What are the effects of a lean condition

We all hear about jetting. I have read the jetting work sheet "I think Motoman web page" and Mark Cantrell has some very good advise.

What are the repercussions of a lean condition and rich condition, I know too rich a mixture will cause fouling but have not heard if the lean condition will or could cause engine problems.

I have also noticed that while we all ride the same bikes we have very different jetting solutions if you look at alt as the start line. We all know that aftermarket parts will affect the jetting.

I guess the bottom line is what am I looking for "at" to determine my optimum settings. My bike runs well and the plug looks good slightly brown around the electrode insulation but a little black down in the plug well probably a little rich. I have a need to try and get machines, engines or whatever else working their best, I guess that is what I'm trying to do here.

Stock Jetting,clip position,fuel screw


I’ll start by saying that opinions vary on this topic. Some guys are more methodical about the process they go by to arrive at the “best” jetting, others are like me and go totally by the seat of the pants and don’t take notes etc. But one constant is that YOU are the one that is on the bike, so really you are the only person you have to rely on when deciding what is the “best” jetting.

With that in mind IMO what you need to do is extrapolate which direction you are likely going to be headed (richer or leaner) by your climate and altitude, making use of all the input available here (which I know you have already been doing), then buy a few jets and start experimenting. That is really the only way you are going to be able to determine the difference between being rich and lean. If you make one change at a time (that is a critical rule) and pay attention to your bike I think you will quickly make the connection between your right wrist, what is happening in the carb, and how your bike is running.

Having said that I also think that some descriptive accounts from other people DO help, so in that spirit I’ll offer this:

Obviously being overly lean can lead to overheating and even cause engine damage, but IMO that only happens in extreme cases, not to guys who are trying to find the best main jet. Anyway a typical indicator of a lean condition is surging at constant throttle. The bike is running out of gas, get it? So it is surging. A typical indicator of rich jetting is that it doesn’t run as well when leaner :) If you were to drop the needle two clips (and thereby virtually assure that it will be too lean) the bike would suck (too much) air and stumble when you “come on the needle.” Raise it two clips and it will stumble a bit at about the same throttle settings. See? It is a much more difficult thing to explain than it is to discern with your butt in the saddle.

I think plug readings on this bike are a waste of time because even if you disable the accelerator pump (because squirting fuel in the cylinder is obviously going to affect the plug read) who knows which carb circuit is responsible for what you are looking at? Sure, you can isolate the main jet by doing a “plug chop,” but at best this is just another tool to corroborate how you think the bike is running.

If you want to learn more about what carb circuit does what, and I recommend that you do to give that “connection” I was talking about a good head start, then read this FCR Tuning Guide. It is a bit technical, but still easy to understand.

Finally, while you are doing your jetting, make sure you run the same gas and ignore whatever the bike does until it is FULLY warmed up.

Hope this helps.

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