I have been rebuilding an 85 250r motor and am not sure were to start jetting, The cylinder has been bored for a 67.50mm piston(stock 66mm) and I will be adding an fmf pipe and boyseen reeds the stock jet is 140 main. so how many size up would be a good place to start. and will the bore require a differet plug? any info will be appreciated, thanks

 

I put one in my Rancher. The kit itself is very easy, but getting to the carb and removing it (reversing this process as well) takes the most time. Be careful on removing small screws for the carb. covers as they can strip pretty easily

Good luck.
Mark

 

think I might give it a try, thanks for the response.

 

I would compare it to changing plugs on a Chevy 350 and regular maintanance. Took me an hour or less. Nice power gain. make sure to open up the air box more air is a must!

 

I plan on running with no lid, so I should be all right, thanks for the input

 

Jetting (main) is very easy physically to do, but can be frustrating to get "right."
With the airbox lid off, the pipe/silencer, go up about 3 sizes in your main jet. If .140 is stock, .142,.145,.148,.150, go to the 148. Do a plug check. If everything runs well and your plug is a lite tan color, well you got lucky. If you engine sputters on top-end and your plug is black/oily, go down one size. If your engine runs really good top-end but your plug has very little change or leaves white stuff this means it's too lean and you will need to go up one size and try again.
Nevery worry that you will damage your engine from running too fat, it just won't run good if it's too fat, but if you run it too lean for very long you can fry a piston right out of it.