Help 2016 YZ250 Jetting, NEED EXPERT JETTING ADVICE

[MikeT]

Hi guys, I just got this 2016 YZ250 (2 Stroke) and it ran fine when I picked it up but I was told of a problem it had in the past. Apparently the previous owner took it to a hare scramble when it was very cold out (like 26 F) and it leaned out and ruined a piston and cylinder. He showed me the old parts and replaced them with new Yamaha parts, he still has the OEM boxes.

Before I ride it, I wanted to see where everything was with the jetting so this doesn't happen again. Well, I opened up the carb and found that the main jet on it is a 172 (stock is a 178) and he had the clip on the needle raised one position, both of which make it leaner as we all know. The bike has a GYTR (FMF) pipe on it and a GYTR silencer.

I have never had any aftermarket exhausts on my bike and I am not sure if they make the bike run leaner or richer. I pulled out the plug and it was dark, but that could be just from idling and not doing a proper plug check.

I will be riding MX in 80 or so degree weather and my elevation is about 700 ft above sea level. Can anyone tell me where I should be jetting wise with the conditions I mentioned? I am just looking for a starting point so I don't blow this bike up on my first time out.

Thanks

 

[MikeT]

Forgot to mention, it also has Moto Tassinari VForce 3 Reeds on it with a spacer

 

[jaguar]

To be safe just put a fatter main jet in it and if its runs irregularly then you know its too fat. Too lean would be a noticeable loss of power. So buy 5 jets and do that. When by feel you think you have the right size jet in there then ride it hard for 30 minutes and then pull the plug out and see what color it is. Darker than a medium brown indicates too rich and lighter indicates too lean. Unfortunately the needle position also has a small but significant influence on maximum gas flow so all along testing you may also be raising or lowering the needle so it runs good at mid throttle.
Be sure to check out my post in mods & maintenance about fuel/oil and jetting.