I'm playing with jetting on my recently overbored 98KX250. As I understand fuel signal, it's the ability of the motor to pull (vacuum) fuel into the combustion chamber. Therefore, an overbore kit should increase the fuel signal, based off the fact that lung capacity has been increased. Further rationalization would lead me to believe that an overbore kit will require leaner jetting, because the motor is breathing in more fuel per stroke than before. With all this said, most posts I read regarding jetting of overbores indicate a need for fatter jetting, which is contrary to the reasoning above. Any feedback would be appreciated.
Increased Fuel Signal relation to jetting
- Thread starter MXTex
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marcusgunby
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- Jan 9, 2000
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Do what the engine needs not what the theory says-theorys are good for books but dont make a engine run right;)
KXaggerator
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- Feb 4, 2001
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I found on my '98 that I did go slightly leaner.
steve.emma
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- Oct 21, 2002
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You have changed the displacement of the engine but not the size of the carb. So if the mixture was basiclly right before then now more air will be drawn through the carb than before so it will draw more fuel with it causeing the mixture to be to rich, therefore you do normally need leaner jetting to get the mixture right again. However i do agree with Marcus, you need to try it one size at a time and see how it works.
It has to do with air speed. Air density changes and rpm levels affect jetting as well. Like the other post - do what the engine wants. Other things that affect jeting is you driving style, track conditions - mud - hills etc. We had a Rotax 250 once with a cylinder reed valve carb and rotary valve carb. (twin carbed single cylinder) The jetting was the same as with one carb and both carbs were jetted the same??? Like Marcus said - do what the engine wants Bob Hogg
