Getting Started

So, the day is going well. You've finished three mowing jobs on time and it's only noon, when suddenly, your mower quits. Stone cold quits, just like you shut off the switch. Now what?

There are several things that could cause such a problem, but time and space limitations prevent covering all of these. Instead, this article will cover the likely cause of a sudden-death engine stoppage where it cranks but will not start — revealing a problem in the ignition system.

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Ignition systems are electrical in nature, which puts them on the list of “scary-things-I-don't-wish-to-work-on” for most of us. While it is agreed that there are things best left to professional outdoor power equipment technicians, there are some things you can save time on by checking in the field before taking your machine in for repairs.


If your engine won't start, first check the fuel and oil levels, then check for spark. One way to do this is to remove the spark plug and look at its condition. If it is soaking wet with fuel, you may have an ignition problem, but it could also be a fuel problem. For example, if the inlet needle in the carburetor is stuck, you will have too much fuel going into the engine (a fuel problem). Likewise, if there is a loss of compression, you would have the same sort of experience.

Check the gap between the electrodes. If it is non-existent or if the plug is closed up, there isn't a gap for the spark to jump. If so, you've found your source of trouble. If the plug looks like something may have struck it, pack it in — you'll need to seek help from a professional.

Should you find that the plug is oily or greasy, changing it may allow you to run your machine for a while, but oil on the plug means oil has been getting into the combustion chamber and that can mean major trouble.

If the plug is reasonably clean and still has the proper gap between the electrodes, you may want to do a field spark test. Take a spare plug and hook it to the plug lead. Install the old plug back into the engine. If there is spark, and you have fuel present, you will have a fire- so put the old plug back in to prevent any fuel from being sprayed onto the new plug.

Ground the new plug against the engine block or frame, make sure the choke is off and crank the engine over. If you see a bright strong spark, your problem may be elsewhere. Install the new plug and try to start the engine. If you don't see a spark or see a weak spark, the engine needs the attention of a mechanic.


It can't be emphasized enough that not having a spark does not necessarily mean you have bad ignition components. As there are several types and manufacturers of equipment, there are also several ways that the ignition could be wired in with safety switches and the like so the wisest course of action is to have a repair manual for your equipment or the services of a good dealer who is familiar with your equipment. Many a customer has blindly shopped based on price not considering items like service after the sale and has lived to regret it. Your best source for any equipment you intend to make a living with is a local independent dealer — they generally sell high quality equipment and they are more familiar with the inner workings of it than a box store manager.


In the case of two-cycle engines such as those found on trimmers and blowers, there are no safety switches to deal with, but the ignition system has gotten more complex on these units as well.

For example, at least one major manufacturer started using an ignition coil with built-in clock and adjustable timing (by the coil not by the operator.) At cranking speeds, the internal clock realizes the slower rotation and tells the coil to retard the ignition. Once the unit starts, the timing adjusts and advances for maximum power.

This is a long way from points and a condenser to be sure, but some troubleshooting basics are worth remembering. Unless you have a two-piece ignition, the only function of the wire hooked to the ignition coil is to ground it when you shut off the unit meaning that a common troubleshooting tactic is to unhook this wire and then try to start the engine. This is not a good idea unless precautions are taken and certainly is not a permanent option, but it will tell you if the problem resides in the ignition system or the shut off circuit.

If you have a two-part ignition system that uses both an ignition coil and a trigger, consult your local dealer. The diagnostics for such an arrangement vary from manufacturer to manufacturer, so there is no one solution to be shared here.

If you should find there is no spark and that safety circuits are not to blame, try one more avenue before buying a new ignition system: Remove the coil, sand the place where it mounts, sand the backside of the coil where it meets the block and reassemble. Be sure to leave at least 1/10,000 gap between the coil and the flywheel — this varies by manufacturer. Failure to allow enough clearance will result in a horrible failure and broken components, no matter who the manufacturer is. The base of the coil must be properly grounded or it will not fire. Many a new coil has been bought by those who overlooked this important step.


Finally, here are a few myths about ignition systems that are not true. First of all, it does not matter if the flywheel is rusty. Magnetic fields (the very source of power for our ignition system) are not affected by rust. While it may ease your mind knowing that your mower's flywheel has no rust on it, it will not make it work any better.

Secondly, the color of spark is no indicator of the strength of the ignition system. I know many of you hold the belief that the color of spark should be a “hot blue,” but the fact is the only proper way to test any ignition system is with a proper tester. The color of spark produced has more to do with the elements present in the air than in the strength of the ignition system.

The modern ignition systems we see in use today rarely cause trouble, but when they do, the services of a competent and knowledgeable repair shop are worth more than they will ever charge you. While doing it yourself is tempting, time is money and a service technician who knows his stuff will save you a lot of money.



Because ignition systems serve a rather important part in the functioning of an internal combustion system, it is necessary to go over the basics of how they work. When the engine has compressed the fuel/air mixture, at just the right time, a spark is produced across the gap of the spark plug, which ignites the mixture, driving the piston down and producing power. No spark means that you either have a diesel engine, or that you have a gasoline engine that won't run. In the typical small engine, the ignition system consists of an ignition coil (with the spark plug wire attached) and a grounding circuit for shutting off the engine. The flywheel of the engine contains a series of magnets that, when they pass by the ignition coil, induce voltage. What takes place next is the circuit within the coil is broken, usually by a transistor or semi conductor, which causes the magnetic field to collapse into the ignition coil. As it collapses, it increases geometrically in strength, developing the several thousand volts needed to jump the gap of the spark plug and make the engine run. Years ago, breaker points were used for this purpose, but they have given way to more modern types of controls such as transistors and micro-processor controlled systems. The result is that modern ignition systems are much more reliable than older systems; but with no replaceable wear parts, their replacement cost is higher. Therefore, performing a full diagnosis before blindly buying parts is a good idea. There are several variations on this theme (thus the need for repair professionals), yet one constant remains: All equipment produced in the last several years has a safety system, which often is directly connected to the ignition system. An example of this would be a seat switch, which shuts off the engine if the operator leaves the seat.

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© 2015 Penton Media Inc.

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