What to check next?

You have a vacuum leak, check the intake and carb mounting gaskets if it revs as you say you should be able to hear the hiss. I hope this helps. Take Care.

Pumping the primer bubble while the engine is running should cause the engine to over fuel and stall (if the tuning is correct). Sounds like its not fueling enough, pumping the primer is forcing fuel through the carb causing the engine to pick up rev's, but this is soon exhausted which is why it wont rev on the throttle.
* Check that the fuel line is not damaged and drawing in air. (if this is intact try the next step)
* Strip and clean the carb. Replace any diaphrams if needed.
* Check gaskets seal between carb, air filter and cylinder.

Just out of curiosity what standard carb settings did you use.?

All these people are missing a basic thing: check the compression. If you did all thus work yourself, you may have reused the head gasket or not installed the new one correctly. It will run with no load, but once force in exerted, the pressure finds the leak and exploits it. There is a reason 2-cycle engines run at 130-160 psi normally.

Carb Jets are gummed up.

There are a few things to check, if the muffler has a spark arrester gauze in the out let this may well be blocked, remove the muffler and the gauze, does the engine now rev? if yes either the park arrester is blocked or the muffler itself is blocked, if this does not cure it, check the flow of fuel from the tank to the new carb, replace the filter in the fuel tank and try again, if this does not help try running the saw with the air filter removed, if it revs normally the filter is blocked.

It is fuel starvation, things to check,when it starts to die slacken off the filler cap, if it picks up again it is the breather in the cap not working and causing a vacume in the tank, next check the rubber boot between the cylinder and the carb fixing plate, this als houses the impulse for the carb, if you remove the carb it is possible to carefully pull it out through the open inlet hole in the carb fixing plate, any signs of damage and replace it.

u require a carb service, also check your carb to engine manifolds..as they are prone to becoming de-laminated & or cracked.

If you have started the engine out of water, you may have locked up the lower end, the prop gear.  If you think it is the carb or a fuel problem, try and get your hand over the carb when you rev it up. if you put your hand over the carb right as you rev it, then pull it off, she will pull more fuel in and should go on to rev up. if this works you still have a port cloged in the carb. 

Remove the water trap bowl at the bottom of the petcock, (gas valve). Any water or trash in the bowl? Drain a cup of gas from the tank. Any water or trash in the cup? Dump it, clean it and re-mount it, ( not all bikes have a water trap bowl ).

Drain the carburetor. There should be a screw on the lower side of the carb float bowl. Remove the screw then replace it after the fuel drains. Turn the gas back on and wait a minute for the carb to fill with gas. Install a new stock NGK spark plug and try to start the engine. If the bike doesn't start and run properly then shut off the gas and remove the carburetor from the engine.

Remove the float bowl and clean the entire carb with a spray carb cleaner from the auto parts store. Wear protective goggles to avoid getting spray in your eyes. Spray into all the little airways and fittings in the carb. Remove the two screws on the outside throat of the carb and spray into the screw holes as well.
< < READ CLOSELY > >
Be sure to put these two screws back in the same hole they came out of. IMPORTANT > do not tighten these two screws down. Only screw these in until they LIGHTLY seat. Now turn each screw one and one half turns outward. Put the rest of the carb back together, clean the air filter and install the carb. Let the float bowl fill then start the engine. Set the idle speed with the idle screw. This process should get you back on the road.

Since you mentioned uneven idle revs, be sure the carb and manifold are on tight and also tighten the cylinder and head. Air leaks can screw up the idle.

Please rate this solution. Thanks speedfreak41

Hi, Sounds like a sticky float valve in one or more of the carbs. When sitting, the carb overfills causing the revving when you start it, then it dies cause the carb isn't filling. Also check the exup valve in the intake, it could also cause the exact same symptoms but because of too much back pressure once it starts. Good Luck to you, hope this helps Fixya!