Honda TRX ATV Forum banner

1 - 7 of 7 Posts

·
Registered
Joined
·
9 Posts
Discussion Starter #1 (Edited)
I have a bone stock 2002 400ex and today I got a dg muffler for it and put it on, the air box is 100 percent stock, still has the lid, and I screwed the fuel screw out a half of a turn, but to I need to rejet? And if so would a 150 or 152 or 155 main jet be about right since my airbox is stock? I'm around 200ft elevation. Thanks
 

·
Super Moderator
Joined
·
3,824 Posts
You may not need to rejet at all and the fuel screw only affects idle - ride it and if you determine it need to be fatter on the top end when wound flat out, put in a 150 or a 152 main jet - the only way to know is to ride it and see how the jetting affects it. Just changing the exhaust does not affect jetting a great deal - it's changes to the intake that affects jetting in a huge way.
 

·
Registered
Joined
·
9 Posts
Discussion Starter #5
Well it seems like half of the people on other forums say it's totally fine, and the other half say it's running way to lean and its gonna cook the motor, I just wanted some clear clarification. One thing I dont understand though about carbs and jetting is if the main jet controls fuel when wfo and the pilot controls fuel at idle, and the pilot screw is for low rpm, then what adjust the air and fuel mix for all the rpms in between?
 

·
Super Moderator
Joined
·
3,824 Posts
The slide and butterfly valve controls air flow and the slide needle and slide needle jet control the fuel from about 1/8 to 3/4 throttle - all the circuits overlap, but above 1/8 throttle you could cut the fuel to the idle circuit and the engine would never know - likewise, over 3/4 throttle you could remove the slide needle and the engine would run just fine.

There are numerous designs of carbs and they have been refined continuously for about 100 years - while the function is set, the operation has been tweaked in various ways. Then there is fuel injection - FI is just an electro-mechanical method of supplying fuel to air and being controlled electronically, is more precise than any design of carburetor. A carburetor has 3 to 5 stages of control from idle to WOT while EFI can change the fuel to air ratio more than 100 times per second and can be programmed to compensate for incoming air temperature, humidity, altitude, fuel quality and degree of throttle opening. It is not instant, but is so quick that it seems instant.
 
1 - 7 of 7 Posts
Top