There are many individual components on a motorcycle that can, if broken or damaged, stop the engine from starting. But in essence, an internal combustion engine needs three things before it will run:
- Fuel (a mixture of gasoline and air in the correct proportions—approximately 14 parts air to one part gasoline)
To check the fuel flow, a mechanic should remove the carburetor float bowl drain screw (where fitted); however, he or she should be extremely cautious, as gasoline is of course flammable. Many carburetors produced after 1970 have a line attached to the drain plug for this purpose. Checking the fuel is flowing in this way will also ensure it is getting into the carburetor. Once fuel has entered the carburetor, the level is controlled by a float acting on a tapered needle valve.
Problems associated with the fuel level include damaged or leaking floats, incorrect float height settings, and a sticking or dirty needle valve (typical fuel will run out of the overflow tube if the valve is stuck open). Incorrect float height settings typically affect the mixture and therefore running efficiency of the engine rather than interfere with the starting process.
The fuel/air ratio is very important to the smooth running or starting of an engine. Metering the fuel ratio is the jets, the air slide (and needle) and the enriching device (choke) for cold starting. Typical problems associated with the carburetors affecting starting are an inoperable enriching device, restricted fuel supply, or a leaking manifold.
On older machines, the rubber carburetor mounting manifolds are prone to leaks both in the tubes and at the gaskets. Spraying WD40 onto the rubbers when the engine starts will prove there is a leak as the engines speed will generally increase.
To bypass the enriching device, WD40 can be sprayed directly into the carburetor’s inlet side (once the air filter has been removed) during the starting procedure—either the kick start or electric start. However, WD40 is of course flammable. Therefore, the mechanic must exercise extreme caution when trying this.
On multi-carburated motorcycles, the carburetors must be balanced or synchronized. Once the motorcycle has started, if the choke needs to be on slightly, the primary jet is either partially or completely blocked.
Adequate compression of the fuel-air mixture is essential for good starting and running characteristics of an internal combustion engine. The compression pressure varies from model to model and also between 2-strokes and 4-strokes. However, cranking pressures of less than 90 lbs. /sq. inch generally indicates an internal problem. However, the mechanic must establish the manufacturers recommended pressure before deciding on any corrective course of action.
Poor compression pressures on 2-strokes can be caused by damaged/broken piston rings or pistons, leaking cylinder head or cylinder gaskets, and leaking or damaged crankshaft oil seals. Note: Before experiencing poor starting characteristics, the owner/rider may have noticed excessive smoking from the muffler when the crank oil seals were becoming worn.
The compression pressure on a 4-stroke is controlled by valve timing, the seal between the valves and their seats, valve clearance adjustment, pistons and piston rings, and the cylinder head gasket.
To determine the cause of poor cranking pressure, the mechanic must carry out a leak-down test.
Poor starting is often caused by a dirty or faulty spark plug, particularly on older 2-strokes. As this is one of the easiest checks, the mechanic should remove the plug and carry out a spark test by laying the plug onto the cylinder head then turning the engine over with the ignition on.
However, extreme caution must be exercised during this procedure as the spark can ignite any mixture ejected from the open cylinder. The spark is created by high voltage electricity and can shock the mechanic, and besides an explosion or fire risk, any ejected fuel can damage the mechanic’s eyes.
Note: Although a spark plug may produce a good spark on the outside of a cylinder, it may not be sparking under the extreme conditions when fitted. Having a spare spark plug (one that has been previously tested in a running engine) is good practice.
If the plug sparks properly (a crisp blue spark is good), the mechanic must check that the spark is occurring at the correct time—controlled by the ignition timing. Depending on the ignition type (contact points or fully electronic), the manufacturers will have specified the correct timing point of ignition. This timing point is either in degrees before TDC (top-dead-center) or a measured distance. (Calculating the measured distance from TDC is simply a case of calculating the number of degrees versus the piston movement derived from the crankshaft stroke).