A trusted source just told me it did. I ran in twice a few weeks ago in my fuel and used it to clean my throttle bodies....

 

If its used properly, then I would say no. I've used it in various engines from my motorcycle, boat, car, lawnmower.... ect. for years without any problems ever.

I would imagine if you used more than what is reccomended, then it I would think that it could in that it may make the gas less combustable and could potentially foul out your plugs.. thats just a guess though, I've never heard of it causing issues even when too much is used.

generally, when someone uses it, they are using it because they already have a problem, such as running issues usually due to lack of maintainance. Then, all of sudden they use it and they foul the plugs. I would think its more than likely just coincidance if someone has an issue after using it. Although, I guess its possible that the fuel system could be so full varnish, that the seafoam breaks it up and then it flows through the system, then causes the plugs to foul. But again, not due to the seafoam itself, but due to lack of maintainance. A properly kept bike will have no problems with using seafoam once in while like its intended. its not a cure all to an already neglected bike... but it can even help those bikes from time to time.