[mightslip]

Your problem sounds like Pilot jet/Slow jet. The tiny holes are nearly impossible to clean. replace the rubber O-ring and needle as well.
You can try Berryman carb dip as Bweigh suggests but be careful to remove all rubber and the floats, that stuff is strong and any non metal part will be ruined.
My suggestion. order a jet kit ( they contain all the jets in your carburetor.) Swap them all out and as Troy suggested check your float height. Good luck.

 

[mightslip]

I f you do some looking around you can likely still find a service manual for your bike which will include part numbers etc.

 

[mightslip]

Mikuni 20mm VM Series Universal Round Slide Carburetor - VM20-273 Dirt Bike - Dennis Kirk

Find the Mikuni 20mm VM Series Universal Round Slide Carburetor - VM20-273 at Dennis Kirk. Shop our complete selection of Dirt Bike parts and accessories including the Mikuni 20mm VM Series Universal Round Slide Carburetor - VM20-273.

www.denniskirk.com

 

[mightslip]

You can try going cheap but it has been my experience they rarely work correctly.

VM20 Motorcycle Racing 20mm Carburetor For 100cc-130cc Dirt Bike Carb Universal | eBay

VM20 Motorcycle Racing Mikuni 20mm Carburetor For 100cc - 130cc Carb Universal. A carburetor is a device to mix air and fuel for internal combustion engines in the proper ratio for combustion. The fuel and air mixture is too rich if it has an excess of fuel, while too lean if there is not enough.

www.ebay.com

 

[mightslip]

In the spec sheet I posted above your carb is a VM20 ( That is your barrel size )

here are 2 videos that should help you.

Kevin Cameron Explains How To Tune Carburetors