Good article on Carb Tuning

makotosun

makotosun

Member
I know that there is a great deal of interest in carb tuning on the board, and I certainly am one of those interested. I also have some old 2 stroke bikes that I am currently tuning and was pointed to this article on carb theory.

http://justkdx.dirtrider.net/printcarbtuning.html

Beware that this is written for a two stroke motor, but the general principles should be applicable to any carb, including our favorite little minis.
 
6

69ST

Well-Known Member
It's a very good article but there are some differences between 2-stroke & 4-stroke tuning parameters. Without getting into too much detail for the moment, just be aware that air:fuel ratios need to be much leaner for 4-strokes. Stoichiometric (chemically ideal) air:fuel ratios for 4-stroke motors are - 14.7:1 at idle & part-throttle, 12.7:1 at full power WOT. 2-cycles inherently make more power for the displacement, but 4-cycle engines are much more efficient and cleaner-running. It has been my experience that motors used in CT70s won't rev-out cleanly with 12.5:1 A/F and that idle & part-throttle A/F ratios in the 13.8:1 - 14.3:1 range give the best overall balance.

Some very good & detailed explanation in the article, they just need some interpretation in places to apply to 4-cycle bike engines and other brands/models of carburetors. If you already know some of the basics, no worries. For those who are still green in the area of carburetion, when in doubt, ask questions.
 
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