Get the engine up to operating temp, and do the "pinch test"... Pinch the fuel line about 1 inch from where it enters the carb, and PAY ATTENTION to what happens. Count the seconds till it dies. 3 seconds, with a slight rise in RPM just before cutting out is perfect. If it dies in under 2 seconds (which I HIGHLY doubt from what you've described) your low speed needle (LSN) is too lean. (Low speed needle is located at the end of the slide where the linkage attaches). If it runs for 4 or more seconds before dying, the LSN is too rich. If you are unsure which way to turn to richen or lean the mixture, clockwise turns the screw in and leans the mixture, counter-clockwise richens the mixture.

It sounds to me like you are probably too rich on the low end, which will cause it to be sluggish at first, then take off quicker (assuming the HSN is tuned right)


Also, Im sure everyone here will agree with Kevin. Take the time to learn how to tune your engine yourself. It can be expensive having the hobby shop always tune your engine. Especially around here, where the temps and humidity are always changing, which will affect the tune on a Nitro engine.