I have a husky rancher 50 at high rev it keeps flocuating up and down

Check the fuel lines for cracks/breaks. If they are o.k., then:

Youwill need to disassemble the carburetor and spraywith carb.cleaner. Make sure the small screen is free of debris.(Should be able to see daylight through it.)

Use a Den Tek Brush, (http://www.drugstore.com/products/prod.asp?pid=165357&catid=1152),to clean the small portals in carb. These portals canbecome blocked. Reassemble and start.

If the carb. parts are more than 4 years old, you may have to replace them.
Parts include: small screen, needle valve, diaphragm, & gasket(s).

The problem is more than likely the diaphragm, but it won't hurt anything to try the first couple of things; you may be able to save some money.

If you need further assistance, just let me know.

Jim