Project Two 50 is sponsored by and uses Amsoil... this is fantastic stuff!!
A few things to consider when changing fuel/oil mixtures is to ensure that the jetting is set up correctly for that mixture. This is probably the most important aspect of running lean mixtures.
When they used the new fuel/oil mixture using Amsoil, did they completely drain out the old fuel/oil mix? Many time leaving older mixtures in your tank/carb can cause this kind of problem as well.
The other thing to consider is the current state of the engine. Is it fresh? Was it rebuilt in the 1990's? What kind of riding do you do? As you can see, lot's and lot's of questions.
Currently in the Project Two 50 machine we are running Amsoil Dominator at 100:1... but know that we have had some special coating applied to all the engine internals.
The bike in this video is running at 100:1
http://www.facebook.com/video/video.php?v=125358884150676#&comments=So far we have over 3 hours of ride time on this machine at 100:1
Will an older air-cooled machine run at these fuel/oil ratios? It's unknown, but before I would mix fuel that lean, I would make sure the bike was fresh and the jetting was spot on.