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Classic Velocity Blog

Carbs and Coffee

Classic Velocity

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The carburetor is a device that executes a simple concept. Mix air and fuel so that they can be passed on to the combustion chamber. Simple. In the early days, they just dumped fuel and air in indiscriminately, and hoped for the best. Even as cheap as gas was back then, it didn't take long to realize that a lot of fuel ended up coming out of the exhaust unconsumed. Smart engineers then began to devise ways to control how much fuel, and how much air would be available at any given time, for any given combustion chamber. That meant that the carburetor needed to become a bit more complicated than a funnel.

Subsequently, some fantastic designs began to emerge for metering out fuel, the Bernoulli principle was applied, and carbs gained passages and mechanical moving parts which responded to throttle controls. Engineering and metallurgy evolved, fuel became progressively more expensive, and the carb became more sophisticated. I am going to stop this story in the early 1950s which is when Bing Vergaser designed the carb on the BMW R26 that inspired today's topic. It is a simple device operated by a cable and a spring, which operates a cylindrical slide, which in turn exposes (or covers) a jet. It has a separate idle control, which is another small jet, and it has a fuel storage chamber with a float which controls when to top it up and when to stop topping it up. The only other parts are a few gaskets and o-rings to ensure good seals on some of the chambers.

The R26 is a single cylinder motorcycle, so any issues with fueling are immediately noticeable and dramatic. It is the ultimate manifestation of one fuel injector per cylinder. Once I safely coasted over to the side of the road, it was obvious that there was nothing obvious. Going by the old adage that most carb problems are actually ignition/timing, I checked the coil and plug for spark. Looked good. Points ? Looked good. I kicked it, and it ran perfectly for a half mile before dying. Then it started again. repeat. I drained the float bowl. Two more repeats got me back to the garage. The next day, I got a strong cup of coffee and went out to the garage. It reaked of gasoline. The R26 had a small wet spot below it, but it was obvious that gas had leaked onto the gearbox case, and dripped to the floor and then evaporated. The petcock was fine, so the only gas available to leak was what was in the carb. As I am sure you know, it does not take much gas to smell like a Shell tanker has crashed inside your garage.

With the second cup of coffee, and the garage aired out, I had the carb off the bike and it seemed like the float was the culprit. It had somehow taken on fluid by condensation or by a pinhole leak, and no longer weighed the prescribed 7 grams. An o-ring was also suspect. I ordered a rebuild kit and a float from Bing. The resulting package had only a few parts as you might expect, but it was the right stuff. For a moment, I thought it was surprisingly expensive for what they were. However, I am pretty sure that in 60 years, you will have no chance of diagnosing any issue with today's motorcycles unless a dashboard icon tells you what is wrong. In 60 years, there will be no rebuild kit for your 2015 motorcycle's fuel injection system, and it will cost you a good bit to print a new one at the cyber depot. While waiting, you can inhale some caffeine and download Rocky XXXVII to your neural network.

Perhaps in 60 years no one will care about such matters, but I am personally glad that I can still diagnose a 60 year old machine, almost incinerate my house, order parts not delivered by a drone, and scald myself with hot coffee.