Engine missfires over 6000 RPM.

[HannesDTX]

Hey guys, so i got my 2006 DT125X about a month ago. It has ran perfectly except a couple of fouled plugs (2-3).
But yesterday it started missfiring around 6000RPM and i cant get over it. The bike starts jerking but it doesnt feel like a carb issue. I thought it was just another fouled plug so i changed that out to a NGK BR9ES. I also cleaned the carb for good measures but no luck. I can also hear the powervalve priming when turning the ignition on.

What could it be?

[Calum]

I mean it could literally be anything.

You shouldn't be fouling plugs, ever...

So something isn't right there

Is it smoking excessively?

Are you losing coolant?

Are you losing gearbox oil?

I've had something similar when I fitted a cheap chinese knock off 'racing' coil, so make sure you have a genuine 3RM coil fitted.

Make sure the exhaust is cleaned through.

Make sure the airbox and inlet tract is cleared through.

Measure the HT lead resistance make sure it's within spec.

[HannesDTX]

@Calum Not leaking any fluids. No more smoke than the regular twostroke. Also a knew airfilter is fitted.

[Calum]

Have you got access to a compression gauge? Can you measure the health of the engine?

[HOTSHOT III]

@HannesDTX Check all the things @calum said, plus are you sure you've done a FULL carb clean? A lot of people just put them in an ultrasonic cleaner for half an hour and expect to bolt them back on. Doing this doesn't sort the blocked emulsion tube which is a leading cause of DTRs refusing to rev out beyond 6000rpm, precisely the symptoms your bike has.

Do a full carb clean including the emulsion tube (the brass tube the main jet screws into). A lot of people (including bike shops) miss this and don't realise it comes out because you have to screw the main jet back in without the brass washer and tap it gently to remove it from the carb body. The space between this and the carb body gets blocked with moisture, dirt etc and blocks more and more holes in the tube (which is how the air atomises the fuel) until it just starts fourstroking and won't rev out one day as this controls the mid-throttle fuelling. Just chucking it in an ultrasonic cleaner usually isn’t enough; you have to actually remove everything from the carb body, physically make sure all the jets and air passages are clear, then reassemble. Make sure the air supply to the emulsion tube is clear as well, it's the small brass tube on the right of the carb body just in front of the carb-to-airbox rubber. You should be able to blow through it, but only just.