevation and ride mostly around 6000 ft, sometimes 9000 ft. 190 main jet, stock (25 I think) pilot, jet needle one lean from center. 64 to 1 premix ratio synthetic oil, non ethanol 85 octane fuel.

[David Lahey]

No surprise there. Sounds like you have only gone leaner than standard on the needle position and main jet, but have standard jetting from throttle closed to 1/4 throttle. Increased altitude always makes a bike run richer and the recommended plug for TY175 B7ES is based on the assumption that people will flog them on roads.
Trials use with good jetting a B6ES is good
Road riding B7ES or B8ES is good
Road racing B8ES or B9ES is good
For those high altitudes you ride, the ideal jetting is probably leaner than what you have.

[Takhli44]

Thanks David. The bottom end is adjusted pretty well with the air bleed but as you suggest the bottom is a bit rich which might be causing the fouling. Might try a B6ES. Do you have any experience on extended tip vs. standard tip plug in these bikes?

Forgot to mention I'm running a Boysen 2 stage reed valve.

[David Lahey]

No I have not used extended tip plugs in a TY175 motor.
I use extended tip plugs (BP5ES) in Bultaco and Montesa trials bikes and have no problems with them