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SXF450 Post Rebuild Head Scratcher

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Rebuilt my 2016 sxf450 for some rounded dogs and a bent selector fork on 5th. While apart added 6th speed to the box. transmission all works perfect but have managed to build in an engine fault.


Fault: Revs build slowly, almost choked past 1/2 revs but progressively as if restricted. engine is down on power but otherwise behaves normal, start idle and low rpm no real complaints. Immediately on release of throttle engine cuts to idle - quicker than any bike I've ever worked on, but then idles fine. No strange noises, knocking, ratling or grinding... definitely some resistance in there directly related to rpm


So far I've:

Checked and rechecked the valve clearance.

Cleaned the injector

Replaced inline fuel filter

No air leaks in intake

Swapped to another bikes fuel tank (+pump)

Advanced and retarded the timing 1 tooth either way(3 positions checked) flipped the crank 180 degrees to the cam timing.

Swapped out the barrel and piston - no change

Had the ecu in another bike


I inspected all the components and measured IAW manual during rebuild - only 50 hrs since main bearings, conrod where replaced.

Engine doesn't drag on clutch, clutch disengages and bike freewheels correct in gear.

Plug out engine turns by hand as expected...


Still need to measure oil pressure, if the gallery was blocked could cause excess drag at higher rpm but don't have the tool.


Any ideas of what I've F'd up?!


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