First you have to have a pressure gauge. Then you need to remove the diaphragm cover, the diaphragm and the gasket. Take the pressure gage and pressurize the inlet to the carb, when doing this you need to have the internals wet with fuel by this i mean pour a little fuel in the cavity where you remove the diaphragm then pump the gage up untill the pressure pops. You will see this when air starts coming thru the main needle. This is the pop off preasure to decrease the preasure remove the small spring under the lever and clip it with finger nail clippers one spring rotation at a time reinstall and test again untill you see the gage pop between 12 to 15 psi. Then run the engine on the ground and see how she does if you still have the issue remove the diaphragm cover and gasket again and bend the lever up just slightly and try it again. Some will tell you to bend it down, you can try that but it my experience that makes the engines sometimes loose fuel in the carb and run too lean and can run out of fuel, by bending it up you are allowing more fuel into the cavity and then you can adjust the needles in to get the engine to run how you want.

Be careful with the spring under the lever they are slippery little buggers and if you drop it you will never find it.

Check this site out it will help http://www.geistware.com/rcmodeling/...n_pressure.htm

here is another that will help http://tech.flygsw.org/walbro_tuneup.htm