Carb jetting - Yamaha ATV Forum
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post #1 of 2 Old 06-01-2019, 05:20 PM Thread Starter
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Join Date: Jun 2019
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Carb jetting

I have a 2013 Yamaha yfz 450 which I put a 498 big bore kit on and ported and poslished head from BP Raceing and stage 2 camshafts. I put a dynojet jetting kit and put a 160 jet in it, which the fuel mixture screw at just about 3 turns. The bike starts and runs but doesn’t have the same power as before it backfires after decelerating.
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post #2 of 2 Old 06-03-2019, 09:53 AM
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Location: southern oregon
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what is your elevation. air box? pipe? backfire on deceleration is generally an indication of lean. if you dont have any other mods try 165 and 2.5 turns out. 3 is to far, if you need do be that far out you need to lift the needle or go a bigger main. i have a 502,port and polished head, no air box lid, and a slip on and i run 170 22.5 2.25 turns out.
there is no magic number for jetting all machine are different and you will have to rejet, run it while and check the plug and adjust. use a new plug each time so you can get it right. you can save them and use them later but you need a fresh plug for a proper reading

jetting a big bore is a little different and there is not much use full information on the web so will give you bigbore jetting 101 the general consensus on the web is bigger bore bigger jets and that is very wrong. changing your jetting adjusts the ratio of air molecules to fuel molecules not the volume of air fuel mixture entering the cylinder. that is determined by the air box, carb size, intake and cam duration. the main goal of jetting is to get the fuel air mixture perfect for combustion. too rich or too lean it wont burn correctly. the variable for adjusting the fuel air mixture are atmospheric pressure (elevation), temperature and speed of the air moving through the venturi of the carb, this is where most people make their mistake when jetting a big bore. again the common belief big bore big jets. in reality you will almost always need to drop a main jet with a big bore kit. if nothing else changes when you add the big bore kit the larger bore diameter will create a greater vacuum and move air through the venturi and over the main jet faster and thus pull more fuel through the same size jet causing you mixture be rich.

a couple other thing to remember when using the dynojet kit be sure to use it all and not mix and match other jets you may have. they use a propitiatory system and it wont work with factory style jets. their kits are ok but for optimal performance sigma or jd jets is a better option they use factory style jets and custom needles. do your self a favor and order the r&d flex jet for your air mix screw it will make tuning so much easier and you can tweek it through out the day as temps change. you can use the r&d with the dynojet kit. let me know if you need anything else.
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