Lean Surge, or how tune your main jet on your scooter

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Lean Surge, or how tune your main jet on your scooter

Post by noiseguy »

From Expressly Mopeds, http://www.network54.com/Forum/26489
Brian did a lot of work around the Jan 2002 timeframe on hopping up Spree. This is one of his articles on jetting. Good reading for any scooter:

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Carb Hints
by Brian

First, this will only be the basics that will hold true to most moped carbs, ford carbs and holey carbs.

Those two screws, they are only for idle. One is for idle speed, it will usally limit the amount the slide will drop into the throttle body. The other is for idle mixture. Why have a separate idle mixture, let me explain.

Your main jet feeds a tube, this tube has usually two holes in it. At at idle the slide is practically completely closed, air does not physically flow high enough past the tube to draw in fuel. Hence a seperate idle fuel circuit. This circuit, is only designed to pass a very little amount of gas for idle and it is designed to work at a higher vacuum than normal operation.
At idle there is more vacuum psi than running at open throttle. Because of those two separate design factors, turning the idle mixture screw does not affect normal carb operation.
Normal carb mixture is affected by two things, one the size of your main jet, which determinces the volume of gas going up the tube. Two, the setting of the metering needle, which measure how much of that gas is allowed pass into the throttle body at what throttle position.
Adjusting the Carb. First if you are not experiencing lean surging, then do not go to a larger jet. Lean surging is easy to identify, you crank the throttle, the bike dies out which is exteme lean. Or the bike engine literraly surges in rpm for milder lean surging. How to check, if you manually choke the bike and it runs better, the mixure is too lean.
If you run too rich the bike will not run good at all. It will start but there wil be very little power. So only increase jet size if you are experiencing lean surge. As long as you have a factory jet, you should be okay. Only go to a larger jet if you are experiencing lean surging.
Setting the carb, For idle, crank out the idles mixture 2-3 turns. Then set the speed at a decent level. Then slowly turn in the idle mixture screw. When a sudden noticeable decline in idle speed is reached, adust the screw out just enough to smooth out the idle. That is it.
If you don't believe the separation of the idle mixture and running mixture then do this. Start your ped. Run it at 1/4 throttle, have a buddy close the idle mixture completely. Guess what, the ped is still running but it won't idle if you release the throttle.
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