View Full Version : Help Jetting a 400 EX?

08-01-2001, 10:01 PM
Can someone please explain to me how/where to change my jet?
I just bought a FMF Powercore IV S/A and want to change the stock 148 jet to a 152 jet. Is there an easy way to do this? I heard that theres 4 phillip screws on the bottom of the carb. that you take off, and then there's a brass nut that you unscrew where you can change out the jet. Is this right? I could only find 3 screws under the carb? Any help would be great! Thanks

08-01-2001, 10:09 PM
Have you done any airbox/air filter work? I would recommend drilling some 1" holes in your airbox lid to make the most of your pipe.

As far as jetting goes....there is a 17mm plug you will see on the bottom of the float bowl, take that out, and get a 6mm socket and take the main jet out. It takes about 5 minutes to change the main jet. BTW it's a good idea to drain the fuel out of the float bowl first... Good luck.

08-01-2001, 10:16 PM
i attached a pic here, i no it isnt mine but i am just using it to help someone out, i put arrows right buy the bolts that need to be taken out. the arrows are pointing to 2 phillips head screws and on the other side of the carb there are too more, take those 4 screws out and take the float bowl off, then right in front of you is the main jet. obviously you no what a main jet looks like because you have the new one in your hand. un screw the old one and screw the new one in SNUG not real tight. if you torque on the jet it will turn another brass screw inside and mix up the mixture. hope this helps.

08-01-2001, 10:22 PM
well theres 2 ways to change the main jet, i listed one way and "wheelie" listed the other. i am assuming you are changing your pilot jet too? because if you are changing your pilot jet you will need to follow the steps i listed, because you can not access the pilot jet from the 17mm plug. the piulot jet is right next to the main jet, the pilot sits in a hole and needs to be taken out with a flat head screwdriver.

08-01-2001, 10:25 PM
I changed mine twice (once from a stock 152, and again from a 152 to a 155) without removing the float bowl or carb.

Sorry, I didn't see the most recent post until I wrote this one, I didn't have to change the pilot......

08-01-2001, 10:39 PM
Thanks 400exRacerX and Wheelie for the info. I did put 4-1 inch
holes in my air box lid with a Uni air filter. I will try and install the jet in the morning!

08-02-2001, 05:53 AM
Originally posted by 400exRacerX
un screw the old one and screw the new one in SNUG not real tight. if you torque on the jet it will turn another brass screw inside and mix up the mixture. hope this helps.

What happens if that "other brass screw" did turn?
I was changing the main jet the other day, and the "other brass screw" came out with the jet.

I put it back in by tightening it slowly untill it bottomed out (I think it bottoms out against the needle seat). At that point, I tightened the lock nut and installed the main jet.

It rides fine, but I have been wondering if something could be messed up. The Honda shop manual has nothing about this "screw".

08-02-2001, 02:52 PM
i've wondered the same thing about the needle jet?it has threads and a locknut oyu would think that you can really fine tune by using the threads and locknut,as far as position with the needle and this needle jet.

08-02-2001, 03:13 PM
What did you guys use to turn the 4 screws with? I can
hardly get my hands in there yet a philips screwdriver!

08-02-2001, 06:33 PM

It's nearly impossible... if not impossible to take the bowl off with the carb on the bike. Even if you were able to do it, you'd find you have to remove the carb to get it back together correctly. There is a small plastic anti-slosh ring in there that is held on only by the bowl.

If you are only changing the main jet, just remove the 17mm nut on the bottom center of the bowl and use a 6mm socket to reach into that opening to remove the main jet.

If you are changing the main jet and pilot jet you'll want to remove the carb, or at a minimum loosen it from the intake manifold and the intake tube and get it turned sidways while on the bike. You'll have a much easier time of getting the bowl back on with the anti-slosh ring in place.

Hope this helps!

08-02-2001, 09:14 PM
OK great guys got the main jet switched out to a 152 and shes running great. Thanks for the help everyone. This is a great web sight! :D