Yamaha blaster - Yamaha ATV Forum
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post #1 of 2 Old 06-20-2018, 08:59 PM Thread Starter
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Join Date: Jun 2018
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Yamaha blaster

Hello im in oh my elevation is about 455 around 80-90 degrees I’m about to bore my blaster .040” get the head ported, boyesen reeds, no lid on air box, has stock exhaust I’m just trying to get some suggestions on what size main and pilot jets should I run I know it’s not going to be exact I’m just trying to get a ball park
Scotty Alexander is offline  
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post #2 of 2 Old 06-25-2018, 10:26 AM
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Join Date: Feb 2015
Location: southern oregon
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start with exactly the same as what you had before and see how it runs. my guess is it will be hard to start and need to go down one size in the pilot and stay the same in the main. either way by staying with you last setting will start you slight rich and give you a safe place to start. i know it is counter intuitive but you will generally need to use smaller jets in a big bore machine. i will try to give you the short version of a very complex math problem. when you are "jetting" you are trying to create the perfect mix/ratio of oxygen molecules to fuel molecules so atmospheric pressure and jet size are you 2 variables, with the jet size being the one you control. now for the tricky part and where cc's have an effect. your carb was designed to flow a specific volume of air or fuel air mixture depending on which side of the jets you are looking at based on the vacuum created by the piston moving down in the cylinder. so to create you perfect mixture you have to balance vacuum, fuel quantity, and air density (there is more oxygen by volume at sea level).

when you add cc's to the system you create more vacuum. the amount of fuel a jet will flow is based on the size of the jet and the pressure differential between atmosphere and the partial vacuum in the carb. as the pressure differential increases so does the amount of fuel the jet flows. this creates a richer than ideal mixture and reduces performance. since the only thing you can control is the jet size you need to use a smaller jet to get you back to that perfect mixture.

i dont know what the standard bore for a blaster is but if that is just a .010 .020 standard over bore you wont need to change a thing and you just wasted 5 minutes learning something you dont need but if it is a big bore kit this will help you a lot. i spent a month jetting a yfz to learn what i gave you in five minutes. let me know if you need anything else.
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