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Guide To Four Stroke Motorcycle Engines, Part 6

Updated on December 31, 2009

Four Stroke Engine Service & Repair

The first step in any repair job should be to identify the real existing problem and its cause. Spend a few dollars for the specific shop manual for your machine. The specifications and the procedural information it contains will speed the repair process considerably. The successful repair is one that not only fixes the damage, but lessens the likelihood of its recurrence. Therefore, the good mechanic begins his task with a thorough diagnosis.

The question that immediately presents itself along with the machine is: "Will it run?" If not, then are the ignition and fuel systems operating properly? Is the valve adjustment correct? Is the fuel fresh? If all of these check out, then it's time for a compression test to determine the basic engine condition.

Compression Test

A simple compression test is performed when you try to start the engine with the kick starter. If you notice that there is virtually no compression resistance to kick through, especially when you already suspect a compression problem, there's a good chance your gauge readings will be low.

Remove the spark plug and insert the gauge into the plug, hole being careful not to damage the soft aluminum spark plug threads. With the gauge in place, open the throttle and kick the engine through about five times, or engage the electric starter for a 2- to 3-second burst.

If the engine shows less than 90 to 100 psi compression pressure, you should look further into the upper end for your troubles. Begin by squirting about a teaspoonful of clean engine oil into the cylinder and rotating the engine several times to distribute the oil around the rings. Now you're ready to take another compression reading with the rings temporarily sealed.

If your readings jump up to 120 to 190 psi, you know a re-ring job is in order. If the reading remains extremely low, however, you know that compression is probably escaping through the head gasket or valves. In either case, if you have taken a careful compression test, with the valves adjusted properly and throttle open and you still get low readings, you know the top end should be disassembled and repaired.

Cylinder Leakage Tester

Another type of compression test employs the cylinder leakage tester. This device pumps air into the cylinder and isolates the problem by tracing down the escaping air. Note: Never use any sort of leakage tester on a two stroke engine as the crankshaft end seals will surely blow out.


Sometimes the compression readings are good, but the engine is noisy. Often these noises come from cam chains, primary chains, rattling clutch hubs, or loose valve adjustments. Eliminate these areas of potential noise by inspection and adjustment before dismantling your machine for an upper-end overhaul.

The tone of the noise is usually a hint of what is causing the problem. Ticking and light tapping noises indicate that the offending part is small, perhaps a rocker arm or a valve. Heavy slapping and clunking suggest heavier parts, such as pistons, rods, crankshafts, and clutch assemblies, as the offenders.

These heavy slapping noises may also come from cam or primary chains slapping against engine cases. Constant grinding or "whirring" noises indicate the problem is occurring at a rotating shaft rather than a reciprocating part. Hissing or puffing sounds suggest compression loss through leaking gaskets. Such leaks must be caught early and corrected before the sealing surface is damaged by blow-by, which mean re-machining or replacement.

Continued in Guide To Four Stroke Motorcycle Engines, Part 7

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