Craftsman 536 881 851 snowthrower continues to stall out and mainly need to keep the choke toward the choke and not run. I have changed the oil, changed the spark plug which states a champion RJ19 but that is not what i took out as it was a Champion RC12yc i replaced with a first fire ff-10 which was the equivalent to the Champion RJ19 until i took out a different plug. The unit also started to backfire. Could the plug be the problem here or something else?

I had the same problem in my craftsman, the problem was in the choke. the spring was broken and it runs fine now after replacing the spring.

If it stalled. Add more gas. Mine does same thing.

Are you sure you have the right fuel mixture? Many of these require 2 Cycle oil to be added to the gas. If fuel is right, you may have a vacuum leak at the carb (or crack in base where it mounts to the engine). My weedwacker needed to be run on half choke for about 10 min before it got hot enough to run with choke off.

sounds like the carb will need removing and having it ultrasonic cleaned

Check age of gas. Cannot be more than 30 days old. Make sure to use fresh gas w/synthetic 2-cycle oil @ 40:1 ratio (1 gal.) Gas to 3.2 oz. Oil). Do not use old or stale oil/gas mixture. Always use the proper oil/gas mix. If you do not, your engine will suffer rapid, permanent damage. May need to clean or replace carburetor.

Make sure your Air cleaner is not obstructed. Clean it!
Check your spark plug for the proper gap. Adjust your
LOW speed throttle screw. It may not be getting enough fuel flow. Check your fuel filter screen (Inside your gas tank). Good Luck

Trimmer Starts but stall out as soon as throttle is used and choke turns off. if I hold the throttle and pull the choke it runs but will stall as soon as choke is let go.

See if there is an oil pressure guage on the dash. When you start the machine it probably has a low oil pressure shutdown which shuts the engine off when you release the start button. This is an interlock. Watch the oil guage and keep the start button engaged until you see oil pressure, release the button and it will continue to run at that point. Your second issue may be a function of the choke. If you are having to choke the engine to start, you may have to play with the choke position to keep from overfueling the unit which may cause it to stall and possibly backfire as it has to much fuel to be burned on the compression stroke, and is igniting occasionally on the intake stroke. Hope this helps. Marvin

Sounds like somewhere you have a air leak. Most likely it's somewhere near the carburetor intake or maybe the carb itself. Could be a line from the primer if it has one and i do believe it should that could be leaking or loose. Good luck.

You need to play with the timing adjustment, either on the spark plug, or the gap between the contact and the magneto.
That'll do it ;)
Don't forget to rate, thanks