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Discussion Starter · #1 ·
Hello, can a 450R carb fit on a 400EX? Would it be much of an upgrade? And what all would it take to get it to fit?

I have a '00 400ex, its all stock. It runs prety good (other than cold natured), but I'd like a little more power, I dont want to put a load of money in it though. I may eventually look at the over bore cylinder stuff like the install and go stuff but not for a while. Whats your thoughts on a 450R carb swap? thanks
 

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You can get a 450R carb to fit, but it will not give you more power. It has a larger venturi which will give you more top end rpm, but low end grunt will suffer due to the larger venturi having a lower velocity of air flow at the lower rpms which in turn causes a lower vacuum in the intake making it harder for the air flow to pick up fuel. Typically you will need a faster idle to get it to idle and to not lean out upon throttle closing causing engine stumble or to even die upon reopening the throttle like in a turn.
 

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Discussion Starter · #3 ·
Ok, thanks for the info. "Low end grunt" is what im really looking for so thats out of the picture. Would re-jetting be a better solution? I really dont know a whole lot on this subject. I can usually get something to run good, but when it comes to adding more power Im lost.
 

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Easiest road to performance improvement is to change the exhaust. For more low end power - bore. Re-jetting will not improve power - jetting is done only to achieve proper performance from idle thru top end. Re-jet after making other changes to restore proper throttle response.
 

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Discussion Starter · #5 ·
I give the fuel mixture screw about a quarter turn and it helped throttle response and give it a little power at low rpms just above idle. So far I cant tell that it smokes any worse, and seems to not get as hot when riding in the woods(1st and 2nd gear). Don't seem to have effected fuel consumption much either. Also starts a little easier without the choke. Im guessing these things run a little lean at factory settings:confused:. I figured that was mostly a problem on on-road approved bikes.

I'll be looking at slip on pipes eventually, for now I have the factory baffle trimmed inside and the screen reinstalled. It deffinatly helped some. Looks like I have a few more problems to address before I get into performance mods. (swing arm pivot bearings and pin, front a-arm bushings, lower steering shaft bearing, rear brake rotor/pads, caliper mount, rear wheel hubs, and chain/sprockets) More power will happen soon though:cool:

Are there any other carb mods to help power using a factory carb?
 

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You do not typically get any power gains with just changes to the carb. The modification you made to the exhaust may have necessitated the tweek on the carb, but in Japan, 93 octane fuel is 93 octane, not 89 octane boosted to test 93 by adding alcohol. Therefore, most inported engines benefit from a tweek on the fuel screw. My 04 250 is completely stock (except for the UNI air filter) and it runs spot on. I'm not changing anything. If I decide I need more power, then I'm buying a bigger machine.
 

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Discussion Starter · #7 ·
You have a good point ;). I just want more hp especially down low, while keeping it stock (relaible) as possible. I'v ran the exhaust how it is for a couple years. Removing the baffle helped a fair amount but made the quad anoying to ride, I trimmed the reduced pipe out of the baffle with the larger dia. piece inside still. Helped some on power and sounds decent (not too loud).
 

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Discussion Starter · #9 ·
I looked at a 400ex that a guy had swapped a xr600r engine into. Maybe thats what I should do:D
 
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