Four things are required for internal combustion to work: compression, air(oxygen), fuel and a spark to ignite it all. This means there are only a few possibilities that are causing your problem.
1. Compression, do a compression test, this will show up any problems with valves sealing and or piston rings/bore wear. Not the most likely culprit for the problem you describe.
2. Air, is the air filter clean? If the engine can't get enough, or alternatively gets too much(manifold gasket or carb gasket leaking perhaps?), it will be difficult to start and won't give good performance, possibly to the point of what you describe.
3. Fuel, is the engine getting enough? A poorly fuel pump or blocked fuel filter can lead to fuel starvation giving symptoms like you describe. Also blocked jets in carbs can cause similar problems.
4. Spark, is the coil performing properly? Are the HT leads in good condition? Is the timing set correctly?
These are the basic steps to sorting your problem. My guess would be ignition problems, either a coil failing or timing out by a significant margin but it could just as easily be a leaking manifold gasket.
D.
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