Ok guys,

I just picked up a new yz250f and I jetted it today. Here are my current spec's

Main-168 (178 stock)
Needle- 3rd clip (4th stock)
Pilot -stock, messing with the fuel screw 2 1/2-3 turns out.
leak Jet -stock

Now here is the problem, I have a terrible off idle to 1/4 throttle stumble and it does not start very easily. This seems very odd. My quick fix for this would be that it is running lean down low, but I'm at 5500ft. Do you guys recommend a richer pilot. I'm not going to mess with the leak jet just yet.

Tomorrow is the first day on a motocross track and I wanted to get this resolved.

Erik

I would think you want to run leaner on the pilot.....turn in screw to 1 turn out as a starting point. Not at your elevation but run a 40 pilot. Stock is bigger I believe. Your main is still possibly rich but you may not notice.

George this is why I feel this is odd, Normally i would think it should be a leaner pilot, but it exhibits all the lean traits. Popping on decealeration and hard starting. Turning out the fuel screw makes it a bit better.

Erik

Then the next thing would be bigger pilot, I guess. I'm assuming its pretty cold.

With the fuel screw being 2 1/2 to 3 turns out i would say you need to put a bigger pilot jet in and start from there. Being that its cold there you need more fuel to mix with the oxygen. When the temp warms up there that pilot will probably be about right. I would try to stay away from running 2 1/2 - 3 turns out cause the fuel screw can fall out.

Yes it was cold out today while i was testing. Since I don't have a 45 pilot, I will gut it out tomorrow. It will be warmer so I will see. I guess every bike is different and I bet if this was summer comes arounf the bike would hopefully run real good.

Erik

Guys just to update you on the jetting. I had the bike running unbelivable today with a 48 pilot and 3/4 turns on fuel screw. No hesitation at all. No popping. Smooth as can be. I'm going to go a 45 as the temp increases. It Still in the low to mid 40's here, but all I can say is wow the bike was running good.

Erik

Hey man, I just got my first ride on my 07 250f today and its a lean mother,Im at 4400ft,but as my other bike 03 wr450 it was way lean every where also . richen up the leak jet and it will start very well ,as your other jetting your main sounds too lean ,but your there and know how its performing for you.good luck .