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Discussion Starter · #1 ·
IDLE JET & AIR SCREW
This is the preferred way to adjust the air screw - Start the engine and let it warm up some. Turn in the slide stop screw for a slightly high idle speed. Then turn the air screw back and forth to find where it idles the highest. Then turn it clockwise till the idle speed starts to significantly decrease. That setting is slightly rich which helps with acceleration off idle. Screw out the slide stop screw for a normal idle speed. Now here’s where I deviate from normal. Most people say to use a bigger idle jet if the air screw is less than 1 turn, and a smaller jet if it’s more than 2 turns. I say that engine idle will be fine over a wide range of idle jets with the air screw making the needed compensation, so why not select an idle jet that will best assist off idle power? If you don’t then you are not making use of free power. Of course then a slide cutaway change will cause you to need to retest for the best idle jet again, but whoever said perfection was easy?
Apart from setting the idle jet for best off-idle power, you can evaluate the idle jet size by measuring the slide opening when it’s set for smooth idle. This page explains the topic: SIGNIFICANCE OF IDLE SLIDE OPENING

UNDER 1/4 SLIDE OPEN
Needle Jet Flow Area and Slide Cutaway
Surprisingly, a lot of idle fuel comes up thru the needle jet. If you don’t believe me then just clog the main jet and see if you can start the engine and have it idle. The “needle jet” is the hole the needle moves in. The flow area is the open area between the needle and the needle jet. It controls the jetting from throttle closed (around 1/8 slide open) to 1/4 slide open (not 1/4 throttle) r when the needle taper starts to clear the needle jet. You can enrich that jetting by installing a skinnier needle or fatter needle jet (with a wider hole). Increasing that flow area means you’ll have to decrease the idle jet size (and vice versa). What should the needle jet area be? Good question, never before answered because it and the slide cutaway affect the same throttle range. So up till now this has been another blind “pin the tail on the donkey” 2 stroke guessing game. Fortunately for you I invented a fantastic jetting calculator that has taught me all the tricks of the trade about carbs and I know the ideal ratio of needle jet area to main jet area, and how to calculate the best slide cutaway based on that ratio (in case you just don’t want to change the needle jet or needle). The ideal ratio is 0.6, needle jet area divided by main jet area. Needle jet area is the jet hole area minus the needle area at its straight section. And the formula for calculating the ideal slide cutaway height is Cutaway = (0.14 x carb size x main jet area) / (1.87 x needle jet area) The needle area = 3.14 x (dia/2) x (dia/2) You just need to know the needles diameter.
Starting at I69 of my free spreadsheet (download from www.dragonfly75.com/moto/various.html) you can find the main jet area for most carbs. And from www.dragonfly75.com/moto/needles.html you can find the needle widths at their straight section. For needle jet hole diameters go to www.dragonfly75.com/moto/NJets.html

OVER 1/4 SLIDE OPEN
For most carbs the ideal needle height is such that right at 1/4 slide open (not 1/4 throttle) the beginning of the needle taper is right at the opening of the needle jet. I recommend setting it up that way and then testing it. Just use a digital caliper to find the taper beginning and then use a felt marker to put a dot there. Then put the needle/slide in so that you can position the slide any where you want to. With the dot facing forward, move the slide up and down and hold the position where the dot is right at the opening of the needle jet (usually 2mm below its top). Then measure the slide opening height. Adjust the needle height so the slide is 1/4 open for taper beginning. Then ride and test it. If that throws off your mid throttle jetting then ideally you should replace the needle with a richer or leaner one, depending on how it runs now at half throttle. otherwise return the needle to its previous height. With large needle height changes you also need to change the main jet since it affects mid throttle jetting.

HALF THROTTLE JETTING
This is mostly the territory of the needle taper angle. Too much angle and it can run rich with a congested sounding exhaust note and possibly some sputtering. Rich at half throttle will love it when you crack the throttle fully open because its leanness balances out the mid throttle richness. Lean will bog if you don’t slowly open the throttle. What complicates this is the dual and triple taper needles. If needle selection becomes too complex or needles cost more than $20 then consider getting my $20 jetting calculator which lets you use the jetting results with different slide cutaways and different needles to save you time and money. A racing bike is OK with slightly a richer needle than a street or trail bike. After you install a different needle then you need to recheck the main jet size because they both affect each other.

WOT JETTING
Jetting for high load situations (that you commonly experience) needs to be richer than for normal riding and so it’s best to jet for high loads such as going uphill or thru sand. Otherwise, if you jet for easy riding, you may wind up seizing the first time you try to ride thru a high load situation. You can test for either best top speed or best acceleration. Forget the misplaced idea that there is one perfect air/fuel ratio that gives a certain plug color. It just doesn’t exist. Factors affecting the needed air/fuel ratio ; squish band area and volume, engine compression, type engine oil, carburetor size, ignition timing, pipe back pressure. There’s only two guidelines for jetting: 1) best power, 2) acceptable piston heat. If you jet for best power but use a low quality oil then you may need richer jetting to cool the piston more. If the oil is being baked black onto the piston underside then the heat is too much for the oil and you need to either use a more synthetic oil or richen the jetting to cool the piston. Unfortunately you can’t judge an oil by its promotional literature. I have fully investigated many oils and how synthetic they are is listed on my page dragonfly75.cm/moto/oil.html Main jet sizing needs to be after you are content with the needle and its jet because those also affect WOT jetting.
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Discussion Starter · #2 ·
This method allws you to tune the most difficult carbs. It is also very valuable for street and trail riders for good jetting all thru the throttle range. For racers, maybe just pay attention to the info for 1/2 throttle on up. They mstly need to know how to select the best needle with a different taper.
 
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