A small engine must maintain a consistent speed through varying workloads – the impetus behind a speed-control system. At idle, the speed-control system of a motor has no real function. However, with the throttle opened up, it fine-tunes engine rpm based on the working conditions being encountered by the equipment.

Here is how the governor of a typical rotary lawn mower functions: 1)the operator starts the mower, and opens its throttle to “high”; 2)the mower maintains a consistent rpm level as long as cutting conditions are easy and routine; the governor has no adjustments to make; 3)until the mower encounters a patch of long and difficult grass; 4)suddenly its engine bogs down, threatening to stall; 5)to avert engine stalling, the governor thrusts the throttle open – endeavoring to increase rpm by boosting the amount of air-fuel mixture that flows through the carburetor; 5)this additional air-fuel mixture revives the motor, and stalling is prevented; 6)the throttle remains wide open as the mower labors its way through the long and difficult patch of lawn; 7)with the tough patch finished off and the mower again in shorter grass, the motor ceases to labor and its rpm rapidly rises; the governor closes the throttle slightly to return engine speed to a preset safe level. Until another tough lawn patch appears. And so on….

Governors come in two fundamental types. These are:

1)The air vane governor
2)The mechanical or centrifugal governor


An air vane governor: will be positioned near to the engine flywheel. It is rudimentary in design – nothing but a flat and narrow piece of metal attached to the throttle by means of some linkage and a coil spring. The air vane manipulates the throttle based on how much air is being blown against it by the flywheel fins.

When an engine is not running, coil-spring pressure on the air vane pulls the throttle fully open. This way, when someone goes to start the engine, the piston, on its intake stroke, finds air and fuel from the carburetor readily obtainable.

Here is what transpires once the engine is running: 1)say the machine operator wants to keep it at “idle”; he manually moves a handlebar lever to shut the throttle. The air vane, being asked to do nothing, is relaxed; 2)when the operator engages “full” throttle, the flywheel speeds up; its fins fan the vane with enough air to overwhelm the coil-spring pressure, and the throttle partly shuts; the system is designed so that the greater the force of flywheel air against the vane, the more the throttle will close; 3)if, at “full” throttle, the engine slows and threatens to stall, the amount of air being pressed against the vane by the flywheel fins is reduced, and coil-spring pressure allows the throttle to open; 5)think of it as a continuous tug of war between the air vane spring and the flywheel fins – to keep the engine within a consistent and safe rpm range.

What you should be aware of: over time, these governor springs do stretch and wear out. They may no longer permit the carburetor throttle plate to open as far as it should. This can result in: 1)hard starting – because when the engine is off, the spring pressure no longer holds the throttle wide open; and 2)stalling when an engine bogs down – because the governor cannot push the throttle open as widely as reduced flywheel air dictates is necessary.

You may be able to detect a governor-spring problem: by examining the carburetor throttle-plate position with the engine off. It should be wide open. If the throttle plate is just partially open, the spring has lost some tension and will need to be re-anchored or replaced. To see if it can be re-positioned, look at the spring ends – where they attach to the linkage. There is often more than one anchoring point available as a way to help compensate for normal spring wear and tension loss. You should never change spring positions as a way to inflate engine performance by widening a throttle-plate angle. This will force the motor to operate at rpm levels not intended by the manufacturer!


The mechanical or centrifugal governor: is basically just a gear with small mobile weights attached. It is these tiny weights that, through their position, manipulate the throttle, fine-tuning engine speed. A mechanical governor will be installed in a crankcase cover, in a place from which it can intermesh with the much-larger camshaft(or occasionally the crankshaft) gear.

With the engine “off”, the governor linkage keeps the throttle wide open. When an engine is started, the governor gear inside the crankcase: 1)begins to spin with the cam or crankshaft; 2)as engine rpm increases, the cam or crankshaft spins the governor gear ever more quickly; 3)as the rotational speed of the governor mounts, centrifugal force created by this motion draws the weights away from the governor body; 4)when the weights attain a predetermined position, their motion will transfer to external linkage(outside the crankcase) – and this linkage shuts the carburetor throttle plate enough to keep engine rpm at a safe level; 5)if the engine threatens to bog down or stall, slower rotational movement of the cam or crankshaft slows the governor gear; this causes the weights to shift toward the governor body, and the linkage opens the throttle plate.

What mechanical-governor linkage consists of: you will find, in examining it, that mechanical or centrifugal governor linkage is more complicated than the linkage of an air vane. Here is what it consists of:

1)Governor plate – attached to the carburetor throttle plate, it changes throttle position when the motor is running, and holds the throttle wide open if the motor is “off”.

2)Governor spring and linkage – connects the governor plate to a governor lever.

3)Governor lever – on the outside of the crankcase, is connected to the internal governor components; these components move the governor lever to begin the chain of events that opens or shuts the carburetor throttle plate.

4)Governor spool – hidden inside the crankcase, it is impelled up a short shaft by the momentum of the spinning governor weights; the faster the governor gear rotates in the crankcase, the greater the centrifugal force on the weights and the higher this force makes the spool rise. Based on how high up its shaft the spool goes, or how far on its shaft it drops, the governor lever outside the crankcase will cause the throttle plate to open or shut accordingly.

How this linkage operates: you already know that the weights move outward via centrifugal force as the governor gear is spun by the cam or crankshaft. Here is what follows:

1)As each  weight moves away from the governor body, its bottom, resting against the base of the governor spool, begins to tilt skyward. This motion coerces the governor spool up its shaft; how far up the shaft the spool rises depends upon how rapidly the governor gear is rotating. The spool will be held in place on its shaft until the spinning governor gear changes speeds, and the amount of centrifugal force on the weights alters their position with respect to the governor body.

2)As it rises, the governor spool pushes the governor lever forward or up; if the engine begins to labor and its rpm falls, the spool lowers and so does the governor lever.

3)The governor lever employs a coil spring and wire linkage to turn the governor plate, shutting the throttle. Or, if the spool drops and the governor lever reverses its direction, the throttle is opened.

The advantage of a mechanical governor: there is much less lag time between a change in operating conditions, and adjustment of the throttle; air vane governors are used primarily on rotary lawn mowers – which tend to experience relatively few shifts in workload during normal operation. If a workload shift does occur, the rotary operator also has the luxury of pausing to make manual throttle adjustments without affecting job performance.

Now consider an electric generator – which can experience many different load conditions as it operates. A generator must undergo a throttle tweak or adjustment each time the electric load connected to it changes. Generator operators also lack the freedom to make manual throttle adjustments to accommodate different loads. If a generator was equipped with an air vane governor, throttle position could not be modified quickly enough to suit load changes and chronic stalling would result.

What you should be aware of: most mechanical or centrifugal governors are located inside the engine crankcase. However this is not always true. A centrifugal governor positioned outside the crankcase can be of major benefit if problems unrelated to linkage should arise. The thing to remember is that, regardless of position, all centrifugal governors function in the same basic way. They are designed to allow the rotational speed of either the cam or crankshaft to regulate the throttle-plate setting.

You can change the sensitivity of a centrifugal governor by adjusting the amount of coil-spring pressure on the governor linkage. Examine the connection point between the spring and governor lever. You will usually discover that multiple anchoring positions are available, each one affecting differently the centrifugal force required to move the governor lever and thus shut the carburetor throttle plate. By re-anchoring the spring to increase or decrease its pull on the lever, you can change the amount of engine rpm needed to alter throttle position.


Engine surging, especially during workload changes, is an annoying but routine governor problem. You will also usually find it simple enough to remedy. It is almost always the result of incorrect spring tension.

Often a spring will not need to be replaced. Regardless of governor type, air vane or centrifugal, you will normally find at least two positions at which the governor spring can be anchored to affect its pressure. In the case of a centrifugal governor, the governor lever will frequently boast a sequence of holes into which the spring end can be placed. As you move a governor spring to adjust its tension, remember that: tension increases make it harder for a governor to manipulate the throttle – this may help rectify a surging problem, however too much spring tension can: 1)force an engine to run at an rpm rate that is beyond manufacturer parameters; 2)hamper the ability of an engine to change rpm if it suddenly encounters a heavy load. Keep in mind too that if a spring is excessively worn, it may need to be replaced.


If replacing or repositioning a governor spring does not stop engine surging, then a linkage problem may be the culprit. The affected linkage will connect the governor lever and plate. Remember that the governor plate actually attaches to the carburetor throttle plate. Linkage wear will normally appear in either: 1)the thin section of wire, bent at both ends, that in most engines serves to wed the governor lever and plate; over time, the ends of this wire can unbend and change the degree of tension between these components; 2)the anchoring holes in the governor lever and plate; these holes can wear from circular to oval shapes that alter linkage tension.

How to remedy a connection or linkage problem: the simplest method of repair is to place a lightweight coil spring alongside the wire linkage to reinforce tension between the governor lever and governor plate.


An air vane should be checked if an engine is suffering from surge problems, or if it cannot maintain a consistent rpm during even routine operation. To access an air vane governor, the engine blower housing will probably need to be removed. You will find the air vane connected to the governor plate by means of a coil spring and some flimsy wire linkage.

Air vane troubleshooting should entail: 1)a check of the hinge pin. Movement of the vane should be easy and unhindered; if the hinge pin is not permitting unfettered movement, it will need to be reconditioned; 2)evaluation of the linkage for stretching or other damage; 3)an assessment of governor-spring tension. With the engine stopped, examine the carburetor throttle plate; it should be wide open. If the throttle plate is less than fully open, spring tension will need to be addressed. You may be able to re-anchor the spring at an alternative position to affect its tension – as described above; if not, or if re-anchoring the spring does not fully open the throttle plate with the engine “off”, the spring needs to be replaced. Check your service manual for the right part number.

What to do if air vane hinge is freezing: if you discover that movement of an air vane is being restricted by its hinge, try: 1)pulling the air vane and its linkage from the motor; 2)thoroughly cleaning the hinge components – both the pin which secures the vane in position, and its mating surface – to remove all accumulated dirt and debris; 3)reassemble the air vane and its linkage to determine if free motion has been restored.

You should not lubricate the hinge components with oil: standard lubricating oil will attract dirt and debris during machine operation; this will likely re-freeze the air vane you just repaired – and in short order. If you believe that lubrication is desirable: try utilizing dry graphite powder instead.


You should begin checking a mechanical or centrifugal governor: by examining its external linkage. What you will be able to see is a: 1)governor plate – probably mounted to the underside of the throttle-control bracket; 2)governor lever – which is connected to the internal components of the governor; 3)coil spring and linkage – that connects the governor plate to the governor lever.

Try moving the governor lever back and forth – being careful not to damage it or the linkage. The lever should move freely and with minimal force. If it does – good news – the internal governor components are probably okay. Next, check out the spring and linkage; you should find no discernible play between the governor lever and governor plate. If play is found, it likely indicates a worn-out spring. The spring will need to either be re-anchored at another position on the governor lever, increasing its tension, or simply replaced. Linkage can sometimes also be at fault, depending on its design. If the governor spring seems okay, examine the linkage for play.

How about if the governor lever is binding? this is worse news because it indicates an internal governor problem – which can only be fixed by removing the crankcase cover. When the cover is detached, keep your eye out for a couple of things: 1)that all of the weights connected to the governor gear are present and accounted for; if any are missing, the internal governor components need to be replaced; 2)that the weights all move freely and retain their full range of motion; 3)that the governor spool is able to move up and down on its shaft with both smoothness and its intended range of motion; 4)that there are no broken or missing teeth on the governor gear; 5)that no other damage exists to impair governor function, or to indicate that the assembly should be replaced.


Low-oil shut-down: is an inexpensive but handy means, utilized by virtually all modern generators, of protecting an engine from the damages of insufficient oil levels. It consists of a sensor inside the crankcase that must remain submerged in oil for the generator to: 1)start; 2)continue running.

What you should be aware of: the reliability of these devices varies by generator type and manufacture. They are fallible in that, if they malfunction, they will prohibit a generator from starting regardless of its oil level. If a generator refuses to start, or if it dies in mid-operation with no other perceptible cause, the low-oil circuit should be checked. Here is what to do: 1)be certain first that the crankcase oil level is adequate; 2)separate the shut-down circuit wires at one of their connection points; this will temporarily disable the low-oil shut-down system. Remember to: be absolutely sure that crankcase oil levels are at “full”. Now try starting the generator. If the engine starts, you can assume that the sensor inside the crankcase is faulty.

TIP: if a generator control panel possesses a low-oil warning light, sometimes you can use this to determine if a shut-down circuit is functioning properly. With the engine-control switch “on”, pull the recoil start. If the low-oil warning light in the control panel flickers or flashes on, the crankcase oil sensor probably believes that the oil level is low.

How to repair a bad low-oil sensor: unfortunately, no simple way exists to do this. You can try: 1)flushing the crankcase just as you would to eliminate sludge; this is the method recommended by most engine manufacturers, however it has a depressingly-modest prospect of success; 2)disassembling the engine and replacing the bad sensor; if a machine is under warranty, you may be able to have this done by a dealer; 3)what most folks end up doing – running the generator with the low-oil shut-down disabled. The risks of doing this are obvious. The thing to remember, if you choose to proceed with it – since your safety net has now been removed – is to monitor engine oil levels diligently during operation.

Fuel shut-off solenoids: somewhat akin to the low-oil shut-down circuit in function, but more complex, shut-off solenoids are designed to monitor engine temperature and oil pressure. If the motor begins to overheat, or if oil pressure drops below a preset level, an engine controller funnels an electric pulse, usually +12 volts DC, to the solenoid, and the solenoid stops the engine.

What you should be aware of: there is no universal means of detecting a malfunction in the shut-off solenoid system. It can usually be accomplished with a multimeter, however the procedure varies from one generator to another. Check your service manual for specific instructions.

Governor linkage: is particularly critical in a generator because engine speed must be maintained to produce a consistent 120 volts AC. A generator without throttle control will operate at a standard 3,600 rpm. Governor linkage can sometimes be adjusted to manipulate engine rpm either up or down. Normally such adjustments entail a simple re-anchoring of the governor spring to affect its tension on the throttle. If you choose to do this, keep in mind that:

1)Lowering the rpm will affect the amount of voltage being produced at the generator outlets. Some 120 volt tools and appliances will not be adversely affected if they operate at a lower voltage; others will, particularly computer and electronic equipment!

2)You can discover the voltage amount being produced at the generator outlets with a multimeter.

3)Mechanically increasing the carburetor throttle-plate position with a re-anchored governor spring can force an engine to run at a dangerously-high rpm – so be careful.

4)Anytime you raise engine operating speed or rpm, it should be checked afterward with a tachometer to ensure that the result has not surpassed acceptable limits.

Circuit breakers: provide overload protection. If a load plugged into the control-panel outlets exceeds generator capacity, the circuit breakers will trip and cut off voltage. The engine itself will remain on. A motor will sometimes need to be shut down and restarted to restore power to the control-panel outlets. If just one generator outlet was overloaded, restoring power usually entails resetting the tripped circuit breaker at the control panel – by pushing a button or flipping a switch.