Hmmmmm....not exhaustive by any means but heres some things I've found in the past:
  1) Water in the fuel tank
 2) Water in the Carb bowls
 3) Crud/deposits in the fuel lines (and the extra fuel filters on the Carb bodies themselves)
 4) Carbs out of balance
 5) Worn carb internals - Emulsion tubes and needles appear to be favourite. 
 Plugs - well I've tried NGK Platinum plugs and found a 'slight' improvement but they were almost £10 each (there are some on Ebay for about £8) or try:  
www.sparkplugs.co.uk  Leads: I imported some from the USA (again via the dreaded Ebay) and cant say I noticed any improvement. 
 Good luck - Frank