91 blaster spark plug

Left side on back of carb. Right where the airbox boot attaches. This only controls air at idle, so it would drive fine if wrong. But go all the way in and then out 1.5 turns to start

Do no believe a previous owner telling you this and that. There's a reason he sold something running this bad
 
Seeing as it's a new blaster (to you) I suggest doing a compression and leak down test. List any mods, pipe, air filter, box lid (Y/N?)
Either find the PDF manual on here, or get a Clymers repair manual.
Go over the machine with a fine tooth comb, write down all issuies and fix in order of importance, brakes being #1
As to the running, start with NGK B/BR8es plug, don't understand "too small" of plug, don't matter, get right plug.
 
If the needle and seat is dirty or the float level is set wrong and allowing too much fuel into the carb, it will run better if you turn the petcock off.

It looks like you have a lot of work to do.

Take the carby off and clean and adjust it well, while it is off leak test the engine.
 
Firstly- Welcome to the forum.

NKS br8es spark plug - if the spark plug was too small, i doubt it would run at all.

sounds like you need to clean the carb and petcock on the fuel tank.
turning off the gas will run the carb low on fuel which in turn gives you the "idea" of it idling better when really its starving the engine and causing a lean condition

you still bought it after you hopped on and it ran like crap?

NGK spark plug