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Guide To Four Stroke Motorcycle Engines, Part 5

Updated on December 31, 2009


The time at which spark occurs is critical to the performance of the engine. Ignition timing affects engine temperature also. Timing that is very advanced or retarded causes very high piston temperatures. A spark plug with too hot a heat range can cause the same problem. These high piston temperatures can lead to a burned piston crown or a seized engine. Keep ignition adjustments and spark plug selection in the recommended factory range or be prepared for upper-end disaster.


As with any other variable, there are hazards to avoid when setting and resetting the air-fuel mixture. A lean charge creates high engine temperatures with excess oxygen around to burn the red hot engine parts such as pistons and valves. A rich mix causes deposits of unburned carbon, dilutes engine oil, and fouls spark plugs.


An engine that is set up with tight piston-to-wall clearances may overheat and seize up some hot day. This dangerous condition occurs when the piston expands to more than the diameter of the cylinder and stops its travel immediately. This seizure locks the rear wheel and sometimes causes the rider to "go down." To avoid this problem, break in a new motorcycle carefully and check the clearances thoroughly before reassembling upper ends. An engine that has been assembled with too much clearance can cause problems too. Loose pistons will rock in their bores, resulting in poor ring seal and possibly cracked skirts. Valves require a certain clearance in their guides to operate properly also. Too much clearance leads to rapid wear and bell mouthing the guide.


Obviously the lack of lubrication will cause quick and extensive engine damage. Improper lubricants can also harm engines. If oil is too thick, it won't reach the critical areas of a cold engine fast enough and rapid wear will result. If oil is too thin, it won't have sufficient viscosity to permit the pump to maintain the proper oil pressure. Use the weight and type of oil the manufacturer recommends, and avoid extremes in oil viscosities to insure long, reliable engine life.

Rider Abuse

The two general types of rider abuse are "flogging" and "lugging." Both damage engines.

A rider flogs his machine by constantly over-revving the engine, shifting too fast and too hard, and using drag race take-off techniques. Since a street engine wasn't built for this kind of treatment, its cast pistons may crack, or they may contact a floating valve. At high rpm broken parts tend to break other parts around them.

"Valve float" is the biggest danger to a flogging rider. Weakened valve springs, changing valve clearances, and worn guides contribute to valve float at high rpm. The valve springs simply cannot return the valve to its seat in time to get out of the up-coming piston's way. There's a collision: possibly a broken piston and valve, or a totally ruined engine. The harder a bike is ridden, the more frequent checking and careful maintenance it must be given.

"Lugging" is another story. The rider who lugs his engine is the fellow who putts around town at 1800 rpm in high gear with two up, resorting to a down shift only when chain snatch and engine bucking make it imperative.

All motorcycle engines with the exception of some of the over 1.5 liter v-twins are made to operate efficiently at higher rpm than the family sedan, so keep it buzzing freely in the ranges the factory recommends.

While there's no need to keep the tach needle on the red line, don't force it to do hard work at low rpm. For normal riding, a good shift point would be about 50% of red line rpm or when the engine sounds "half-way wound up."

Lugging leads to excess carbon, overheating, and bearing wear. In addition, it damages the chain, the sprockets, and the clutch assemblies.

Continued in Guide To Four Stroke Motorcycle Engines, Part 6

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