No spark with plug in, take out and have spark to ground

It pays to check and test systematically instead of throwing new parts at a problem - a new spark plug doesn't guarantee a good spark any more than a new carb guarantees fuel is reaching the cylinder...
An engine to start and run needs: -

Compression.
A good cranking speed.
The right amount of fuel being delivered into the cylinder.
A good spark.
Everything happening at the right time.

At least one of those things is absent and we can't help you discover which...

Hi Mike:
It's trying to make you crazy.
I have a model YT4000 and it is a PITA to start when cold.
I pull the air filter, toss in a bit of gas to prime it, and it starts instantly.

I'd suggest that you give yours another try. 3 or 4 times of a couple of seconds each. If it doesn't go, pull a spark plug and see if it is wet or dry. Wet indicates getting gas but no spark. Dry indicates not getting gas.
While you have the plug out, check the spark... Hot & blue.
Hope this gives you some more to work with.

You need three things to make any gas engine run, in this order double check make sure you have proper timing set, spark and fuel. Could be bad gas or a bad ground from motor to frame.

If you have compression then the first thing you check is for spark. If you have spark then you move on to the fuel system. You will have to pull off hoses at a couple of points to see if gas is at the carb and further along. If it is the carb then a simple carb kit will work.

It sounds like you have a air line busted somewhere i had one of my outboards do that it wouldnt prime for anything so i ran everything down checked the fuel pump and cleaned that out tore apart the carbs cleaned them bought brand new fuel hose still scratching my head i took off the air lines for the carbs and put new ones worked like a champ

Spray starter into the carb intake with no choke, if she runs for a few seconds then you know its fuel starvation
clean jets & airways & blow through with compressed air

take the carby in to have it assembled correctly
the positioning of the gasket on the right side of the diaphragm is critical

take apart and clean carburator if that wont help then is time for new one. make sure you are getting a spark first.

With a new plug and fresh gas it should start. Check to make sure there isn't a kill switch turned off or that the gas petcock from the tank to the carb is on.

How did you check the fuel filter? Disconnecting it from the fuel line that goes to the carb is the best way, making sure the gas is turned on under the tank. Bleed off a little gas into a can or something just to make sure the filter isn't plugged.

The last thing I'd do is take off the air filter and spray a little starting fluid in the carb. It may start and run several times if there is old gas in there somewhere. Remember the bowl on the carb is probably still full of old gas. After some fresh fuel reaches the plug it should start and continue to run.

(You might drain the bowl on the carb of ols gas before you go any further)

Good luck!

Sirwriter

Stihl weedeater was running and stopped suddenly. Put in a new fuel filter and cleaner air filter with soap and water. Checked spark plug and it appeared fine. Started the weedeater, ran for a second or so and stopped. Spark plug wasn't gas fouled. Could it be the carburetor needing new diaphram gasket and spring.