i have put together a mod 1975 yamaha gpx with 40mm powerjet carbs. i have the carbs with the powerjet in bowl. im not sure on how to tune the powerjet with main jet as i go up or down in main jet sizes. does anybody know how to tune these carbs or a website that explains how to. i have the aaen book but they dont show it. thanks

Bill

billcat, powerjet has become kind of a generic term that describes a few different systems or products. From your decription it sounds like you have something like thunderproducts sells? you may check them out. If you have the adjustment on top of the carb the idea is this. If the powerjet is turned closed (no added fuel) you would run a stock mainjet. If you clicked it open 1, you go one leaner on the main, open it up 3, then go 3 mains smaller etc. To actually use it to jet up and down in the field you want it baselined so that you have some room to adjust each way. You can reference a temp/alt chart to give an idea where you might start at based on your proposed riding variation. I ran them years ago on a finicky ported piped exciter and they worked pretty good, easier than changing jets anyway. Every sled is different so the 1click=1 main size isn't always exact.