My Fuller FB90T when I turn it on the motor gets slower and slower and smells like something is burning. Is it fiaxable?

Sounds as if there is something stuck in the vacuum causing the beater bar to stop turning while the belt continues to turn giving off the burning smell.

Check if there is debri at the bottom behind the roller where the dust gets sucked in. There is a reset button in this area. Press it with a needle or pen tip. Hold it for about 5 seconds.

There is a overburn protection if their is obstruction in the hose due to which the vacuum shuts down. Resetting it will fix this issue

Try pushing the reset button located at the bottom on the back of the machine. If the reset button has tripped it usually indicates a clog or restriction somewhere caused the motor to overheat.

First replace the belt if it's over 6 months old. Stretch belts lose elasticity. It may still turn the brush roll but when vacuuming the carpet it may be slipping and not turning fast enough. Make sure bags and filters are good.

You may have damaged a bearing or bearing housing and that could be causing the motor armature and field to rub together which in turn causes the motor to draw a significantly increased current to overcome the friction. It could be a simple fix if it is a bearing but if it is the housing then it is likely toast. I used to repair motors years ago and the cost then was not bad. However if the motor is heat damaged, as happened to my power rack last year, it was cheaper to buy a new one than replace the motor. A burned out motor is not worth repairing these days.

For the stack of problems you have, it sounds like it be best for you to ditch it and get a new vacuum. The heavy problem sounds like a motor issue. The burning smell is most likely from the belt being loose on the power head.

Economically, you should just get a new one instead of replacing the whole system guts

most vacuums use a belt that runs from the motor to the front brush. and when the brush gets jammed the motor continues to run and burns the belt (probably what you smelled).
you'll have to turn it over and remove the base plate and replace the belt.
use the old belt to find a match
hope this helps.....jay

A guess: either there is something in the fans, stuck in the tubes or the belt is being held in position by debris causing it to burn.

This could be a motor problem. There is always a slight blue arcing that occurs when an electrical motor runs and can be visible through the exhaust vent, but any excessive arcing indicates a problem. If the carbon brushes are sticking, this will cause excessive arcing and can be corrected by an experience service person, but if there is any electrical failure, the motor would have to be replaced. There are no parts available for these motors and the expense of replacing an armature (the likely culprit) would be more than the motor. This could also be as simple as a worn belt, but usually, belts will not produce the smell your describing. When a motor is burning out, it will produce an acrid odor that will linger for a long time.