Quote:
Originally Posted by austinninjabill
Thanks for the reply. Given your stable of bikes, you've got some experience certainly more than me -- I haven't had a motorcycle since 1977 and then it was a 250 Maico (sunburst head, coffin tank and a 4 speed).
So it pulls strong to the redline with a 108 jet? And the plugs look good (brown, not white)? Most folks say 110 minimum and some a 112 to avoid a lean condition.
Do you think your location (WI -- cooler weather) makes a difference?
And out of curiosity, where did you get the idea/advice for the two washers under the needle in the first place as that is not stock? I wonder who first came up with that adjustment?
Any idea what that does? Guessing it allows more fuel into the mix by raising the needle base, maybe that's why a 108 works for you. Perhaps the 110-112 needle guys aren't shimming the needle or not as much.
So maybe the theory/rule could be for main jets 112 with no shims, 110 with 1 shim, 108 with 2 shims? And for pilot jet, 38 with 2 shims, 40 with 1 shim, 42 with 2 shims?
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I've ridden a 250 Maico like yours, back in the day.
Pulled to redline and beyond to the limiter just fine. Plugs were tan. Non-ethanol gas does give you a richer mixture than gas with ethanol, but I think a lot of people go overboard with the Main Jet size. What exhaust will you be using? That can make a difference as well. 108 is a good place to start. Temperature and elevation also alter the mixture. There are plenty of variables.
Shimming the needles gives a richer mixture sooner, and compensates for a stock mid-range that is on the lean side. 1 to 2 shims is common. Adjustable needles do the same thing, but aren't really necessary. The needle adjustment doesn't effect the mixture at WOT, as they (needles) are completely out of the Main Jet by then. Shims don't effect the Pilot Jet size or Idle Mixture setting either because the fuel is not being drawn up through the Main Jet at that point.
Not sure all the info is definitive, but it has worked for us.
There is plenty info on carb tuning, and much more, here -
https://faq.ninja250.org/wiki/Ninja250_Howto