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Diagnosing small engine problems

How to deal with that cranky 
auger or lawn mower engine

Getting small engines running again after winter storage may require a little effort.

Winter is virtually over — we hope. Soon it will be time to pull the lawn and garden equipment out of storage. And maybe that auger with the small gas engine that didn’t see any action during the cold weather will soon be needed to move seed or fertilizer.

How much frustration and arm fatigue from pulling on starter ropes those small engines dish out this spring depends a lot on how well they’ve been maintained. And importantly, how they were put to bed last fall.

The instability of modern gasoline is often the cause of small engine woes after long periods of inactivity. Leave some in the tank and carburetor for a few months and that engine will likely refuse to start. So if your small engine refuses to cooperate this spring, here’s a look at some of the basic troubleshooting steps necessary to get it running again.

It’s often said the eyes are the windows to the soul of a person. When it comes to a small engine, the spark plug might be the window to its soul — figuratively speaking. If a small engine won’t fire, the best first step is to pull out the plug and take a look at it. Its condition will often suggest the source of the problem and the overall condition of the engine as well.

Checking the condition of a plug can help diagnose an engine problem. This one has some carbon build up on it, but the nose and electrode are dry after repeated cranking, indicating no fuel is reaching the combustion chamber.
 photo: Scott Garvey

If the engine has been cranked over several times and the plug is bone dry, that suggests no fuel is reaching the cylinder, which is the most common problem for small engines that have been sitting for a while.

Of course, always check for the simplest things first. Is there fresh fuel in the tank? Disconnect the fuel line at the carb to make sure fuel is getting to it. Pulling off the carburetor float bowl will reveal if there is fuel inside it. Some float bowls have a drain. If so, just open that. If fuel is making it that far but not into the engine, the main jet in the carb is probably gummed up from old, evaporated fuel.

Diaphragm carbs on things like weed trimmers and chains saws are different, though, and don’t have a conventional float bowl. If there is fuel at the inlet, it’s clear the carb isn’t functioning properly.

Chain saws and weed trimmers use a diaphragm carburetor that pumps fuel through it allowing the engine to operate at extreme angles that would interfere with the operation of a conventional float bowl carb. photo: Scott Garvey

But in either case pulling off the carb and cleaning it with carb or brake cleaner and some gentle blasts of compressed air will often solve the problem. If not, the carb will need to be fully disassembled and put into a cleaning tank.

Having a carb kit containing new gaskets on hand will sometimes be necessary to get it properly buttoned back up.

A very wet spark plug indicates the engine is probably flooded. Wipe the plug dry, or better yet replace it, turn the engine over a few times to purge any excess fuel, reinstall the plug and you could be back in business. Before torqueing the plug back in, though, it’s easy to double check that the ignition system is supplying spark. Just rest the plug along the cylinder head to ground it and turn the engine over. (Make sure it’s well away from the opening. Fuel being expelled from the cylinder could be ignited by the plug and turn your lawn trimmer or other engine into a momentary flamethrower.)

Grounding the plug on the engine head away from the cylinder opening and turning the engine over will reveal if the ignition system is creating spark. photo: Scott Garvey

If there is no spark, try replacing the plug. If that doesn’t help, check that the air gap between the magnet on the flywheel and the ignition and trigger coils is correct, as a rule of thumb that’s going to be somewhere around 12/1000 of an inch. If you don’t have a feeler gauge handy, that’s about the thickness of a standard business card. Coil positions can be adjusted with a couple of screws to correct the gap.

If there is still no spark, the coil pack may be at fault.

Technology hasn’t overlooked the small engine industry either, and a solid-state electronic module that houses a capacitor controls some ignition systems. If there is voltage going into the module but not coming out, replacing it is the only alternative.

To keep two-cycle engines running properly, it’s important to get the fuel-oil mixture right. If the manual calls for a 50:1 mixture ratio, make sure that’s what you blend. In this case if a little is good, a lot isn’t a lot better. Too much oil in the fuel will form excessive carbon deposits inside the cylinder. And that can raise the compression due to the amount of build up on the piston, which reduces the effective size of the combustion chamber.

Extreme carbon build up on the top of this piston from a two-cycle engine was created by excessive oil content in fuel mixtures. photo: Scott Garvey

Eventually, that can kill an engine. Basically what happens is the excessive heat from too much compression ignites some of the fuel prematurely. Eventually the spark plug also ignites some of the fuel, and as the two flame fronts collide it creates a detonation that can damage the engine over time.

If the engine is going to be stored unused for several months, adding fuel stabilizer is a must — or drain the tank and run the engine until the carb empties out and it stops. But for small two-cycle engines with a diaphragm carburetor, it might be best to leave stabilized fuel in it. That can help prevent the delicate diaphragms from drying and cracking over time.

A speciality fuel like Aspen can be left in engines for several months without creating problems. photo: Scott Garvey

About the author


Scott Garvey

Scott Garvey is a freelance writer and video producer. He is also the former machinery editor at Grainews.



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