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Need help with jetting....Newbie

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Just bought a 99 125 yz, I spent the first weekend going over everything and resetting the carb.  Previous owner had a NGK 8 plug in it that fouled our first day out, so I put the 9 in it.  It also fouled after about 3 hours of medium riding. ( I'm not a very good rider, just trying to keep my boy interested) Anyway I ended up leaning out the needle as far as it would go and have rode it a couple of times on the same plug, response is good ( a little down on low end torque IMO), and gets with the program on the pipe.  I'm mixing yamalube at 40:1 and the plug still is a little rich.


Compression check was 110psi cold and all jetting is stock. I think were at about 600 feet elevation it's in the 90's now.

Just wondering if you guys could give me a getting base line to start from.  Kinda felt like I was loosing the plug on our last ride, It didn't want to get in the power band until I hit second and cleaned it out? If that makes sense.


ON another note will the power valves sticking cause a low compression reading? I haven't check those yet, I think 110 is a little low.




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the br9 heat range plug you've installed is the std one, the one you removed is the next hottest, if your jetting is too rich , it will quickly foul the colder plug, take longer with the hotter plug, if it fouls the hotter plug fairly quickly[ providing its not being ridden out of powerband] then you will have to rejet your carb

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