Tasks that Motorcycling Once Required

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Once upon a time and for a few months, I rode a pushrod 500 AJS single. Part of its starting drill was to make sure that its oil pump had not lost prime (It wouldn’t pump if the oil leaked out of it). Remove the cap from the oil tank to reveal the open end of the oil return line just inside. Pull the compression release, then rotate the engine with the kickstart lever until oil can be seen to return to the tank in little blorps. With oil return verified, you may proceed with retarding the spark (to prevent kickback from damaging your leg or the arch of your foot), priming with the “tickler” atop the carburetor’s float bowl, then positioning the crankshaft at just after top center on the power stroke to give your leg maximum time to whirl the 35-pound crank. With a degree of abandon, throw yourself into the air to come down confidently on the lever. Visualize success.

The engine should start, but many alternatives existed.

A favorite photo from the 1960s shows the late Arturo Magni, then head of MV’s roadrace team, using a camp stove to preheat oil before attempting to start one of the team bikes.

Today we have multigrade oils which act thin at cold-starting temperature, yet retain sufficient viscosity for best lubrication once hot. But with old-time single-grade racing oil—possibly castor-based—the danger was that its high cold viscosity would prevent it from circulating once the engine started, leading to possible scuffed pistons or valve tappets from lack of prompt oil circulation.

Preheating such single-grade oil reduced its viscosity enough to assure immediate circulation and lubrication of critical parts.

Another photo shows the NSU team in the early to mid-1950s, using a Luftwaffe-surplus engine heating unit (really just a portable hot-air furnace) to preheat the engines of its several team bikes. Large-diameter flex ducts carried hot air from the unit to the several bikes, the idea being to warm the parts until intake tracts were warm enough to vaporize gasoline, form a fuel-air mixture, and so start easily. Systematic.

Modern bike engines just start and run because their digital fuel injection systems, having temperature sensors, automatically adjust fuel delivery for variations in air temperature. Turn the key and the engine starts. But in 1960s motorcycle GP racing, each track might have its own official gasoline, which might be av-gas, which evaporates less easily than pump gas. Best therefore to start with an engine pre-warmed by hot air or hot oil.

For a time after the coming of digital fuel injection, cold starting remained under human management by use of a manual cold enrichment system known as a choke. It enriched the mixture in the same manner as putting the palm of your hand over an open carburetor during cranking: Cutting off airflow in this crude way sucked in enough extra fuel to start (a cold-start mixture has to be enriched because the colder the air, the smaller the fraction of the fuel it can evaporate). For starting with a choke, you operated the choke lever, started the engine, and then gradually moved the lever as the engine warmed and more and more of the fuel delivered to it evaporated. This is why engines being cold-started smell of raw fuel: you are smelling the unevaporated fraction that passes through the engine unburned. And this is why the EPA up there in Ann Arbor now wants the exhaust cat as close as possible to the engine’s exhaust ports—it wants the cat to light up instantly and burn that unevaporated fraction.

Today a combination of automatic enrichment and a throttle positioner or “variable intake air leak” allows the fuel system to both enrich for cold starting and to provide an initial high idle to prevent rough running or stalling. As the engine warms, you will see idle rpm decrease as the system returns gradually to normal idle rpm.

A modern engine’s spark timing is set forever by the relationship between a fixed crankshaft position sensor and a set of poles attached to the crank and rotating with it. By knowing crank position, the digital ignition system can fire the spark at the time called out in one or more 3D spark maps carried in memory. X and Y on these maps are the two important variables “N” (engine rpm) and “Alpha,” the throttle angle. “Z,” the height from that XY plane, is the ignition timing in degrees before top dead center (BTDC). Z is determined by running engines on the dyno and finding the best value for each combination of X and Y. That info is stored in everyone’s ECU.

Until the coming of such digital noncontact systems, the firing point was determined by a cam, physically lifting a contact arm such that current flowing between two contacts or “points” was mechanically cut off. Because wear naturally occurred between the cam and a plastic “rubbing block” that was part of the contact arm, every thousand miles or so it became necessary to reset the contact gap and the point in the cycle at which the cam opened the points. The owner or mechanic set the gap between the open points using a “feeler gauge” (consisting of many thin steel leaves, each of a different marked thickness). Then the timing of points opening had to be reset to the correct spec.

Resetting contact gap on a set of ignition points was a recurring and common occurrence for a motorcycle owner in the days before digital ignitions. (Jeff Allen /)

In many cases, the ignition cam was driven by an “advance mechanism” that used centrifugal weights to allow timing for engine starting to be retarded enough (occurring later in the cycle by a few degrees) to prevent kickback. Once started, with the engine accelerating into its power range, the centrifugal weights would fly outward against their little return springs, moving the cam to a more advanced position correct for operation above idle.

Another refinement, more often used on car engines than on motorcycles, was “vacuum advance.” The smaller the throttle angle, the less dense the fuel-air charge becomes (a “partial vacuum”), and the more slowly it burns. If spark timing were fixed, the result would be later occurrence of peak combustion pressure—resulting in poor fuel mileage. Therefore an intake vacuum sensor was provided (we used to see these on the sides of auto engine ignition distributors) that would advance ignition timing in response to high intake vacuum.

Today those functions are built into the spark maps of digital ignition systems.

Another part of the 1,000-mile tuneup was inspection and regapping of the spark plugs. The gap between the spark plug’s center wire and its ground electrode had to be small enough that the ignition’s energy could jump the gap. Hours of operation, however, caused the gap to grow as spark energy evaporated metal from the very hot center wire and ground. If the gap grew large enough, spark blow-out and misfiring resulted.

During the tuneup the plug gap(s) would be measured and reset to specification. Or, if the gap electrodes looked well rounded off, the plug(s) would be replaced with new.

In modern ignition systems, gap erosion has been all but eliminated by limiting the current allowed to flow across the plug gap(s), and making the plug’s center wire of a high-melting-point metal such as iridium (spark plug electrodes were platinum-tipped in  World War II–era large aircraft piston engines).

All these changes have added up to the low-maintenance motorcycle engines of the present.

Back when 98 percent of motorcycle engines were air-cooled, oil changes were required more frequently. With air-cooling (and in the absence of a cooling blower) engine temperature goes up and down with the weather. This means that cold-engine commuting allows combustion products (moisture and acids) to build up in engine oil, which never becomes hot enough to boil them away. The hotter the weather, the hotter the oil—perhaps hot enough to prematurely degrade the temperature-sensitive additives in it. In either case, the “remedy” was more frequent oil changes.

Modern liquid-cooled engines operate at constant temperature, taking the sharp edges off the problems described in the previous paragraph. Oil lasts longer.

Less wrenching = more time for social media. Progress?

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