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Main Jet/Main Air Jet ratio?


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Not really.

Sizing of the air jets is done to get the corresoponding other jet in a range, hene most bikes using a 45 pilot. But the air jets operate in an differenet 'order'. For example, the slow air jet has a greater effect as the throttle is opened whereas the pilot jet has a diminishing effect. Notice that for the main jet and main air jet, the effect of both increases as the throttle is moved wider.

FCRTuningguide.jpg

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Thanks I will try that .I've read main air jet on average should be up to about 10 percent either side of main jet size depending on how hard the bike is ridden .Is there any value in that?

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Ok so it came restricted from yamaha with no maj which makes it a 200 and the yamaha deristricting or performance kit supplied a 110 with a 50 pilot and 175 main , now that I'm using a 45 pilot and a 170 main which maj should I be using .Which is standard the 110 or the 200.?

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maybe this will help explain why they did this:

If you find that the engine runs perfectly at WOT near the torque peak, but becomes lean toward redline, select smaller main air jets. Conversely, if the engine runs perfectly near the torque peak but richens toward redline, select larger main air jets. Changes in air jets may require changing the main fuel jet. Say you have good performance at engine speeds leading to the torque peak under wide open throttle with a 150 main fuel jet and 200 main air jet. Should you need to richen the mixture at full revs and full throttle and select a 180 main air jet, you may need to go down to a 145 or 140 main fuel jet to keep the lower rpm mixture the same as with the 150/200 combination. Generally (there are always exceptions), once the main air jets are properly selected for the intended application, they need not be changed again unless you change the intake restriction (modify the airbox or use different filter) or change engine or exhaust specification. Varying atmospheric conditions can usually be dealt with by fuel jet, fuel screw, and needle changes.

The part that really sucks about FCR carburetors is that they come with primary type emulsion tubes as opposed to the bleed type emulsion tubes every other four stroke carb in the whole universe ever used. What this means, essentially, is that the above paragraph doesn't actually have any connection to reality. It has been my experience that if you for whatever reason have something like a 200 main air jet in the carbs, and the mixture goes lean at high rpm, and you manage to get the mixture sorted out by putting in different (smaller) main air jets and main fuel jets, then the carbon monoxide numbers might be really pretty and even from mid rpm through redline, BUT THE ENGINE ISN'T MAKING AS MUCH POWER AS IT DID WITH THE BIG MAIN AIR JETS. I think this has something to do with a bigger air jet allowing the fuel coming up through the needle jet to get emulsified more before going up into the venturi, therefore allowing better atomization, and more complete burning of the fuel supplied into the cylinder, since lots of little bitty fuel droplets have more surface area per unit of volume and burn more completely, as opposed to a few big fuel droplets having less surface area per unit volume and not only burning less completely, but because they burn less completely you need to have more of them to burn the oxygen available, which means the CO and hydrocarbon numbers go up because lots of unburned fuel is hanging around throughout the combustion process, soaking up heat and getting pumped out the exhaust port. Or something.

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Thanks for the help ,it sure can be a challenge fine tuning.Its running good and clean now but lacks power, i would like to know what a yzf 450 standard main air jet is,and pilot air jet too for reference,(i think the ACV on the WR might make it way different) anyone?

Maybe that can give me a better starting point.they dont come restricted.

Edit: did some extra searching yzf has a 200 maj and 100 paj compared to the wrf's 70 paj.

Edited by Racegun
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