Cranks but dies after 2seconds....why

Environmental wackos got alcohol mixed with gas. Alcohol causes rubber diaphragm to swell and not pump fuel properly. Buy new carb on eBay and run out of gas or put syabil in tank when not used

Your carb is starving gas from your saw. You will have to remove the carb. and take it apart, check the diaphragms for hardening or splitting, clean all passages and screen. There is a lot of small parts that have to be put back together correctly, also the diaphragms and their gaskets have to be installed right.

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Leaking head gasket or intake gasket

Try a new spark plug & fresh fuel first,
& clean the air filter

if you replaced the diaphragm and gasket below the carb and are completely positive they are installed correctly and not damaged, then the carb is defective and they usually warp or the primer system goes bad

You probably need to clean the carb and possibly replace the gasket/diaphragm underneath it. If you are a do it yourselfer then you can do this. Otherwise take it to a professional. If tackling this yourself, first remove the gas tank and carb as one unit, be very careful to note where the springs are hooked up. Drain the gas. Remove the 5 Philips head screws holding the carb on. Underneath the carb will be a rubber diaphragm and gasket (careful; there's a tiny spring under there; don't lose it!). Spray out the carb with carb cleaner and spray out the little holes and cutouts on top of the tank. Replace the gasket/diaphragm (about $3 USD) and put it all back together. I hope this helps and best of luck to you.
James

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

if this is a vertical crankshaft 3.5 then you may have the gasket and diaphragm inverted, the diaphragm must be against the tank, and the gasket against the carb for the fuel pump to work, also check the pickup tube screen for being plugged, water in the tank bowl or a film or varnish or oxidation on the main jet sock.

If this is a horizontal shaft, like a go-kart, all the stuff about plugged pickups and main nozzles applies, only big difference is if there was a lot of corrosion in the bowl under the carb, the main jet could be permanently plugged, also there should be a removable jet on the control lever side of the carb that will probably need cleaning as well, also when removing the carb from the tank make note of any fuel in the bowl, if there isn't any, make sure the pickup tube isn't plugged or split, the diaphragm spring is installed, and the bowl doesn't have a hole rotted through it.

Fuel starvation - 'lean fuel' condition
Basic things to check:
1) spark plug
2) spark arrestor in muffler
3) fuel lines and tank filter
4) turn adjustment needles out (counterclockwise) 1/4 to 1/2 turn
Chain Saws:
5) check impulse tube and intake boot for cracks or leaks
*if all of the above are OK and/or do not improve performance then:
Remove Carburetor
6) replace gaskets/diaphragms
a) use a spray carb cleaner to clean jets, holes, and body of carb
b) fuel pump diaphragm goes against the carb body,
then the gasket towards the outside
c) metering gasket goes against the carb body,
and the diaphragm goes towards the outside

I’m happy to assist further over the phone at https://www.6ya.com/expert/rolandw_c013176d62ee17ae

Fuel starvation - 'lean fuel' condition
Basic things to check:
1) spark plug
2) spark arrestor in muffler
3) fuel lines and tank filter
4) turn adjustment needles out (counterclockwise) 1/4 to 1/2 turn
Chain Saws:
5) check impulse tube and intake boot for cracks or leaks
*if all of the above are OK and/or do not improve performance then:
Remove Carburetor
6) replace gaskets/diaphragms
a) use a spray carb cleaner to clean jets, holes, and body of carb
b) fuel pump diaphragm goes against the carb body,
then the gasket towards the outside
c) metering gasket goes against the carb body,
and the diaphragm goes towards the outside

I’m happy to assist further over the phone at https://www.6ya.com/expert/rolandw_c013176d62ee17ae

Fuel starvation - 'lean fuel' condition
Basic things to check:
1) spark plug
2) spark arrestor in muffler
3) fuel lines and tank filter
4) turn adjustment needles out (counterclockwise) 1/4 to 1/2 turn
Chain Saws:
5) check impulse tube and intake boot for cracks or leaks
*if all of the above are OK and/or do not improve performance then:
Remove Carburetor
6) replace gaskets/diaphragms
a) use a spray carb cleaner to clean jets, holes, and body of carb
b) fuel pump diaphragm goes against the carb body,
then the gasket towards the outside
c) metering gasket goes against the carb body,
and the diaphragm goes towards the outside

I’m happy to assist further over the phone at https://www.6ya.com/expert/rolandw_c013176d62ee17ae