Timing may have slipped. Has the idle changed at all?
If the idle is ok, check the total timing with an adjustable timing light.
Check that you're getting full throttle, make sure both the primaries and secondaries are vertical. Could be the secondaries are jammed.
Do you run a good fuel filter? With today's gas, modern fuel tends to clean the fuel system well, but it also rots rubber pieces. Check your fuel filter or filters. I've had a Fram canister filter clog after only about 200 miles.
