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Old 10-12-2013, 09:06 PM   #279
Studly Adventurer
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Joined: Jun 2009
Oddometer: 593
Originally Posted by Sporting Wood View Post
Idle air performs several functions within the carb in addition to supplying idle air to the pilot jet. It's size also determines vacuum pressure inside the fuel bowl. Larger air jets = closer to atmospheric pressure. Smaller = more vacuum. Bumping up to larger not only supplies the idle circuit with more air, which is a good thing, but it does so with the side effect of lowering the pressure within the carburetor, a bad thing. This results in slower response for the needle and sometimes less suction to draw fuel through the main jet, all things being equal. With a restrictive filter, increased vacuum somewhat compensates for this though imho, at the expense of proper atomization, efficiency and fuel mileage...

Using these free flowing setups puts a premium on the need for vacuum. Increasing idle air has a dampening effect on carb operation and seems to drastically move the low end operational parameters in the absence of a restrictive airbox or Uni prefilter. We see a lot more midrange lean issues and tall needle positions to compensate as a result. Not to say more IAJ is totally wrong... There is a certain black magic to getting the balance of air volume to vacuum to fuel correct. A lot of it overlaps and in certain circumstances can be made to work, in the right setup. My experiments showed me though, that it makes far too wide a difference, totally resetting the baseline to which all the rest of the carb is adjusted and shortening the range of idle circuit operation. For my purposes, slight IAJ swap like going from 50 to 52 or 55 is way plenty, more than enough and even then not at all necessary.

Conversely, lowering the float increases air volume making more air available to get sucked up through either circuit without much change in vacuum or complete baseline changes. In other words, this results in leaner low end operation without as much bleed over effect to the main circuit and the speed and rpm at which the needle goes up.

Well after several thousand miles over the summer i gotta say this is the best mod i have done to the bike second to suspension rework. I have decided i am going to give the 50 IAJ's a try again and drop my needle and see where i land. Current setup is 99% what i want, but when stone cold the bike can be a bit of a PITA to start even with enricher's. Have a ever so barely noticeable surge when cruising, i mean i can barley feel it.

I bet based on your EXPERT testing if i drop down to 50 IAJ's and change needles to slot 4 (possibly 3) instead of slot 5 i will fix those two issues.

Current Setup (Aggressive Street Sport Riding):
FMF cans
Part Factory Pro kit and Part H2W kit for my jetting
Main Jets 170R / 168F
Pilot Jet #45
Needles Factory Pro Ti needle 5 position from top
IMS screws 2.5 turns (Flex-Jet installed for easy adjustment)
Air Jet #70 as supplied by H2W to replace stock #50
Float Height 3.5MM
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