2006 450R misfiring issues

We have had a 2006 CRF450R for about 18 months which we don't use every week as its just for occasional practice track usage.


After initially having a couple of faults to sort out it has been faultless for a few outings.


A few weeks ago before winter set in it began stalling occasionally when going into corners but would start back up ok. My 27yr old son who rides it most noticed it was nice and steady at start-up/tickover and at WOT but would not hold a midrange throttle without misfiring and backfiring through the exhaust pipe.


I tried replacing a few items after reading posts and searching on the usual forums but each time I swopped something out the misfire remained. After a few hours working on it today I am getting to the end of my patience with the bike and would like your thoughts on my next attempt in a few days time when this other stuff turns up by post.


I have sent for a new header to silencer seal and clamp along with exhaust repacking materials after a friend came around and noted straight away that it was excessively noisy considering it sports a STD can. He reckons it might be an air leak in the exhaust somewhere that is causing it to stutter at mid throttle openings, could this be the cause?


So far in order of replacement/testing:-


Valve clearance checked (spot on)
Fresh fuel
Iridium spark plug x twice
Disconnect kill switch
Coil, plug cap/lead etc
Sent stator away for testing, resulted in rewind
ICM and harness swop
TPS tested as per manual (4.9K ohms) so tried it unplugged
Carb stripped (again) new main and pilot jets fitted (and AP squirt checked)
Carb connector boot checked for cracks/leaks


The engine is mechanically spot on and has only done a few hours since piston kit, clutch pack, steel inlets/recut seats, all valves checked and lapped in again (all steels now). Engine has been taken apart again since and everything checked again and all OK.


Have I missed something really obvious?


This episode has knocked my confidence a little as I am usually the 'friends dad who is good at fixing crossers'


All comments welcome and I have a thick skin so weigh in if I'm well off the mark with my efforts so far


Thanks guys



PS. I have 5 Hondas in the garage :D

Clogged main jet, clogged needle circuit, improper float level, in order of likelihood.

Another wear point often missed is the needle jet emulsion tube. Over time, the needle and the tube wear, causing jetting changes at mid throttle.

Yes, but that makes it run rich. Sounds like he's encountering a lean misfire / popping.

This might sound dumb but your hot start is closed right?..

Clogged main jet, clogged needle circuit, improper float level, in order of likelihood.

Main jet was clear but changed for an almost new slightly 'richer' one, needle circuit blown through with airline (8 bar), now as for the float level ............ I don't have a gauge but with the carb held on its side and allowing the float and needle valve to gently seat the taper onto the brass conical seat the height to the highest (lowest when upright) part of the floats pyramid shaped bottom seemed OK at around 8mm BUT I noticed it was VERY EASY for the float to compress the spring loaded pin of the needle valve. I realise this is a lightweight action on a purpose but just tilting the carb very slightly would allow the float to compress this spring and allow the float to almost disappear into the upper part of the float chamber. This may in fact be perfectly normal but it just seemed really lightweight. The bike doesn't flood or drip from the overflow unless it is leant over at quite an angle so I assumed this was something normal that I just hadn't noticed before.


Another wear point often missed is the needle jet emulsion tube. Over time, the needle and the tube wear, causing jetting changes at mid throttle.

Needle was removed and has no visible signs of wear, emulsion tube was removed and no wear or 'ovaling' was noted


This might sound dumb but your hot start is closed right?..

Hot start has almost new cable and adjustment checked, plunger lightly greased and with a completely free return action to 'closed'




I can't do any more testing until weekend as the exhaust has been removed and opened up for re-packing when the materials arrive and then fitted with new mid-pipe seal and clamp


Thanks for the help so far, it is much appreciated



Raise the needle one spot.

I'd do a leak down test while the pipe is off. Just because the valves are adjusted doesn't mean they are sealing. The exhaust won't make any difference. Also check can timing. Good luck.

I'm going home tomorrow and my son has re-packed the silencer and installed the new mid-pipe seal and clamp ready for refitting to the bike.


I will try the re-sealed exhaust firstly


then I will try lifting the needle (it ran fine before this misfire arrived though so I don't expect this to work TBH)


Cam timing is spot on and all 4 x s/steel valves were lapped in only a handful of hours ago during an inspection



I think I am going to try and borrow a 'known good' carb just to eliminate having an internal problem that I can't test/see, would a 450x carb work ok?


Thanks again


Ian :uk:

Try swaping out ignition module. This alters ignition timing. Check ground and positive lead on coil. Replace resister in spark plug cap with copper or stainless rod.


Don't give up!

Make sure solder joint on stator is'nt rubbing on inside of the flywheel causing a ground. (Common problem with bigger flywheels. Have to file solder down.)

After reading this again, I think its an electrical issue. But having swapped the harness, stator (rewind) and the ECM already, that only leaves the coil.


I wonder if the plug cap is bad?

When you're getting your butt kicked it is always something you've already eliminated or replaced.

Check the clearance of the flywheel and the solder joints on the stator. This will cause intermittent problem. If the crank bearings arethe least bit worn the flywheel can ground out on the stator. You will see rub marks on the stator solder joints.

The exhaust has made no difference, lost interest for now ...............

check the pickup.  there are 2 points there and when you have metal shavings in the oil they will magnetize to the points.  eventually they will touch causing backfiring about half throttle. 


pickup is next to the stator. 

check the pickup.  there are 2 points there and when you have metal shavings in the oil they will magnetize to the points.  eventually they will touch causing backfiring about half throttle. 


pickup is next to the stator. 

Hi, there are no shavings at all in the oil or even inside the casings etc


When the stator was removed I inspected it very thoroughly and the oil was spotless as were the flywheel and pickup. The stator was inside the parameters of the basic tests but was sent away for checking at west country windings in the south of England (where everybody goes in the UK). They supposedly tested its output and I was told it was LOW and would need a rewind, on re-fitting it made no difference at all to the misfire




I am left with the feeling it MUST be a carb issue even though I have been through most of it. I really need to borrow a carb before I lose my mind!!!




Edited by mxscotty

If your main jet is too big it will cause the bike to cut out almost as if your hitting your rev limiter early. Sounds like what your going through, try putting in a smaller main jet. You don't have to remove the carb to do this, just loosen the boots and turn pull the bottom out when you can get a better look at it. 17mm float bowl bolt and 6mm main jet.

Ok, Ive read your post three times, so I think I understand what youve done except for...


The coil, lead and plug cap. You tested it, or replaced it? I know youve replaced the plug twice.


One other thing..the cam chain tensioner, have you ever replaced it? The OEM parts can be bad and you'll never know it, unless you physically assult the thing to see if it locks in place. Otherwise, they can back off, dump the timing, reset themselves like nothing ever happened.

I tried a borrowed plug cap early on in this saga and then replaced the whole coil, ht lead and plug cap set. As for the tensioner? There are no rattles and it will rev to the limiter ok but it will not hold a steady part throttle without misfiring and sometimes cutting out. I will attempt to make a short video of it tomorrow and post it up. Thanks!!

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