1986 xt600 dies at above 4000 rpm when warm
 

My XT has been giving me trouble lately. It starts and idles fine. While it is still cold I can run it up through the gears to 6500 RPM or so, with no trouble, but once it is warmed up, say after 5 or 10 miles, it will die above 4000 rpm(usually backfiring in the process, but after coasting a bit it will usually refire on its own). If I stay below this, it seems to work fine. I have cleaned all the passageways in the carbs, as well as the fuel strainers in the tank. The ignition coil secondary has a resistance of around 20k when cold. This is quite a bit higher than the published number of 3.5-5K. The plug looks good.

mix and spark
 

My feeling says something wrong with the second carb; diaphragm ok, slide moving ok, needle in correct position, indeed fuel arriving at the jet?
Sure that fuel arrives in the float bowl - tank filters are only to catch bolts and mice, there is another little nasty one on top of the float needle seat?

Air filter clean?

If all the above checks out, check the ignition part; any possibility to exchnage part with an identical XT, coil, TCI, etc?

Good luck,
Auke