2008 kx450f sputters when tapped out

I bought a '08 kx450f. I replaced the exhaust to fmf full set up and jetted it to #175. It was already a #175 before i guess. But now when imon the track and tapping it out in any gear it sputters and falls on its face. Obviously not the rev limiter. Its only while i am on the throttle not when I let off for a turn. Does it need a different leak jet or does it need a #180 main jet? Thank you. Also it seems the bike runs a little warmer

come on somebody has to know...

Hi Danny

It's not the leak jet...for wide open throttle it has to be main jet or something electrical..I would start eith going up or down on main jet(probably down first..too lean would pop not sputter)

Hope this helps


Check to make surd your petcock filter is all clear I had a kx 80 that got clogged once cleaned it ran great

I also put a JD jet kit in my 08 and it seemed to run better and cooler

Sputtering is rich or a spark mis fire.

Lean is a surging bogging type of thing.


How's your air filter?  New spark plug?


If only the exhaust changed, and it's sputtering when tapped out, then I'd go down on the main to a smaller 172.  Perhaps your FMF system is more restrictive than the stock 08 (super loud) system. Wouldn't surprise me.  Which would also explain why the bike seems to run a little hotter.


Main jet swap in the 06-08 Kawi is a 5 min job with the right tools. Gotta love that.


Constant wide open only uses the main jet. Well, pilot circuit is working but not relevant.

Edited by numroe

Once it seems to run smooth, if you want your 08 to haul real hard at any RPMs and minor throttle opening, then try the 06 CDI with some 12 cams (same as 2010 and similar to 06 and 07). You'll probably lap slower, but hang on!

Edited by numroe

thanks gonzokx. thanks mx5112002 it runs warmer with the jet kit. I am taking it to the track on saturday. I will bring my jet kit. Thank you numroe, Yes the air filter is clean. I need to put a new plug in it. I totally am putting the 06 CDI on it too lol and the 12 cams. The FMF is pretty loud. I had a P.O.S. Drd pipe on it before and it cracked from heat -_- and there was no packing left in it also baffle snapped and was rattling around. Litteraly sounded like I taped a pop can to the header haha. Yes the jetting is easier then changing the oil ha. I will let you know the outcome this weekend.



Check to make surd your petcock filter is all clear I had a kx 80 that got clogged once cleaned it ran great

I don't use a filter haha. I haven't even noticed one on any other bikes around here.

I don't use a filter haha. I haven't even noticed one on any other bikes around here.

Its in the tank they come on it stock you have to pull the petcock off the tank its two 8mm bolts

Any time you put a high flow exhaust and or filter you gotta re-jet the carb. If you let the bike get more air it must then need more fuel. I'd go bigger on the main and maybe even a clip position but start with the main jet

Edited by JacobKoehn

ok i did say i already re jetted it to 175 but now i switched it to 180 and I will see tomorrow. going up one notch onthe needle made it worse and hotter. and going down one notch made it worse aswell so I put it back. I will know tomorrow for sure.

Good you made sure, but when wide open the needle is doing almost nothing.

Whether it was rich or lean on the main depends a lot on your perception of sputtering.

Make sure you take a 172 with you tomorrow too. 

You certain your new FMF muffler doesn't have a quiet tip or spark arrestor installed?

Also check the coil ground wire isn't loose.

turns out the clutch was sliping causing the bike to over heat and run aweful. I got a new top end and clutch put on and fine tuned the carb. runs ridiulously nice now. my first moto is on sunday and i am super stoked.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now