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So I've heard you should rejet for most performance upgrades. Well I have a older hmf pipe. Just ordered a new one so shouldn't have to rejet for that. But I've looked into getting the 250r cdi on my 400ex. Will it make a difference. And if so would I need to rejet? Also is the 450r carb swap worth it? Money and time to put on.
 

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Always remember TRX450R Carb's are Different from 2006 and up they changed to the FCR Carbs. But the 04-05 TRX450R carb is a Big Big upgrade for the 400EX. You can Make an FCR Carb Work on the 400EX but I would Recommend something like a 37mm FCR from like a YZ250F or a CRF250R. I actually have a 37.5mm YZ250F Carb for Sale 160.00 I need to get ride of I asked another guy if he wanted it too but Who ever get back to me first can buy it.

Now for the 250 CDI I would just get you the Big Gun Rev Box they raise the Rev Limit and Ignition Timing Curve.
 

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Jetting is an air velocity issue. If you make a modification that changes the velocity of air passing through the venturi of the carb, then you have to rejet

Reducing exhaust restriction do not affect intake velocity much, but reducing intake restrictions affect air velocity greatly. Especially in the higher RPM's. Boring and stroking an engine has little effect on jetting depending on the amount of increase in displacement and the type of carb.

Changing the ignition usually has no effect on jetting, but the jetting has to be changed as required. So, if you change the ignition and it needs rejetted, then rejet it!
 
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