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9. Powerplant

Competitive race bikes are now available with two distinct and very different engine types - two- and four-stroke - that require very different levels of expenditure to rebuild. This reality is that because of the extremely high levels of power output, the new generation of four-stroke race bikes not only requires maintenance at least as often as (probably more often than) current two-strokes but also costs more. Whereas a top end rebuild of a non-exploded two-stroke will cost a couple of hundred dollars, a top end job for a four-stroke race bike is even more. A four-stroke with a season of racing will most certainly need valve replacement and a new piston. If this is not done, chances are it will blow up or worse and easily require as much as a couple of grand to get to running condition.

There are two quick ways to check the top end of a two-stroke. First, feel how much compression is available with the kickstart lever. There must be a very noticeable resistance, right at top dead center; and if there isn't, steer clear. Second, if you can talk the seller into letting you pull the exhaust pipe, look in on the piston through the exhaust port. With a flashlight, check the condition of the piston. There should be no vertical marks, and especially no shiny aluminum buildup, indicating a previous seizure. Note that this isn't definitive, as the piston can be seized on either side of the port and not visible unless you pull the cylinder; but you can get an idea of the wear and maintenance on the bike by assessing the condition of the piston. Dirt that has entered the engine will also show vertical marks; and a smooth, shiny surface on the front of the piston indicates a lot of use.

On four-strokes, starting is the key indicator. As the engine gathers hours, the intake valves especially will tend to recess into the seats, with the sealing surface of the valve mushrooming out. This will decrease the vale clearance and eventually will cause difficulty in starting because of the tight valves. Also look for blue or black smoke exiting the exhaust when revving the bike. Blue smoke is the result of oil getting into the combustion chamber and indicates worn rings or valve guides and seals; and it means the top end is badly in need of service. Black smoke points to an overly rich fuel mixture and may also signal a worn top end, as the rich fuel mixture tends to wash the oil off the cylinder, speeding wear on the piston and rings. Check out the top end as closely as you can, since it is not cheap to repair.

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