Burning smell?

The smoke will be coming from the motor.

It's more than likely that the motor's brushes are not making best contact with the commutator and heat is being generated. Left to run long enough the build up of heat will destroy the motor. And with each successive use it will become worse- you will not know exactly when it will self destruct.
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There is no smoke without fire - they say.

Have it serviced/repaired or buy a new one.

Good luck...

I have just worked it out "forever" means for a long time -it does not mean the particular model...

Peccavi
January 2013