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Old 09-30-2019, 03:08 AM
Beginner Class
Join Date: Jun 2011
Last Online: 07-12-2020 12:17 PM
Location: Paraguay
Posts: 35
Default jetting misconceptions destroyed

What I have recently learned is that the main jet doesnít dominate the upper throttle openings - it works in conjunction with the needle (the distance between it and the needle jet) and also influences the idle jetting because some fuel is flowing up past the needle jet even when the throttle is only 1/8 open.
If you donít believe me then just put in a slightly skinnier needle and plug up the idle jet with silicone sealant or just stick a toothpick into its hole. Then you will see if the needle flow area isnít too far off then the bike will idle. The stock carburetors for motorized bicycles are that way. They donít even have an idle circuit.

Also I no longer consider using the needle clip position as the best means to adjust the mid range jetting. You can do it that way but then the off idle jetting suffers because the clip position controls when the needle taper comes into effect somewhere close to 1/4 slide open. The best way to change mid throttle jetting is to change the needle taper angle, not the clip position. This page fully explains: The Importance of Needle Height

This graph was derived from making jetting changes using my new jetting calculator which gives a more realistic version of each parts role in jetting:

Hereís my guide to carburetor tuning: 2 Stroke Carburetor Tuning
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