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Discussion Starter · #1 ·
I recently put in a 80mm big bore kit in my 250x. Still running stock cam, fmf megamax II exhaust. I bought a newer model 300ex carb which is 32mm compared to the 29mm stock 250x. i have a 40 slow jet and a 140 main. it runs well, but i have a 400ex carb and i am wondering if it would be a noticeable difference if i put the 400 carb on, properly jetted of course. if it's not going to make a good difference, i'll just save it for my stroker engine i am working on.
 

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Who told you the stock 250X carb was 29mm? the 250x and 300Ex carry the same exact carb. my Honda service manual indicate the carb as 36mm the 29mm is the slide inside of the carb I believe. but if you want performance get a flatside carb. The 400EX carb will make a noticable difference but you will have to stretch the boots over it to get it to fit. How does your quad perform with the 80mm kit? can you tell a difference in power?
 

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Yeah whats the power like.I was gonna do this sometime.Maybe.LOL
 

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Discussion Starter · #4 ·
i have the 87-88 250x honda manual and all of the 300ex manuals. In '04 the manual states that the 300ex went to a bigger 32mm carb, why I don't know. The 80mm big bore kit I have is from JE. Can't notice that much on bottom end, but it is a lot stronger mid to top end and that is with the stock cam. I have been looking and calling powroll, they offer the kit also but you have to get someone to bore and sleeve your cylinder where the JE kit comes ready to install. I am really happy with it.
 

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Discussion Starter · #6 ·
I was wrong about the carbs. I rechecked my manuals and this is what they say. 87-06 250x/300ex = 29mm venturi (bore diameter), 07-09 300ex = 36mm venturi, 99-04 400ex = 38mm venturi. So I guess the carb on my engine is a 29mm. I am going to put the 400 carb on it soon and see what kind of difference it will make.
 

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If your going from a 29mm to a 38mm.Be care full with your intake boot or you can rip it.If you are not sure what carb your running have it mic'd
 

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If your going from a 29mm to a 38mm.Be care full with your intake boot or you can rip it.If you are not sure what carb your running have it mic'd


They will fit but like you said you have to be really careful, I suggest a heatgun to soften up the boots.
 

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Agree^^^^^^^^^!!!
 
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