Check all the basics. Wet, oily or black plugs, plug wires jumping spark, bad timing, loose carb, fuel flow, air cleaner. Do you have a Webber ? The Webber mounting plate screws are notorious for coming loose. Does the tail pipe sound like its sputtering. If all that checks adjust the valves.
Worst case...blown head gasket. Oil in water / water in oil, but would probably also run hot if thats the case.