make sure the Gas cap hole is not clogged and also the rubber insert.
Are you depressing the choke each time you try to start it?
Are you releasing the decompression lever after it starts?
Is the decompression valve closing? You may have to remove the cylnder head to check to see if the decompression valve is leaking. This is not hard just three hex head nuts. while you're at it, you can decarb the dome of the cylinder head. Then lay it on the bench and pour a little gas on the inside of the decompression valve and let it set for a little bit to see if th gas leaks out. If it does, you have a defective decompression valve and the valve or cylinder head will need to be replaced.
The head nuts should be torqued at 7 1/2 lbs.