Hello everyone. 2002 400ex, HMF slip on, UNI filter, (3) 2" holes drilled in the air box lid. When i bought the wheeler a few weeks ago, the kid told me it had a high compression piston and a cam as well but, I've owned several EX's with cams and high compression pistons and this one isn't nearly as strong so I'm 99% sure its internally stock. Here's my issue, when I got it, it was a pain to start and ran super lean. I tore the carb apart to find that it had stock jetting (148/38). Currently I'm at 155 main, 42 pilot, needle on the 3rd clip and fuel screw 3-1/4 turns out. It starts great and runs pretty good, the plug chop showed that I'm pretty close. The problem is that, if the quad is idling and I snap the throttle open, it just bogs, backfires and tries to stall. If i roll on the throttle, it's fine. Should I go down a size on my main or go up or what? I have a 152, 158, 160, 162, 165, and a few more larger ones up to 180. I also have a 40 and 45 pilot i can use as well. The gas is fresh, oil and filter are fresh, air filter is brand new and oiled properly, and the carb is spotless. I am at roughly 1,200 feet above sea level as well. My initial thought was to go to a 160 main and turn the fuel screw back in some but, I wanted a few opinions first.

 

^^^^Very good description. Question, does that particular Keihin have an accel pump? I'm thinking it does. If it does, have you verified that it is functioning correctly...pump not torn, no holes, and seated correctly in the housing...and actually provides a "squirt" of fuel?
- I would think the 42 pilot should be about right but 3.5 turns out sounds
a bit much, but leave it for now.
- The 155 main may be about right but not because of the slip on, more so
because of the holes in the air box. Many slip on's can run fine with stock
jetting.
- I think you may also want to go richer on the needle....try the 4th clip, if it
improves, then try the 5th position.
Have you tried applying a little choke.....I mean just a little and see if it
decreases the "bog".

 

It does have an accel pump and it seems to be in perfect working condition. Applying the choke, to any degree, doesn't make much of a difference, unless it's full choke, then the motor dies. I started at 2.5 turns out on the fuel screw and the bog was awful. It got considerably better, the more I turned it out but, I was told to never go past 3.5 turns.

 

Going past 3.5 turns doesn't hurt anything....but it's a bit uncommon to open up past 2.5 to 3 turns. Open it another turn....if it runs great...leave it but, it's sounding like to might need to go up another size on the pilot jet.