It should idle easily for 30 seconds on the starting bench with NOglow plug igniter attached. It should transition smoothly from idle to full throttle in approx. 1 second with no stumbles.
1. check glow plug - a glow plug may work but is not optimal. Iuse an #8 plug for all my 40 / 60 2-strokes. Can't tell ya how many times an "expert"tells me to swap the plug and "TaDa" - smooth operation. One of these days I will rememeber this and NOThave to be reminded by an expert.
2. are there any air bubbles showing in the fuel line between the high speed needle and the carb? Idon't know if this engine has separate carb / needle assembly with a connecting fuel line - if they are 1 piece - check the fuel line into the carb/needle assem ... If there are air bubbles between the needle & carb then air is getting into the fuel line between the engine and the needle assem OR between the needle assem and the carb OR thru the high speed needle assem itself (o-rings). Oh yea, had a situation where my fuel tank was solidly mounted without foam. Engine vibration caused foaming in the tank that then transferred to the needle assem and then to the carb. No fuel line problems just needed to isolate tank with foam. Some anal retentive types use stainless thin guage wire and safety wire the fuel lines to the various nipples to ensure "no air leaks" - be careful 'cause those little wires can puncture the fuel line (don't ask how I know this)
3. steal (OK- be nice and ask to borrow) some new fuel from a friend and replace your stuff with the new stuff.
4. All else fails - sell it to someone you don't like

BTW - Ihappen to use Hobbico power panel that has an amp guage for my wired glow igniter. Allows me to turn off the juice to the glow plug without having to physically remove the igniter.