Hi all,

I'm having a serve issue where my bike (2010 KX450F) is bogging down and backfiring bad after the top end was rebuilt (only piston rings were replaced, valves weren't touched)

Heres a video of what its doing:
I had this problem when i first rebuilt my topend 3 months ago and i ended up fixing it by cleaning out the air idle passageway in the throttle body as it was gunked up with air filter oil. But ever since i had to replace a blown head gasket 4 weeks ago its doing the same thing but my throttle body is spotless. My bike has been at the dealers workshop for the past 3 weeks and got it back yesterday after they told me to put a new rear tire on as they thought because it was loosing traction so easily that it was messing with the cdi box?? Threw a new tire on but not to my surprise this didn't fix a thing. It worked good for the first 30 mins but got worse and worse the more i rode it and happens more frequently if im going over rough terrain, whoops or going up the upramp of a jump (still don't know how i managed to save the nose dive haha ). It happens randomly though - one minute it will work good the next its almost unridable so this is pointing to electrical/EFI issues. So if anyone else has any ideas what it could be i would love to know, here is a list of what the dealer has checked:
All electronics and sensors show that they are fine with not faults on the diagnostics kit
Air temp sensor and air box was cleaned
All wiring has been checked for any wires touching or shorting out
Changed spark plugs
Entire throttle body cleaned
All electrical contacts cleaned
Fresh Fuel
Fuel pump filter screen from what I can see is clean


I'll give abit of a history with what happened:

70hrs - Top end rebuild (only changed rings as valves were perfectly in spec) Took it for a ride and had the bogging down issue, checked the throttle body and noticed the air idle passage way was gunked up with air cleaner oil. Cleaned it and fixed the problem

74hrs - Blew head gasket, replaced head gasket with same cheap one as i thought i torqued the head bolts incorrectly

90hrs - Blew gasket again noticed head gasket failure was caused due to a slight bur ontop of barrel mating surface, got a mechanic to sand the surface flat and used genuine head gasket and base gasket. And this is as far as i have got now as i got the bogging issue.

My bike has never been on a race track, always serviced at 5hrs intervals and never had the "oops" flogged out of it.