The 'cam chain myth' came about when the results of the Daytona bikes' failures were revealed in late 1970, after Honda won it. The truth of this design is: above about 7500 RPM, if held there for a long time (like a Daytona oval), the stock (rubber) idler wheels in the cam chain tensioner assembly can become chunked out by the impact of the chain hitting the rubber, at least on the early rubber formulations. As the rubber improved, so did this problem. Once the damage starts, the bits of rubber could get jammed into the spaces between the rollers and the pins of the chain, blocking out the oil. Then they would fail after a while for lack of lubrication, as the pins would wear thin and break.
In racing, one of the very first 'fixes' was to use a steel roller instead. Later this gave way to hard nylon rollers. In production bikes, circa 1976, Honda issued the --410-- series cam chain, which has both thicker pins and harder rollers with larger oil clearances inside, a miniature copy of the Diamond XDL drive chain that was/is uber-successful on this bike.
In all the years of working on these engines, I have yet to see a cam chain that "failed". I have seen a couple that were rusted so badly from sitting & then being started back up, that they tore up the sprockets and tensioners, which then led to their demise from the same mechanism as the Daytona racers: lack of lube. I have seen one (just one) that failed due to use of Pennzoil, which waxed the links closed like Pennzoil often does in short-haul driving, again causing lack-of-lube failure (Pennzoil/Quaker State can do this with timing chains in older Detroit engines, too). I have seen a couple where the chain broke because the rider missed a shift so badly that it buried the valves in the pistons, sorely overloading the cam to where it twisted, poor little chain gave its all.
But, in a normally-ridden engine with regular oil changes, cam chain adjusted once in a while to reduce the slack, and touring at speeds up to 100+ MPH on a regular basis (even mine), the chains do not wear out, nor break, as their load is barely 90 PSI at 8000 RPM. Someone has to break them, and it is not easy to do! Mine went 90k miles since it's last change, and it meets the Honda spec for about 55% "worn", so I changed it last summer when I rebuilt everything else.
Mostly, the (jealous) rumor-mongers started this to slander the enormous success of the 750. Kawi KZ riders used to sneer at the Honda's "reliability problem", but the Kawis spend a LOT more time in the shop being repaired or freshened up in comparison. Suzy was a little better, but no one (except possibly the pre-1973 BMW R60) holds a candle to the 750 SOHC4 reliability, not even Honda's Gold Wing.
