Jump to content
Sign in to follow this  

KTM flame thrower

Recommended Posts

2003 KTM450EXC

42 Pilot

172 Main

JD Blue needle 3rd clip

Fuel Screw 1 turn out

sea level to 1000ft

BK Mod, timing perfect, 1 sec spray. Using an oring to hold it all together.

Firstly my bike runs great. No off idle bog, pulls strong from bottom to top.

But it pops on decel when riding normally and to the extent that when its revved sitting in the garage it backfires large blue flames on decel!

Also when the bike is warm and idleing and i adjust the fuel screw its peak revs are at 1/4-1/2 a turn out. If i wind it out much more it idles rough/erratic, its sweet spot is definately nearly all the way in. In fact i can virtually seat the fuel screw.

This is telling me i need to go down a size on the pilot, but a 42 'should' be ok. I havent tried a smaller pilot. (I dont have a smaller pilot atm!)

I have tried the Red needle in the 5th clip. My header is sealed correctly using hi temp RTV. The carby has been cleaned several times and is spotless.

Air filter is brand new. The boots are tight and sealed correctly. The oring on the fuel screw is brand new and the spring, washer, oring are in the correct order. I have spent hours messing with the fuel pumps timing, duration etc and im happy that area is ok. (it is pretty well setup as per jd instructions). The valves are in spec.

Any suggestions?

Id really like to get rid of this decel popping and id like the fuel screw to drop the revs when its wound in and peak at about 1 1/2 turns like it should with this setup.

Thanks for any help, this is my first post here so be nice :thumbsup:

Share this post


Link to post
Share on other sites

So to cure it do i need to go down to a 40 pilot? Im wondering if my 42 pilot is still a 42....perhaps somebody has opened it up. Anything smaller than a 42 on this bike seems very odd from what ive researched.

Id like to think the exhuast is sealed properly. Ive sealed it several times over the last year and its always backfired, surely i havent got it wrong everytime. If its sucking air could it be from the rivets holding the end cap on? Thats the only place i can think it might be leaking!

Share this post


Link to post
Share on other sites

I am currently having the exact same problem with 06 525 exc with JD jetting. Have changed needle to red in 5th position. Still popping on decel. I have 42 pilot as instructed by JD. Fuel screw all the way in or out does not seem to make a difference. I have not checked for air leaks yet. Planning on red RTV at joints and new gasket at head exaust flange. Also have 40 and 45 pilot jets on order. Also saw pictures at JD's websight showing pipes at flange at head not being parallel. That might worth checking. I am running out of ideas. Bike runs great except for decel popping and a little surging at about 10 MPH. Please let me know if you get anywhere.

Share this post


Link to post
Share on other sites
I am currently having the exact same problem with 06 525 exc with JD jetting. Have changed needle to red in 5th position. Still popping on decel. I have 42 pilot as instructed by JD. Fuel screw all the way in or out does not seem to make a difference. I have not checked for air leaks yet. Planning on red RTV at joints and new gasket at head exaust flange. Also have 40 and 45 pilot jets on order. Also saw pictures at JD's websight showing pipes at flange at head not being parallel. That might worth checking. I am running out of ideas. Bike runs great except for decel popping and a little surging at about 10 MPH. Please let me know if you get anywhere.

Hi Dave

Ive all but fixed my backfiring, it had nothing to do with the header seal. (my header and complete system has been sealed with rtv for a long time and it has always backfired)

My exhaust valves were slightly loose. I tightened them to .005 and the backfiring is gone. Ive got no idea why this has anything to do with the back firing but i can asure you it has. To confirm what i learnt, i went back and loosened them again (just so it was a loose feel on the feeler gauge rather than the usual snug feel) and it back fired again.

So my exhaust valves are very sensitive to this. I now use the 1/6 turn method and slide a feeler in there afterwards to confirm it is nice firm feel on the gauge.

However ive never been able to make my fuel screw act correctly. I havent tried going to a 40 pilot and i doubt i ever will. (everyone else seems to use a 42 pilot as per jd instructions)

Let me know if you learn anything.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

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

Reply with:

Sign in to follow this  

×