Hey, I'm new to the forums and I currently own a 2007 400ex and the only performance add on I have is a full Yoshimura RS-3 exhaust. What jet should I run and how can I tell if I need a jetting change? Also, what kind of cam should I do if I mainly do trail riding? What are the best price to power increasing add ons out there? Thanks in advance!

 

Hi Chrone247, welcome! The best starting place for jetting would be to contact Yosh directly. That being said, if the exhaust is your only upgrade, usually one or two sizes up on the main and one up on the pilot will get you close. Pretty sure the December edition of atv/utv action actually had a great jetting write up in the "Trouble Shooter" section.

You check jetting generally by doing plug chops, or more specifically by renting dyno time where they use an exhaust sniffer to determine air fuel ratios.

As for cam, you need to ride the machine a bit and determine where you need the power. This is another good question for Yosh when you call about that jetting as well. Additionally, if you decide to go with a high comp piston and/or big bore kit I would ask that builder/manufacturer what they recommend. It is best to go with one quality builder for this, as sourcing parts here and there is a recipe for disaster. Matching a high end pipe with a low range cam for instance may not leave you happy with performance in either area.

Since you already have a pipe, the next logical step is a better flowing intake and probably another rejet. Realistically, the 400EX is a great trail quad stock. An intake and exhaust are as much as most trail machines ever get.