Had a good Gold Wing customer. He complained of an odd noise on deceleration. Sure enough, wringing the rpm up then snapping the throttle shut would, as the rpm fell below 3,000, produce a loud tapping noise before the engine quieted again at idle. So I took a cylinder head off. Couldn't see anything wrong. Then, looking down, whoa! There it was. Marks on the piston crown where it was gently closing the sticking exhaust valve. All four cylinders, it turned out. Seems the customer had been using only premium gas and as a consequence the exhaust valves carboned up so bad they were beginning to stick in their guides, as confirmed by the appearance of the valves when later removed.

Hard to believe? Not to a career mechanic. This scenario actually repeated itself almost 40 years later. This time it was a Kawasaki ZX-6R that was missing at high rpm. A handful of mechanics had examined it and couldn't come up with a cause even after replacing the ECU and eventually disassembling and inspecting the engine. The hero that day got a good look at the valves, and noticed something the others missed--excessive carbon buildup on the exhaust valves. What was happening, it was proved by oscilliscope measurement, was the ZX-6's ECU, programmed to not allow the engine to pollute, was detecting the miniscule change in dynamic cylinder pressure resulting from the carboned-up exhaust valves' slowed closing, and was turning off the ignition coils to prevent HC emissions increase, creating the loss of two cylinders at high speed. Didn't know ECUs did that, did you? Decarboning the valves solved the misfire. True story, and I often related this episode to my Kawasaki dealer students when explaining that vehicle ECUs contain things we really don't know much about.

There's more. During the early 1980s when Honda's first-generation DOHC fours were very popular, I was on the warranty contact team at Honda's headquarters in L.A. A segment of CB750F and CB900F riders who were club racing their machines were experiencing a high rate of valve failure. It was eventually discovered that the octane boosters these riders were using accelerated carbon buildup in their engines and this caused the valves to fail prematurely.

Excessive carbon formation would continue to plague these bikes and also the related model, the CBX1000. Many of the CBXs I have had in my shop for service during the past several years have suffered from enough carbon accumulation that cylinder leakdown numbers initially read at 30 to 50 percent.

At about the same time as the GL1000 episode mentioned at the beginning of this article, I disassembled a high-mileage CB750 whose owner complained of oil consumption. I did not find enough wear to account for abnormal oil use, but I did find something else. Every single oil drain-down passage in each of the pistons was completely closed up. With carbon. Seems our conscientious customer used "only the good stuff" when fueling his Honda. Nothing but premium for 50,000 miles.

Read my other articles about combustion and octane and water tuning and all of that. The bottom line is, Honda didn't make any of these vintage models to run on premium.