Mikuni and Dynojet Main Jets
FactoryPro and DynoJet kits have different jet sizes. This is because Mikuni jets (used by FactoryPro) are marked according to their flow rate i.e. 150 = 150 cc of fuel per minute, whereas, DynoJet jets are marked according using a unique DynoJet system (possibly the diameter of the hole of the jet i.e. DJ150 = 1.5mm). Mikuni jets come in size increments of 2.5 for sizes 50-200 while DynoJet jets come in size increments of 2 for sizes 74-150 and in increments of 5 for sizes 150-210. Although you may find Mikuni to DynoJet size comparison charts on the internet, they are not accurate. I have no experience with Dynojet kits so I guess it's a personal thing, however, being a Mikuni carburetor, I tend to favour Mikuni jets.
Main Jet Selection Methods
Before testing to see whether or not you have selected the best Main Jets, warm up the engine for 15 minutes to ensure you get consistent results. Don't be concerned with mid & low end power problems just yet as they will be addressed at a later stage. There are two of ways to test to see if you have selected an appropriate Main Jets other than putting you bike on a dyno and testing it. They are:
Roll-On Method. Select the Main Jets that gives you the highest top speed by making full throttle runs between two points or markers at a race track. The Main Jets that gives the highest speed at the second marker are the best. The gear selection and speed must always be the same at the first marker when full throttle is applied. The distance between the markers must be enough that small differences in performance can be detected.
Roll-Off Method. In 4th gear and over 6000 rpm, open the throttle to the Full position and let the engine accelerate for several seconds until it is pulling hard then quickly roll off the throttle back to the 7/8 position. If the engine gains power as you roll off, the Main Jets are too small and if it staggers slightly or hesitates, the Main Jets are too large. If the engine winds down smoothly and evenly, then the Main Jets are likely correct.
Top End Power Test (Full Throttle)
To test to see if you have the best Main Jets fitted, do a couple of 4th gear full throttle runs from 6500 rpm to Redline. Remember, if you have a desnorkeled or race filter, split Mains may provide better top end power.
Too Rich - If the engine appears to pull harder at high rpm when first do this but not as hard after a further 15 minutes of hard use, then the Main Jets are probably too large (too rich). Replace with smaller Main Jets and retest until you find the Main Jets that pulls the hardest on this test.
Too Lean - If the engine doesn't feel like it's pulling as hard as it should based on your stock test ride (it may stumble and feel like the engine is taking a while to wind up to Redline), then it's likely the mixture is lean and the Main Jets are too small. If your bike starts to run very badly and seemingly misfire after 7000 rpm, then it's likely the mixture is very lean and the Main Jets are a couple sizes too small. To test a very lean mixture, block off about 1/3 of your airfilters intake using race tape. If your bike runs better after 7000 rpm, then you need bigger Main Jets. Replace with larger Main Jets and retest until you find the best.
NOTE: Remember, a rich problem only gets worse after the bike warms up whereas a lean problem will seem to “correct” itself (or may even disappear altogether).The Jet Needle is a long tapered rod that primarily effects carburetion from 1/8 thru 3/4 throttle by controlling how much fuel can be drawn into the carburetor venturi at any given throttle position. The thinner the taper, the richer the mixture. The tapers are designed to give different mixtures for different throttle positions. Jet Needles have notched tops with 4 - 6 clip positions. These are used to make an engine run richer or leaner within the 1/4 to 3/4 throttle range by raising or lowering the Jet Needle inside the Needle Jet accordingly. There are two main components of the Jet Needle which control your mid range power - the needle clip position and the needle taper.
Jet Needle Taper
As you can see from the picture below, Jet Needle taper and thickness is important and has an effect on emulsion in the Off-Idle, Mid and Top power ranges. The Mikuni, FactoryPro and DynoJet Jet Needles all have different sizes and tapers from each other so their emulsion characteristics are also different. This means that they may have different needle clip settings even though they have the same Main Jet. Jet Needles can have more than one taper and companies usually offer 3 to 4 Jet Needles to choose from (richer to leaner). Since tapers can't be physically changed (other than replacing needles), the only way you can change the Jet Needles effect is by adjusting the needle clip setting to change when the taper becomes active and inactive for a given throttle position.
As an example of just how much the taper effects your carburetor settings, the FactoryPro Stage II Jet Needle (#0984h5-60v2) is richer at full throttle / 3000 rpm than a FactoryPro Stage I Jet Needle (#0988h-64v2). It's also richer at full throttle / 5000 - 7000 rpm, by the equivalent of about 2 clip positions (60 is richer than 64)
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