adjustable main jet

jaguar

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#1
With few jets available here and the fact that I like to mess with my engine (porting, timing, etc) I came up with the idea of an adjustable main jet and was able to successfully pull it off. I didn't mention it before because I wanted to make sure it was reliable. Well, after a year of use I can say it is. What is sweet about this is the quick adjustability for weather or altitude changes. If the bike isn't running right then just pull the carb and the bottom drain plug and adjust with needle nosed pliars.
What I did is make the internal gas passage bigger by drilling it out some. Then I found a fine thread machine screw with a slightly bigger diameter than the gas passage. Then I drilled a hole for the screw at a 90 degree angle to the gas passage so that as the screw is turned in it blocks the gas passage more and more.
Just thought I'd share this with you. So when it becomes available to the masses, you heard it here first!
 

jdbrusch

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Nov 11, 2001
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#2
Good idea,do you have any way of limiting how lean or rich you can go with the screw?
I deal with small 2-stroke engines at work and have always wondered why they did not manufacture dirt bikes with a carb with external adjustment screws for the high and low speed circuits(we have the pilot screw,low speed ),you could even put in a screw for the mid range,most of the motors I deal with now have limiting blocks so you cannot lean or richen them out to far to damage the motor.The terrain we have here runs from sea level(warm) up to snow level (cold),you could simply give the circuit you wish to adjust a tweek for peak performance.
Might be ideal to fab up one into the drain plug with a external high speed screw(main).
 

jaguar

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#4
The maximum limit is determined by how big you drilled out the gas passage. Besides that it can have a wide range of adjustment and can adjust in smaller increments than by replacing jets.
no picture. just imagine a screw going into the side of the main jet.
It works great and never changes adjustment from engine vibration.