is it my timing advancer??

ok guys here my problem... the bike has not been driven only started up because i have rebuild the motor and trying to get all the bugs out. the bike starts up good and idles about 1000 rpm. whenever i rev the throttle over 3000 rpm, it wont go back down to idle. RPMS will go up to about 4000 and stay. then when i shut the bike off and turn it back on, it idles at 1000 again like normal. if i only rev it up to 2000/2500 from idle it will return as normal. i have already checked for vaccum leaks just had carbs cleaned with new intake boots... is there any way of checking to see if its the advancer?? i have access to a timing light but the manual doesnt say where to use it... please help.... this is a movie of what happens i took on my phone...

Your manual should tell you how to take out the timing advancer and lube it. Have you done that?

It can be the advancer. I worked on a KZ400 which did a similar thing, but it would eventually come back down on it's own. The springs were just too weak.

Yours may be sticking.

To check the mechanical timing advance, use an inductive timing light. put the inductive clamp around #4 plug wire and hook the power leads to your battery terminals and start the bike. With the points cover off, point the light at the timing marks. Rev the engine and you will see the advance go from the F mark and rotate to the fully advanced position. If your advance isn't working right... springs bad or it is dirty, you will not see the counter-clockwise rotation back to the F mark position quickly when you release the throttle and go back to idle.