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Engine Timing

The two areas of concern in regards to timing on a 4 stroke engine are the ignition system and valve train, which we will try to address in this section.

Ignition Timing
Ignition timing is simply the distance between the leading edge of the flywheel magnets, in relation to the position of the ignition coil and piston travel. Modern 4-stroke engines generally do not have an adjustable timing feature. Instead, they rely on the flywheel key to keep the timing set in a fixed position for purposes of optimum ignition timing.

Although modern electronic ignition systems are generally very durable, there are a couple of potential problems that you need to be aware of:

Air Gap
The air gap refers to the space or "gap" left between the flywheel and ignition coil. This "gap" is calibrated to a specific thickness (or range) by the manufacturer and must be maintained for proper ignition timing and optimum performance. Variation from the manufacturer's recommended specification will often result in weak spark and poor performance.
 
Damaged Flywheel Key
This type of problem is most often seen on rotary mowers after the cutting blade impacts an unseen object. In this situation the flywheel key becomes sheared (cut in half or partially sheared), which causes the ignition timing to change considerably (advanced timing). In extreme cases, the flywheel and crankshaft may also be damaged, so a good inspection is in order in this situation.
 
The most common side effects for a damaged flywheel key are engines that are hard to start, won't start or those that "kick back" when trying to start. Even a very slight amount of damage to a flywheel key can create big problems in regards to ignition timing and replacement of the key is almost always required.

Valve Timing
The timing of valve train components should also be considered in relation to ignition timing and piston travel and in this case it has more to do with "when" each valve is being opened or closed.

For example:
An intake valve that opens too soon will allow combustion to escape (pop back) through the carburetor and when opening too late, it will not allow enough fuel mixture into the combustion chamber. In both cases the engine will suffer from performance problems or maybe not run at all.

An exhaust valve that opens too soon or fails to close fast enough will cause a loss of compression and subsequent performance loss.
 
Valve Adjustments:
Unless there is an internal problem with the engine or damage and/or excessive wear to the valves or other external valve train components, adjustments to valve timing is simply a matter of adjusting the gap or "lash" between the small end of the valve and whatever component directly causes it to open. (Valve tappet or rocker arm in most cases.)
 
Generally for the non-professional, adjustments to valve timing on overhead valve engines (OHV) are pretty straight forward and easily accomplished with a feeler gauge and a few common tools. However, because of all the differences from one manufacturer to another, you should always consult a repair manual for the specific details for your engine.

Although sometimes "do-able", for non-overhead valve engines you will probably be better off taking the engine to a shop due to the complexity and common need for specialty tools.
 

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