The Harley-Davidson CAC 500cc single port speedway model is very rare and not much is really known about its history. One came up for auction in Las Vegas a few years ago and the owner talked about it to Rick Newlee. It appears that there were 12 of these machines built, the whereabouts of 9 of them is known but nobody is sure what happened to the other 3. It appears that H-D factory rider Joe Petrali was the man instrumental in getting these machines built. Joe had more or less begged Harley-Davidson to build the bikes, but they were not really interested. Joe hung around the H-D plant and pestered them until they gave in to Joe's wishes on the condition that he built it himself (or hired his own people). Other conditions were that he could only use the plant on weekends and he must leave the workshop in good clean order come Monday. He also had to put the Harley-Davidson name on the bike (just in case it became a success!). After a few of the CAC machines were built and tested, Joe tried to persuade Harley-Davidson to put the CAC machines into production for a second time, however the tests did not go as well as expected and the engines didn't make the power he had hoped for. It appears that Joe had copied the British J.A.P. engines to some degree, but had a problem with the cams and the cam timing. Harley-Davidson decided not to get involved and the project ended.
Joe Petrali, also known as "Smokin' Joe" to others, left a huge mark in the world of motorcycles. His career in motorcycles spanned from the mid 20s on into the mid to late 30s riding in everything from board track races, dirt track races, land speed records and hillclimbs. Joe is part of the AMA motorcycle Hall of Fame and a full account of his successes are available to read here
Joe Petrali on his factory Harley-Davidson racer in 1925
I recently found this publicity photograph of Walter Davidson sitting on a CAC at the model launch.