I had the exact same problem - and it is now back to its full potential.
(from not a technical speaking person)
The rubber in the diaphragm on the carburettor had gotten old and fallen apart (mine 22 years old).
To fix it cost me just over £200 that was £73 + VAT for each diaphragm and £40 + VAT labour.
I wasn't able to do it myself as you need a special star like screw and a newbie for playing with bikes.
This is for anyone searching for this problem as I was searching for an answer.
Joshua - I tried the valve clearances and timings - ran a little smoother but no success until the above.