Won't run at full speed continuously. Cycles speed up and down even at full throttle.

Check and make sure the exhaust outlet is totally open. If it has a metal gauze screen slide it out and wash with carby cleaner and replace.
Blocked exhausts are common problem in 2 stroke engines because users run the motor too slow and it ( + oily residue) carbons up the exhaust. With a blocked exhaust, usually the moror will start and idle pretty ok but will screw up when you try to run at full revs.

In Australia we get mud wasps building nests in there - a quick poke out of the mud and she's running full throttle again.

ps. Don't fiddle with the 2 mixture screws - unless your know what you are doing. You will only make it worse!!.

Basics to check:
1) Spark plug - brown or dark brown - OK
- aluminum or silver - piston/cylinder scored
a) is it carbon or fuel fouled
b) is there spark

2) Compression - 125 to 150 psi

3) Muffler remove for inspection
a) exhaust screen clean or plugged
b) inspect cylinder wall/piston as the flywheel is slowly rotated

Fuel starvation - 'lean fuel' condition
Basic things to check:
1) spark plug
2) spark arrestor in muffler
3) fuel lines and tank filter
4) turn adjustment needles out (counterclockwise) 1/4 to 1/2 turn
*if all of the above are OK and/or do not improve performance then:
Remove Carburetor
5) replace gaskets/diaphragms
b) use a spray carb cleaner to clean jets, holes, and body of carb

I’m happy to assist further over the phone at https://www.6ya.com/expert/rolandw_c013176d62ee17ae