Tuning Info Req.

[John In NC]

I am the 2nd owner of a '99 YZ 250. It has a FMF Gnarly pipe. My problem is that it is not running right from idle to about 1/3 throttle. It also has a tendency to foul the plug. It came with a NGK B7ES, the book says to use a BR8EG. So I think the PO installed a hotter plug to compensate. I keep the air filter clean so that that can be ruled out. The stock jet sizes are MJ 172, PJ 50, power J 80, Starter J 85. I don't know what is in it right now. The info I'm looking for is what would be a good starting point: stock? As that I live about a week from any parts I my require, is there a kit available with an assortment of jets? Or am I stuck ordering one jet at a time. I plan on installing a 12oz fly-wheel weight and would like to do both projects at the same time. Lastly, any recomendations on a place to purchase the items?

 

[skipn8r]

It sounds like a Jame Dean jetting kit is what you want. I've never tried it, but I know of several people who have and with good results. Unfortunately, I scanned his web page and didn't find one for a '99 YZ250. You might want to contact him directly.

James Dean Jetting

EDIT: Otherwise, since you're close to sea level (I assume in NC), you might want to pull the carb, verify what's inside, order and install original jets and go from there.

 

[skipn8r]

I keep on thinking of things to say after I post. The off-idle to 1/3 throttle range is affected by several adjustments, including air screw, slow jet, needle dimensions and clip position, and slide cutaway. The symptoms you describe suggest either too large of a slow jet or an improperly adjusted air screw. I'd suggest playing with the air screw and see what happens.

With the bike off, turn the air screw all the way in and count how many turns until it lightly seats; write down that number to go back to if you get confused. Then back it out to 1.5 turns out, crank the bike and get it to normal operating temp. With the bike idling (increase the idle speed if necessary), slowly turn the air screw both in and out looking for highest idle and best engine response. If you get best idle and response at less than 1 turn out, you need a larger slow jet; if at more than 2.5 turns out, you need a smaller slow jet.

A couple of web sites that break down the function of carb systems are:

http://www.duncanracing.com/techfaq/Tech_keihin-carburetion-jetting.phtml

http://www.dirtrider.net/justkdx/printjetneedles.html

 

[soulmate33]

I would check the reeds also. Could be chipped/cracked.
Check the SFB racing website for a good flywheel.

 

[yamahopper]

I agree with the previous suggestion - try the air screw adjustment first
You may want to try raising the clip in the carb one notch and seek if it runs any smoother for you, may just be off and it won't cost anything to try. It may help your off line repsonse.
Someone may have it set richer than you prefer, if you end up on the last notch then you may be right - jetting would be the way to go.
Choke works correctly and is clean?

 

[John In NC]

Thanks for the information. I'll play with the clip and air screw first. No first I'll install the fly-wheel weight. Having much more of a four-stroke seat time, I'm still learning the two stroke suffle.