I agree with 30YT that it's probably a fuel problem as opposed to ignition. The timing would not change on a running motor. It's typically either good or bad and the things that would cause it to change on a running motor are all pretty unlikely scenarios.
I have seen magnetos(ignition modules) be intermittent and the best way to check that is with a gap type tester that you leave in series with the plug and watch the spark with the engine running. When the engine begins to shut down, if the spark is going away, that's a good indicator it is the cause of the shutdown. If it isn't, I'd make the leap of faith that the ignition is probably OK and your problem is elsewhere(most likely fuel delivery).
In addition to inspecting and cleaning the carb, have you tried loosening the gas cap and running it. If the vent is plugged in the cap it will run for some period of time till the bowl is empty and then stop. Restarting can be difficult.
Finally, regarding the manual. You can get them from HelmInc.com. Honda distributes their manuals through them. Here is the link for your manual.... $28.95. (I haven't seen these on line anywhere for free)
http://www.helminc.com/helm/product2...RA7TCKGGT1A05E
Let us know how you make out...