I have a 97 Trail Boss 250 air cooled 2-stroke. I rebuilt the top end last winter after I lost compression and discovered a torn air boot. A good majority of my riding is on open road routes with long periods of open or near open throttle. In hopes of making the top end job last longer with all of the high rpm riding I increased the main jet from 145 to 160. The thought was that the extra fuel would richen the mix and provide a little bit of addtional cooling. The engine ran very well last year. It is a mph or 2 slower on the top end compared to before the rebuild but it was probably running lean at that time because of the torn air boot. The slight decrease in top end speed may also be because of the larger main jet although I am more than happy to give that up in lieu of extended life on my rebuild. I have not noticed any bogging or other issues with the larger main jet.

I plan to purchase a new spark plug for the upcoming riding season. I am considering replacing the BR8ES with a cooler BR9ES. Again the open road riding with high rpm's for sustained periods is the only reason I am considering this change. Does anybody have experience running a BR9ES in this motor?

I understand that this is not the best engine or bike for this style of riding but it is what I have so I would like to do anything that I can to keep it running strong. Please feel free to contribute any other tips if you have them.

 

the 9 would be a hotter plug han the 8

 

are you having any problems with overheating now? If not, I'd probably stay with what works... I just can't see anything good coming from more fuel and a colder(weaker) spark.... imo

 

I keep forgetting also,so the lower the number the cooler the plug with NGK?

If Open road riding at a fast pace for long distances is your thing, you should look at a twin cylinder ATV next time around.
Even the smaller bore Can-Am 500 twin is more suited for that type of riding than a big bore KQ 750 single.

 

Thanks for the tip on the twin cylinder, I will definetly look into that when the time comes for me to upgrade.

As for the NGK plugs, the lower the number, the hotter the plug. And I have not had any overheating issues but I am always concerned on long rides in 90 degree heat that I'm going to melt the thing down running all the miles with high rpm's. Maybe your right Jmuddin, if its not broke don't fix it right.

 

I think I might give the BR9ES a try this spring. From what I understand the only thing at risk with a cooler plug is fouling, and if it does this I will put the BR8ES back in.