I bought an 07 YFZ 450 last week.   When I first rode it I noticed a bad bog down low in the RPMs.  If you blip the throttle too fast it falls flat.  This happens if your riding, or sitting in nuetral.  The dealer said this is normal for these bikes and that when I put a pipe and jet kit on, it would be cured.  

So,  I put a full system on,  jet kit and a k&N filter kit.   The jetting is 165 main, 48 pilot, 3rd notch on the needle, and 1/1/2 turns on the mixture screw.  I tried changing the jetting on the pilot and turning the mixture screw and it does not make it go away.  I put a new spark plug in, did a reading and the plug looked good.  It pulls great once the Rs are past that point. 

After a little looking, I see that Boysen sale a Quick Shot Accelerator pump cover thats supposed to help with bogging, they say its a must for KIEN carbs.  I just dont want to spend $ on something thats not going to fix my problem.

If anyone has experience with YFZs, or 38 MM Kiens,  I would greatly appreciate some advice.   Thanks In Advance. 

We haven't had any problems with bogging on YFZ 450s, even though they're carbureted.  The power has been very snappy.

Did you buy this quad brand new from the dealer?  Unfortunately now that you've put all that aftermarket stuff on there, they're probably going to blame your running problems on those parts.

Quick Shot Accelerator pump covers do seem to help.  They sure are expensive for such a little part, but people who try them seem to really like them. 

Sounds like an ATV Rider product review is in order...

Just a quick Q, does that jetting match the specs of the exhaust sytem you installed?

For a YFZ to stumble off the bottom is odd, even stock. Sure it's choked up in stock form and a simply GYT-R can and jets cure that easily.

Could be the carb giving it too much gas. Is it cutting out? That could be the throttle position sensor for either.

Bryan

I HAVE AN 07 SE YFZ I ASKED A LOCAL SHOP AND THE GUY SED IF U LEAVE UR LIGHTS ON AT ALL TIMES IT WILL MAKE THE DEAD SPOT GO AWAY  

it just needs some tuning, i have run into this with the 450. it normally takes a lot of head scratching and frustration before you get it tuned in just right.
the keihin carbs are pretty good.
just a matter of setting the carb just right and making a few modifications.  

i no a couple people with yfz's and they put rev boxes/cdi box in them and it helped alot but if you have a full pipe and u need to rejet it. it somw=e times can mess up the performance he has a DG silencer and he cut it down to 4 in and he just bought a fmf mega bomb header and he has a big gun cdi box and it ran great until he rejetted it