So I tried to remove the main from the carb on my 400ex today to see what size jet it currently has. I removed the 17mm cap on the bottom of the carb and then used a 6mm socket to try to remove the main jet. The socket would turn and turn and turn but the jet never came out! It would be easy to turn for about 1/2 a rotation and then get a little harder to turn then back to easy. I'm thinking the main jet is stripped out, any suggestions on how to remove it, and do you think the threads on the jet itself are stripped or the threads on the carb??

 

Ok so it turns out the jet is NOT stripped. I removed the carb and then I was able to get the main jet out. I'm guessing it has an aftermarket jet in it because it does not take the 6mm socket to remove it like everyone says it should. The jet itself has a round head with a flat tip screwdriver slot, and it screws into a brass base that has a hex head but it does not seem to be 6mm either. When I removed the jet with a flat screwdriver I could not find any numbers indicating what size jet it was...

 

glad you got it out!

 

Are you rejetting or just having a look see?
Got a pic?

 

Well I was going to rejet it, because I just bought this 400ex about a week ago from Phoenix, AZ, which is 2,000 feet lower in elevation than where I am... I wasn't sure if the previous owners had it properly jetted for they're altitude and mods it has done (Pro Circuit T4 Slip On and a K&N filter), so I was trying to remove the main jet to see what size it had in it already. I'm still not sure if it is the stock Keihln style jet or a Dyno Jet style or something else.

I don't have pictures, but the main jet looked exactly like the round one on the left side in this picture:


At first I could not find any identifying numbers on the jet when I removed it, so I just took the whole carb up to my local cycle shop to see if they could help with the jets. They found the numbers on the jet, it was just REALLLLY small and I didn't know that lol.

So the main jet that it already had in it was a 155, the previous owners must have had it jetted because these don't come stock with a 155 jet from what I've read, and after talking to the guy at the cycle shop, he said with the mods this quad has and our elevation at 4,000ft the 155 jet should be just about right on. So once I get a new spark plug I'm going to do some plug reads and see how it looks.

It turned out to be a lot of work, removing all the plastics and tank and carb and stuff, just to end up putting it all back together with the same jet lol. BUT, this is my first quad and I really wanted to learn more about it and get use to tearing it down and putting it back together anyway!

 

Never seen a round one like that.
I would replace with Keihln jets. Never used dyno jets and never heard anything good about them. As i understand the numbers do not correspond with the keihln jets and just bad reviews in general.
getting the jetting right can be a pia and take some time. You will know when you get it right because it will run cool and pull pretty good.

 

After searching around on Ebay for jets, it appears both Keihin and Mikuni make round slotted jets like that. There also seems to be three different types of round slotted jets. There is a Round, Small Round, and Large Round, like in this picture:



The one I pulled out of my carb looked more like the normal Round one. Mine did not have a grove around the head like the Small and Large round jets in that pictures. Or maybe I just didn't notice there was a grove, I dunno. I'll look closer next time I pull it out.