Runs for 5 seconds with choke pulled;no crank without choke

Put fresh fuel/oil 50:1 mix in the fuel tank. Turn on ignition, set fast idle and full choke. Prime the carb, if it has a pump bulb for that.

Holding saw so it can't move at you, pull the starter smartly for at most 5 times or until it "pops". If it pops, take choke off and pull starter until it starts.
Else, take choke off and crank it maybe 5 times until it runs, or go to full choke again for maybe 3 cranks.
Keep alternating sets of pulls with/without full choke until you get a pop, or give up. All the above is for the general case- shouldn't take many cranks for modern saws to get one running.

Pay attention for safety to how you hold the saw, like maybe with the rear of the rear handle located inside your right knee.

Check the following, and then clean or replace:

• Fuel Filter - Located INSIDE the fuel tank
• Spark Plug
• Spark Arrestor - at Muffler
• Carburetor - May need a rebuild kit

Also watch this...

and this one...

Sounds like a choke issue. but I am no expert on Bikes.

Full choke, stop switch on run. Pull rope handle 3-4 times or more until it "tries to start" (it will not start, but it will try to for a split second) move the choke lever to the half-choke position and without pulling the throttle, poll the rope handle until the engine fires and runs for 4-5 seconds then fully open the choke to the run position, let it warm up for 25-30 seconds (don't forget to let it cool down about 45-60 seconds before you turn it off!) and then proceed to use it.

As you have done most things, i would suggest you have a massive air leak, either through the cylinder or crankcase gasket, or through the crank seal, i think its time to pull the whole engine down and replace these items.

It should start easily, start with fresh fuel/oil, pump the primer as many times as it takes to 3/4 fill the bulb, pull the choke fully out, crank the engine until it fires once, push the choke fully in and crank again until it fires up, after re fueling a warm engine, pump the primer again until it 3/4 fills the bulb,before cranking the engine pull the choke fully out and in again( this will engage the part throttle ) then crank the engine until it fires up.

The choke system is connected to the half throttle position, if you lift the throttle trigger the choke pull will automatically return, whilst trying to start do not lift the throttle trigger, the starting procedure is, pump the primer bulb as many times as it takes to 3/4 fill the bulb, pull the choke pull fully out, this will switch the saw on and engage the half throttle, crank the engine until it fires once, do not continue to pull,it will actually not run on full choke, if you do continue to crank with the choke fully out, you will very quickly flood the engine, now push the choke fully in, now crank the engine until it fires up, lift the throttle trigger once, this will dissengage the half throttle, and it should now come down to idle.

First thing to do, if you haven't already, change the oil, and remove all 'old' gas.
Then, after that's done, remove the plug, in order to de-pressurise the cylinder.
Then try to crank it again.
If it turns, then clearly, it's not ceased.
Remember, don't loose you're patience, just take your time.
Good luck with that:)

check crankshaft seals,maybe sucking air. most of the time the seal on the starter side. hope this helps.