Umm..check your sparkplugs. With #120 mainjets and premium fuel, they will foul very quickly. Today's midgrade fuels burn at about the same rate as the 1970s premium, and today's D8EA sparkplugs are far colder, to boot. This makes them foul, pronto.
My suggestion if you find black plugs:
1. Change your mainjets to #110 size (or even #105, which works better) for today's fuels, and run midgrade unless touring 2-up with full [camping] gear aboard at 90 MPH. Then the plugs won't foul. Make sure the jet needles in the slides have their clips in the middle notch for the early carbs.
2. Set your air screws on the carbs to 3/4 or 7/8 turn out, NO MORE. Optionally, if you want the original screw adjustment range of 3/4 to 1-1/4, change the pilot jets to size #38 (aka 37..5) in Keihin jetting. This, too, is due to modern gas.
3. Make sure your battery is well charged. It should not drop below 12.0 volts when running. If it does then the spark will get weak.
4. If you have installed Daiichi brand condensors, go find your old ones instead, and install those. The Daiichi condensors are pure crap and will act just like what you have described above. They are not rated to high enough voltage (250v instead of 400v), and they short internally with just a few minutes of runtime.
