Mixture Maladies and Maldistribution

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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/)

Because modern motorcycle engines are computer-controlled and carry one or more exhaust-gas oxygen sensors to keep mixture always correct, many of us have never experienced either the popping-back-and-throttle-hesitation of lean mixture or the black-smoke-and-runs-heavily of rich operation.

Mixture maldistribution is the condition in which the same fuel-air ratio is not supplied across all cylinders, leaving one or more operating lean, maybe a couple just right, and one rich.

During the 1990s’ transition from carburetors (rich in summer, leaner in winter) to digital fuel injection, at Daytona practice I would notice the Yoshimura Suzukis had a single oxygen sensor on the exhaust collector pipe, but solid plugs in each of the four head-pipe sensor stations. That told me these engines had run on the dyno with four oxy sensors so that all four cylinders could be “trimmed” to the same mixture, after which mixture at the track could be controlled by a single sensor.

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Especially tough is to get the correct mixture to all cylinders when fuel is supplied from a single carburetor, as in NASCAR. Pull the four-barrel carb off of a classic American V-8 and hike yourself up on the fender to stare down into the intake manifold. What are all those funny little vanes and other shapes cast into the bottom of the intake manifold, pointing in various directions? They result from the engine development team attempting to make all eight cylinders pull their weight, without one or more giving weak power because the merry little breezes in the manifold happen to carry more fuel to this one, less to that one.

Much of the fuel streaming downward from that four-barrel is in the form of droplets, and when they hit the bottom of the manifold they form a flowing fuel film called “wall wash.” Those little vanes act as rudders to steer a bit more to the weak cylinders and less to the rich ones.

More Power, More Trouble

Fuel maldistribution becomes very serious business at high power. On my first visit to Bonneville I savored the strong, resonant, high-rpm sound of a V-8 roadster accelerating into its run. That sound mesmerized even salt veterans (old salts?), who stopped working to stare and listen. Often, just after the upshift to fourth, the sound would cut: The engine, pulling hard in top gear, had holed a piston. When not given enough fuel, the oxygen in the air burns anything it can find.

At one time, Ford reportedly had a fuel distribution problem so severe that the number-seven cylinder in its NASCAR engines was producing almost no power.

Maldistribution can also kill. It’s spring of 1945, and the B-29 next in line for takeoff is waved off down one of Tinian Island’s four parallel 8,500-foot runways. The pilot eases the four throttles forward with his left hand, and the plane accelerates its load of 20,000 pounds of bombs and 6,700 gallons of sweet-smelling aviation gasoline. The right-hand seat calls speeds and all is normal. Then they hear the dreaded blump-blump of backfiring from one of the four engines—some R-3350-23a radials had large mixture variations, cylinder-to-cylinder. Lean mixture burns more slowly than normal, sometimes leaving residual flame in a cylinder all the way to the next opening of its huge intake valve. That cylinder blows fire into its intake pipe, setting it aflame, rapidly burning aft to the magnesium supercharger diffuser, where the flame lights up the other 17 intake pipes. Taking in fire instead of fresh mixture, that engine ceases to produce torque. This is an induction fire.

The aircraft, now just a few feet above the runway, yaws and rolls toward the failed engine. The pilots instinctively make this worse by trying to lift the wing and fly straight with aileron and rudder, but the effect is more drag. The airplane, barely flying on three engines, settles back onto the runway and breaks up.

Help was on the way, in the form of Bendix direct-cylinder fuel injection (today Detroit calls it GDI, for Gasoline Direct Injection), able to deliver equal mixture to all cylinders. With this system, induction fires are impossible because there is no fuel in the intake pipes. But it can’t help this aircrew, whose only hope is to get out and away from the aircraft as it burns on the runway. Crash trucks and the runway-clearance dozer are coming. This was told to me by a man who was there, our late friend and lifelong Indian enthusiast Bob Nichols.

Fuel Cooling

Since the earliest days of motoring, rich mixture (too much fuel in relation to air) has been used to improve reliability in poorly cooled engines. It does this by keeping pistons cool enough that they don’t outgrow their cylinders.

In the late 1930s, DKW raced supercharged two-strokes. Even with liquid-cooling, the piston controlling the exhaust port ran extra hot, so to help it go the distance, mechanics enriched the mixture. At the Isle of Man this was a problem, because there can be big temperature differences along each 37.7-mile lap. As the bike ran into cooler air, the mixture would move toward best power, generating more heat… That often caused a seizure.

My own experience with air-cooled Yamaha TD1s of the mid-1960s was that the jetting that gave the fastest first three laps soon caused overheating that made following laps, and overall race time, slower. It became normal to jet two sizes richer than best power. Why the power loss as the engine heated up? As the crankcase and cylinders got hot, they heated the air rushing through them more, causing it to lose density—making reduced power. This is why Pro Stock motorcycle drag racers place fans to blow on their engines when they come back after each run; a cool engine makes more power.

In the early 1960s, when piston metallurgy remained fairly primitive, Suzuki’s GP race engineers described their pistons as “swelling like cakes” when hot, resulting in a discouraging number of unpredictable seizures. Why not just jet rich like DKW? They did, but just as with the “Deeks,” when a gust of cooler, denser air rolled across the track, power and heat production would increase and a seizure might occur.

Reading Spark Plugs

In the carburetor era it was the engine tuner’s task to prevent mixture maldistribution and give each cylinder the mixture it needed. This was done by “reading” the spark plugs and then using the mixture information they gave to jet each cylinder as it required. In the case of Kawasaki’s H1-R production roadracer of 1970-71, the center cylinder needed the least fuel (300 main jet), the left cylinder the most (320 MJ), and the right a 310. Why the difference? A 120-degree inline-triple oscillates like the motion of a double-bladed kayak paddle. Because the gearbox was shifted to the right, the left cylinder whirled in a bigger circle than the right one, and the center, moving almost not at all, needed the smallest jet. Vibration, whipping the carburetors around on their rubber sleeves, shook fuel to the back of each fuel bowl. You can see this on an engine equipped with Lectron carbs, which have transparent plastic bowls. As you rev the engine, the fuel climbs the back of the bowl, leaving less fuel height to push fuel through the submerged main jet.

The young Glenn Curtiss won races on the single-cylinder motorcycles he built by hand. When he needed more power he hinged a second connecting rod onto the big end of the first and added an additional cylinder, making a V-twin. The naturally irregular firing order made the second cylinder to fire run rich, as the first cylinder had started fuel flowing through the carburetor. His rough-and-ready solution was to drill a hole in the intake pipe of the rich cylinder; this let in air to correct the mixture. Triumph twins would later be given an even 360-degree firing order, allowing a single carburetor to supply the same mixture to both cylinders.

Today, the onrush of technologies able to unfailingly supply the same mixture to all of an engine’s cylinder relegates all of the above to near-fairy-tale status. Yet the more you know about what those technologies are doing, the more you will appreciate them.

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