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What I have recently learned is that the main jet doesn’t completely 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 don’t 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 isn’t too far off then the bike will idle. The stock carburetors for motorized bicycles are that way. They don’t 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: www.dragonfly75.com/moto/NeedleHeight.html

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:
www.dragonfly75.com/moto/images/jetrange.gif

Here’s my guide to carburetor tuning: http://dragonfly75.com/moto/carbtuning.html

This is not new information, Kehein's own jetting instructions and diagrams have shown this for years. For most people unused to, or uncomfortable with, carb tuning, it just makes it easier to consider the main jet alone to get the carb tuning to a good rideable condition. For those looking for max performance needle tapers and slide cuts are the final pieces of the jetting puzzle. Easiest is to adjust clip position solely because lots of riders don't have multiple needles of varying sizes available. Same goes for slides, get expensive to have multiples in different sizes laying around.

bufftester wrote: ↑05:33 pm Sep 30 2019 This is not new information, Kehein's own jetting instructions and diagrams have shown this for years. For most people unused to, or uncomfortable with, carb tuning, it just makes it easier to consider the main jet alone to get the carb tuning to a good rideable condition. For those looking for max performance needle tapers and slide cuts are the final pieces of the jetting puzzle. Easiest is to adjust clip position solely because lots of riders don't have multiple needles of varying sizes available. Same goes for slides, get expensive to have multiples in different sizes laying around.

While we’re on the subject, how should one measure cutaways? I’ve read that on a lot of slides, the cutaway is actually a “phantom” measurement that’s in the CAD drawings of the slide, and not measurable on the slide itself. This probably differs between brands, but I’ve been pondering this for a while now. If I want to make a 4.0 cutaway into a 5.0, how would I get an accurate result and not screw up my slide??

Easy answer is you can't make an accurate cut on a non-round slide without machining equipment. Round slides the cutaway is easy to measure, but the D-slides the Keheins use are designed in CAD and the measurement points are a bit murky (likely to make it harder for aftermarket companies). The only sure way to get an accurate #5 slide would be to order one. It will be leaner than a #4. Of course, if you cut your slide to a leaner # you can't go back to a richer one, so IMO buying the slide is a better path. But honestly, for 95% of the folks out there, slide cutaways are going to be indescernible, and the bang for the buck just isn't there, again IMO.

I absolutely agree plug chopping only jets your bike to scream down the street and we dont ride dyno's in the either, take your bike to the track or woods and with a few jets and jet it for where you ride where ever that is. FMF's jetting recommendations are a very good starting point with a safety net in place only a couple of small changes needed. Low end highly effects top end and vise versa. its all about getting the sonics of the pipe working at the top efficiency, With the stock needle 2nd position is motocross 3rd is for the casual trail rider. You can have anything you want between those 2 points using .25 shims it is that sensitive and tunable between 2nd and 3rd position. The smaller the engine is the more sensitive to jetting it is.