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Discussion Starter #2

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Most of the thumb screws on the market are for the Keihin carbs on CRF's and TRX450's (or anything with a Keihin FCR carb) - the 400EX uses a Mikuni carb which is not as sensitive to atmosphere and fuel changes as the FCR. You are lucky to have found one for the Mikuni. Be advised - I looked at all the links and all depict the screw assembled wrong - the spring goes on first, then the washer and last, the o-ring - be sure to assemble it correctly when you install it. I keep the FCR screws in stock, but I had never seen one for a Mikuni until you posted it.
 

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Discussion Starter #4 (Edited)
Most of the thumb screws on the market are for the Keihin carbs on CRF's and TRX450's (or anything with a Keihin FCR carb) - the 400EX uses a Mikuni carb which is not as sensitive to atmosphere and fuel changes as the FCR. You are lucky to have found one for the Mikuni. Be advised - I looked at all the links and all depict the screw assembled wrong - the spring goes on first, then the washer and last, the o-ring - be sure to assemble it correctly when you install it. I keep the FCR screws in stock, but I had never seen one for a Mikuni until you posted it.
Thanks for the reply.

For some odd reason, I thought the Keihin carb was the factory carb??? I know that you KNOW what is factory, I guess I was just very confused... (Mine is a 2014 400X, maybe they used Keihins in those?)

And, thanks for pointing the wrong assembly. I, too, noticed that when searching all over the place.

The 400 I just bought has a cheap aftermarket carb on it. I lost the airscrew (vibrated out, I guess). I was seeking a new screw, but wanted an "easy" one.

My neighbor builds and races Hondas, and he's supposed to give or sell me a factory 400 carb tonight. I'll see what he comes with before I order anything.

Appreciate the help!
 

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Carb part number 16100-HN1-A41 was superseded to 16100-HN1-A43 and that carb was used 2005 to 2014

Both 16100-HN1-A41 and A43 are listed as carb number QB11A A and they are both Mikuni

There is a performance kit available that has a carb holder that will accept a Keihin FCR carb, but there is no advantage unless you are flat track racing - the FCR carb has a larger venturi whill enhance top end but at the cost of low end. Typically they are only used if the cylinder is bored out to increase low end (larger bore = more low end | bigger carb and high lift cam = more top end) it's a matter of what do you want and need. You can only go as fast as you can afford.
 

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Discussion Starter #6
Carb part number 16100-HN1-A41 was superseded to 16100-HN1-A43 and that carb was used 2005 to 2014

Both 16100-HN1-A41 and A43 are listed as carb number QB11A A and they are both Mikuni

There is a performance kit available that has a carb holder that will accept a Keihin FCR carb, but there is no advantage unless you are flat track racing - the FCR carb has a larger venturi whill enhance top end but at the cost of low end. Typically they are only used if the cylinder is bored out to increase low end (larger bore = more low end | bigger carb and high lift cam = more top end) it's a matter of what do you want and need. You can only go as fast as you can afford.
Awesome! Thanks a bunch! No racing... Just a 40 year-old man out having fun with his 11 year-old (and sometimes recreation parks with friends).

Again, I appreciate your knowledge and dedication you give to ignorant folks.
 
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