Cold Weather jetting

Was wondering if anyone could help me out on this. I've decided that I really don't want to store the bike this winter and would rather ride it the most I can. The bike's always ran fine but I thought I should take a look at the plug and see if the jetting needed any adjustment.

First off:

Am I right to assume that the mixture screw is what you want to make your minor adjustments (1/4 turn etc etc) and that the throttle stop screw just gets set anywhere it needs to be to get the right idle speed depending on where the mixture screw is turned?

I turned my mixture out about a 1/2 turn from where it was originally set and rode for a bit then pulled the plug and took these pics. I turned it out because the plug was obviously a little lean. So if this looks OK, then I'd just set my final idle with the throttle stop screw correct?

Forgot to add this is a '05 CRF150 and it's a cold weather plug.


Went back out and threw in a brand new cold weather plug and it looks like It's running better with this new setting. Question though, why is the plug staying lighter on one side then the other?


Don't try to read too much into how your plug "looks" and its color. Unleaded fuels don't lend themselves well to "read" the jetting from. Besides that, your looking at an overall riding condition, not a true "plug chop" from a wide open running condition to test main jet only.

The color looks fine. If the bike runs to your satisfaction, then why mess with it? In most instances you don't need to make anything but minimal changes for cold weather. Adjusting the mixture screw will likely be all that is needed unless it is sub zero temps. Even then the changes would be like the next size jet.

Thanks for the reply. The bike's always ran fine I guess I was just getting paranoid. :busted: Am I thinking right though in general terms of how adjustments are made for jetting? I've never messed with carb adjustments really before, always had a uncle's friend mess with these things.


No worries. Too many folks feel the need to "tinker" when changes aren't really necessary. The Keihin PD series carb is VERY tolerant of changes in weather and altitude. When jetted close like I believe yours is, the bike will run well in most any condition in your area, even elevation changes as when riding in mountain areas.

As I mentioned earlier, anything more than a fuel screw adjustment is likely un-necessary, extremes being the only variable. (Like sub zero weather or sustained riding in elevations over 10,000 ft)

If that is what the color is supposed to look like, then my jetting is way rich.


As I mentioned before, don't try to read more into spark plug color than necessary. If your plug appears dark or even black, that doesn't mean your jetting is too rich. You may have a dirty air filter, or you may have just been putting around before you pulled your plug.

Again, unleaded fuel's are not as good of jetting indicators as are leaded fuels and unless you have performed a good, steady state RPM or throttle position test, your plug color will not represent any particular jetting conditions.

Thanks for the tip, I'm still learning.

Where do you live in Indiana? My brother used to live in Elkhart.

Brownsburg, which is west of Indy.

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