Man at the smoke

If you pull the carb, and he runs a single plane, you can use a light and mirror to see the intake valves..Look for wet oil on them if it's seals or guides. If they were "redone" but you can only see new pliable seals, I'd say the guides are junk. It may be only one cylinder..You can also do a leak down test and see which ones the bad one..If you cant tell by the old plugs.