Blue smoke on start up, popping on deceleration?

Hi, I'm abit of a novice when it comes to 4 strokes, this is my first and don't really have a clue when it comes to the valves ect.. I have a 05 kx250f, only had it 2 months, it came with a fmf powercore 4 full exhaust system. Everything else is standard.

First of all, On start up it burns blue smoke, once warmed up it goes away.. I'm guessing these are leaky value seals? Question, how bad is this? Doesn't seem to be losing any oil?

Secondly, it's popping on deceleration, the guy who fitted the fmf exhaust never done any jetting, so I upped the pilot jet from a 40 to a 42, played with the fuel screw but no noticeable difference, do I need to change the main jet?.. Also, since upping the pilot jet it's really boggy on start up, whenever I try to rev it it bogs a little then picks up.. Once I give it a few high revs it stops it and doesn't happen again throughout the day? should I go back to 40 pilot and up on main jet? Or is it just dirt in the carb? (there's no exhaust leak and I have a new air filter in it)

And last but not least.. I can not get a steady idle out of it, it's like it hangs for a few seconds then drops off? When I first start the bike and I adjust the idle screw and get it idling nicely, that settings becomes way to high after a 20 min session, or even during the session, today I had to reduce the idle screw in the middle of a session because I felt like the bike was pushing me round corners on its own? Almost like the throttle was on a tiny amount, but it was fine again once I turned the idle screw out a little, but then it wouldn't idle? And sometimes it's really hard to hot start, but I'm not sure if that's just me because I'm not the best at kicking 4 strokes! Lol

Apart from these little things the bikes running nice, all seems nice, kicks first time every time on a cold start!

Any help or advice on any of my little problems would be greatly appreciated! Thank you.. Sorry for the lengthy post!

Probably still lean on the Pilot. Try a 45. With the bog when you snap the throttle is part of the AP circuit. Check the AP timing and change the leak jet in the bowl to a 50 or 40. On that jet a 40 is richer than a 50.

The blue smoke could be worn rings or stem seals. The valve springs will need to be removed to replace the seals. Its probably a good idea to just plan on a piston kit and stem seals. The top end gasket kit may even come with new seals, not sure though you would have to check.

Edited by Joshx221

Thanks for the advice, just got a 45 pilot so going to try that over the weekend! I think I'm gona get through this summer then send my engine off for a full top end rebuild over the winter.

Idle is probably the fuel/air mixture screw might be missing the oring that belongs in there - does it have an after market fuel screw? Without the oring it hunts idle really bad and may be causing the pop on decel - remove screw and it should have the oring, small washer and spring on the screw - if u pull it out and no oring inspect inside the passage and make sure it's not stuck in there

Also pop on decel could be an exhaust system leak, either at the cylinder/header pipe crush gasket or the gasket at mid pipe

Josh covered the rest I think

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