yz125 vs other 2smokes

Why is it that yz125s run 410 main jets but I've seen many 250 2t's running 170s and 180s and even a cr125 running a 170. Why does the yz125 take such a bigger main jet?

The larger the engine, the smaller the main. This is due to the longer and stronger vacuum signal. Samller the carb boore, smaller the main, same reason.

Main jet is also affected on the fuel/oil ratio, less oil, smaller main.

larger displacement does require smaller jet because of more vacum, but the biggest diffrence in what you explained is that they are diffrent carb. brands and they number thier jets differently.

Good point about the jet sizing, I was assuming same brand of carb but of course (dooh!), that is not always the case.

crappy mikuni on the yz

Okay thanks guys

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