I basically have the same configuration as yours, except for a Vm16.
I had the same problems. (Nc50 1981)
At this point, you have a exhaust issue: you must increase your outward flow.
Gut your stock exhaust or change it.
In 2 cycle engines, exhausts must match engine size. (very important)
At least get your stock exhaust clean as a whistle. (inside of course)
Then, a jet in the 85 range should fit....that's what I have.
If you're four-stroking, you're running way to rich: reduce your jet size.
Check your spark plug, it might be fairly black...which is not good.
It should be light chocolate brown.
Your four-stroking might also be the result of unburned gas back flowing from the exhaust...again caused by your to large jet.
Took me 2 weeks to figure this out.
Keep it up, you're almost there.