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If you are having a spot of trouble with your small generator or mower, here are some simple things you can try before you start stripping the entire machine, or send it off for someone else to strip for spares. As always with machinery, it is best to have an assistant, even if it just pulls on the starter cord for you once in a while...

It starts

Easily...and dies

Check the spark plug and HT cord. The vibration of small machines tend to loosen the spark cable, letting in dust and moisture.

Clean up and make sure the cable clips onto the sparkplug securely.

If the choke is still closed, it may be simple flooding. Either leave to stand a few minutes and try again, or remove spark plug and spin motor a few times to dry out the piston. A wet spark plug is a dead give-away for flooding.

Make sure there is enough oil in the sump. Some engines will allow you to start with a low oil level, then activate the oil sensor when it warms up.

Check fuel flow.

The carburettor stores a small amount of fuel in the float chamber. This is often enough to get the motor started, only to disappoint and confuse you when it suddenly splutters and die, for ‘no reason’.

Check that the tank’s stop-cock is open, the pipes are all connected, not bent sharply enough to block, and never must they leak.
A dirty fuel tank cap can cause a vacuum in the fuel tank. Open the lid enough to let air is, a sucking noise when you open the tank is a clear symptom of this.

Runs with a splutter

Because you keep your air filters clean, the fuel flow is always your first suspect. There are a number of places where the fuel is controlled, measured and dispensed, and the slightest dirt at any of these points will cause trouble:

Make sure the carburettor is fitted tightly and securely, with no rattling. Tightening nuts usually behind air filter cartridge. Look for damaged packings (gaskets) between the carb and the intake.

If possible, undo the fuel line where it enters the carburettor. Good flow here means the tank and all surrounding systems work. Refit fuel pipe. Poor flow indicates obstructions or dirt in the tank, filter, pipe.

Open air filter while running. Hold your hand close to the intake. Can you feel air blowing? Can you feel tiny drops of fuel splattering? Your valves are suspect, see the section on Cylinder Head.

If the fuel flows, and your engine is not backfiring, it may be a small obstruction in the carburettor. You may be lucky, and benefit instantly from the correct application of a commercial carburettor cleaner. ALWAYS read their instructions! Every make is made and used slightly differently.

Removing and servicing the carburettor can become a complicated business, and you may want to deliver your fate into qualified hands. You may find our ideas in the Carburettor Service section useful, too.

Runs but smokes like a chimney

Your piston rings are shot. This requires a complete overhaul of the engine. Hopefully it won’t take a year for this site to cover the entire process…

The engine speed runs low

If every single thing about the machine looks right, smells right and sound right, but the speed is too low, check for the following:

Pull on the governor arm, to see if there is still some extra vooma available. If there is no more speed to be had:

The governor arm itself is connected to the carburettor with a rod and spring. That spring is a rather sophisticated bit of precision engineering. Make sure it is undamaged and properly fitted.

Confirm that the maximum governor throttle indeed does cause maximum throttle opening on the carburettor. If not, see if limiting the play on the linkages solves anything.

On machines with accelerators, check that the mechanism operates correctly. Cables stretch, hooks fall out, bolts rattle loose...

On generators, the accelerator is usually a small screw that changes the tension on your governor arm spring. Unless you see an obvious fault there, do not adjust before eliminating other faults. Factory settings are usually the correct ones.

Are you loading an old machine like you used to when it was new? Everything loses efficiency with wear. Maybe your new welder draws more power than the old one, maybe you should mow the lawn more often, before the grass chokes your old mower. Maybe it is time for new blades…

On generators, the electrical motor has its own bearings, often open and prone to fouling. These can put tremendous drag on an otherwise healthy engine.

The air flow could be obstructed by dirty filter/s, or a faulty choke that stays closed.

The spark plug may be fouled or cracked, inside the metal bit, where no-one can see, making for poor spark at the gap.

If there is no way to raise the speed, even under no load, it may be necessary to Service the Carburettor.

The engine runs too fast

Over-revving an engine is always bad. Switch off immediately, and inspect for obvious faults:

Sometimes an engine will run away because there is not enough oil, or the oil is too thin to activate the governor arm properly, leaving the carb in open-throttle start mode.

The linkage between the governor and carburettor sometimes wear out and end up stuck in odd positions.

Check the proper operation of the accelerator and all linkages thereof.

Sometimes a loose wire, or pipe, a drifting leaf, may get stuck in the mechanism controlling the carburettor.

Sometimes, it is because the blades fell off, the electric generator motor stopped working, something is causing the motor to run without actually working the load.

Revs run up and down

First check that it is not the load switching in and out. Many a confusing situation can be avoided by knowing what other people using the machine, are up to.

Check the fuel flow. A slow fuel feed may starve the engine, it runs slower, just before it dies, enough petrol ran into the reservoir again, the speed picks up, sucks the reservoir empty, it starts dying, the petrol drip catches up again, repeat whirrr…rorrr…..whirrrrr…..rorrrr….whirrrr…rorrrr and so on.

Check the spring on the governor linkage. The rod and spring together form a feedback and damping circuit, finely balanced. It should have no loose play, without visibly stretching out.


Any engine that backfires, has serious issues. The entire idea behind the four-stroke engine depends on things flowing the right way at the right moment. The sooner you consult the Cylinder Head section, the better.

Commercially, there is a product available called “Compression Stabiliser”. This usually takes the form of an aerosol can spouting thick white foam. As a quick prayer to the gods of mechanicals, this stuff can be useful, if used at the first signs of trouble. Read the INSTRUCTIONS carefully, the method requires certain steps, don’t be cleverer than you need be with this stuff.


The poorer the quality of the machine, the sooner will your silencer disintegrate. It is one of those things, hope you find a new one.

Maybe the exhaust bolts came loose? Or the gasket blew out?

If you hear exhaust sounds by the tappet cover, you have blown a head gasket.

If your exhaust is still quiet enough for you to hear strange rattlings and hums from the rest of the machine, well, goodness, let’s wonder together:

With a looong screwdriver, or a mechanic’s stethoscope, try to locate the rattle as follows:

With the but of the screwdriver pressed firmly against your ear, without touching the metal shaft, gently touch the working end of the screwdriver onto various places of the machine. Like a stethoscope, it will magnify noise from small areas for you.

Listen on the tappet cover. You don’t want to hear rattles there, go to the Cylinder Head section.

Listen against the starter bell. A small amount of oil should stop the steel catch from sticking.

On the block, you want to hear no rattles, but the crankshaft bearings may be tested by opening the starting bell, and wiggling the shaft by hand, to feel for play, you want none. On a lawnmower, of course, you just grab it by the blade, and wiggle, expect no play.

On the electric motor, listen for the bearing at the end, possibly take off that bell housing, but beware electric shock. Also, anything you accidentally drop or poke in there, is almost guaranteed to stuff up your windings beyond hope.

The motor starts after many pulls

Check your choke.

Dry off the spark plug after flooding the motor by starting it hot with choke?

If the motor starts with difficulty, and stutters forth, you most probably need to see to the valves. Look up the Cylinder Head section. Relax, it’s a lot easier than a motor car’s.

It is very hard to pull the cord

First check for things caught up around shafts and in blades or other rotating parts.

Start by removing the spark plug. If the motor still refuses to turn over, you have  a fault in one or more bearings, whether it is inside the engine, or on your electric motor.

If the motor turns over easily when the plug is removed, you need to do the valves, see Cylinder Head chapter. 

It starts not

The internal combustion engine needs three things to energise the working cycle: Air, fuel, and fire. Unless your machine has been physically damaged or deteriorated, your problem is probably one of the above components being blocked. A quick check for their good flow is often rewarded with stress-less success.


This might sound silly, but even lawnmowers have some sort of on/off switch. Sometimes it is a little switch, or lever, or key. Sometimes it is a little metal strip that is flipped onto the end of the spark plug to kill the engine, and maybe not lifted back up. Often it is a little tag on the throttle mechanism that grounds out the coil feed. Make sure your machine is switched on.

Cold engines need help, traditionally by ‘choking’ the air flow. This mechanism has to be activated for a cold start, but may cause flooding when the engine has run a bit already or on warm days. See that the choke is set, close it by default for starting.

Most small motors prefer to start with the throttle in a fully open position, usually assured by the governor arm, so confirm the linkages and operation of this feedback system. Open the throttle if necessary.


Are you sure there is enough fuel in the tank? Are you really sure, or are you seeing the glimmer of a thin layer of filthy juice at the bottom of the tank, mostly water, anyway.

The fuel tank has a shut-off tap, usually right where the fuel pipe comes out. Is it open? Usually, up/down is open, crosswise is closed. Usually.

Close the stop-cock and carefully remove the fuel pipe from the bottom of the stop-cock. Open the tap. The fuel should flow fast and free.

If the fuel flows in drips, dirt and water may have blocked the small hydrophobic plastic filter inside the tank, it usually comes out with the stop-cock, which screws onto the tank.

Another reason for sluggish fuel flow, is the fuel tank cap. Petrol caps contain a vacuum valve, when this goes bad, air cannot enter the tank, preventing fuel from leaving. Open the cap and see if the flow improves.

If you are sure there is enough fuel, and the stop-cock is open, let the fuel issue rest for now, but we may return to it later. Next, check for:


The air filter is often neglected. Do you clean it regularly?

Every manufacturer designs their own container for the air filter, but the actual materials are fairly standard; paper or sponge.

There might be a steel mesh component. Open up the air box and see.

Switch the choke, to confirm it does open and close properly.

With the choke open, look into the carburettor to see if the butterfly valve at the deep end is operating properly, by adjusting the accelerator and confirming the vane moves appropriately.

Hold a piece of paper over the intake. Be sure to hold fast, as your assistant gently turns the engine over. The paper should be sucked violently into the carb throat, be sure to hold on. If there is little or no suction, you may end up stripping that motor, but let us assume you got a fright at the suction power. If the paper gets blown outwards, stop what you are doing and get used to the idea of some serious spanner-swinging, go see the section on Servicing the Valves, in the Cylinder Head chapter. You have no quick fix, I am afraid.

Sponge filters may be washed with soapy water, rinsed and dried before re-insertion. Some manufacturers recommend a small amount of very thin oil to soak the sponge. Paper filters usually shake out clean after some careful effort. Especially on lawnmowers and such, the integrity of the filter is of extreme importance. A filter with holes or gaps must be replaced before continuing.

Reassemble the filter box, the coarser filter, or gauze, on the outermost layer. Note that the sponge used for bedding, packaging and so forth, is not always stable when soaked in petroleum products, filter sponge is a specialised material, otherwise you may end up with granules of carbon hopping around in your piston sleeve. Not good.

Try to start her, If she still refuses, check for:


The next step is best done with help, or a spare spark plug. Remove the spark plug cord, and fit to a spare loose plug.

Hold or wedge the outer thread of the sparkplug against an open metal surface of the machine, to ground it.

Have an accomplice gently pull the starter cord, and look for spark. This is best not done in direct sunlight, but some shade should show up a healthy spark, blue rather than red, usually yellow.

In the absence of a spare plug, a thin metallic tool may be touched to the inside contact of the spark plug cap, and gently held close to the bodywork, but not touching, about half a millimetre away. Be prepared for a tingle yourself, which is a good sign! The spare plug way is just so much safer, though…

If there is no spark, you may be lucky, or very, very sad. You may be lucky, and find the machine was not switched on, after all.

Another reason the spark could be dead, is because of the oil sensor. It usually looks like a bolt screwed into the block, with one wire coming out of its rubber centre. Follow this wire, it will plug onto another wire that leads to a small metal box, with three wires. ONLY IF you are sure the sump oil level is on the level, and only then, unplug this wire.

Test for spark again. If you do find spark, you have found a problem. These sensors do sometimes fail, REPLACE immediately, DO NOT run your machine without this safety feature, or risk destroying your engine. Also, make very sure the thing is not activated because there is too little oil in the machine!

If you still have no spark, you are probably saddled with an electrical issue, fetch the appropriate tools and read that section if you need some ideas. If you do have spark, but the machine still refuses to fire, it is time to fetch tools anyway. In the meantime, go read the Spark and Plug section.