Went riding today, started out with a new set of plugs and fresh oil and a 3/4 tank of gas. After a half-hour of riding it fouled out one of the plugs, but it wouldn't stop firing it would just miss. Changed out the plug and it ran great for another half hour and then it bogged down and died. changed plugs and it would start and idle rough on one cylinder. The one it was hitting on this time was different then the one it was running on before. We have plenty of spark, and put fresh plugs in. The fuel tank and both the carbs were cleaned couple of weeks ago, and it's ran good before, except for fouling out the one plug after riding a half hour. The fuel pump is working.

if it is cold where you are like here, ice is what i would look for first.

Make sure that you put gas line antifreeze into each tank of fuel. The snow dust like Paul mentioned will get into the carbs and cause that type of problem. That is why it ran OK last week and now isn't.

The snow dust will get into the carbs and cause that type of problem. That is why it ran OK last week and now isn't.[/b]
i bet the side that has the questionable plug has air bubbles in the fuel line.