Surging and hunting
Generator Surging or Hunting: Why the Revs Rise and Fall
A generator that surges, or hunts, cannot hold its engine speed, so the revs rise and fall on their own and the lights pulse with them. On a diesel the usual causes are air in the fuel, a blocked filter or a governor that needs attention. On a petrol set it is most often a carburettor gummed up by old fuel. We find the cause by measuring speed, frequency and fuel delivery at steady loads.
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- Speed and frequency measured
- Fuel checked before the governor
- Written quote before any work
- Onsite across Gauteng
An engine that cannot hold its speed
What does it mean when a generator surges?
A generator has to turn at a fixed speed to make 50 Hz. The governor adds or removes fuel to hold that speed as the load changes. When the fuel supply is unsteady, or the governor overcorrects, the speed swings above and below its target in a rhythm you can hear.
Surging is often worst with little or no load, because a small change in fuel then makes a large change in speed. A set that runs smoothly under load but hunts at idle is pointing us at fuel delivery and governor settings rather than at the alternator.
If the engine note is steady but the lights still flicker, the problem is more likely electrical. Our unstable voltage guide covers that case.
Fault guide
Generator surging or hunting: what it can mean and what to do
A generator that surges, or hunts, cannot hold a steady engine speed, so its revs rise and fall on their own. The usual causes are fuel starvation, air in the fuel, a governor or actuator fault, or a petrol carburettor gummed up by stale fuel. This guide separates what you can safely notice from what a qualified technician tests. One symptom never proves that a particular part has failed.
Symptom, likely cause and repair
Find the line closest to what you see. The cause is the most likely one, not a diagnosis: testing confirms it before any part is replaced.
| What you see | Likely cause | Usual repair |
|---|---|---|
| Engine speed rises and falls on its own | The governor or actuator is out of adjustment or faulty | Generator fault finding |
| Surging mainly at light load | Air in the fuel, a blocked filter or a weak lift pump | Fuel system repairs |
| A petrol set surges and hunts | Gummed carburettor jets from stale fuel | Petrol generator repairs |
| Lights flicker in time with the engine | Speed changes carried through to voltage and frequency | Unstable voltage diagnosis |
| Surging started after a fuel delivery | Water or dirt in the fuel | Contaminated diesel |
01
What you may notice
- The engine note rises and falls in a regular rhythm
- Lights brighten and dim in time with the engine
- Frequency or voltage on the controller swings up and down
- It is worse with little or no load connected
02
Possible causes
Any of these can produce the same symptom. Testing decides which one applies.
- Air in the fuel, a blocked fuel filter or a weak lift pump
- A governor or speed actuator that is out of adjustment or failing
- Stale petrol gumming up a carburettor jet on a petrol set
- Water or dirt in the fuel
- A load that switches on and off, such as a pump or compressor cycling
03
What you can safely check
Look and note only, from outside the canopy. Do not open panels, covers, fuel lines or the cooling system.
- Does the surging happen with no load, light load or full load?
- Did it start after refuelling, or after the set stood unused?
- Is the rhythm steady, or does it come and go?
- For a petrol set: how old is the fuel in the tank?
04
Stop using the generator and call if you notice
- The engine races far above its normal speed (overspeed)
- Surging together with smoke, knocking or a burning smell
- Equipment in the building switching off or tripping
If it is safe to reach, stop the generator with its normal stop control or emergency stop, keep people away, and call a technician.
05
What a qualified technician typically tests
These are technician checks, not steps to try yourself. Several involve live electrical measurement, fuel or pressurised systems.
- Fuel delivery and filter restriction, and a check for air in the fuel
- Governor or actuator response and speed settings
- Engine speed, frequency and voltage logged at steady loads
- Carburettor condition on petrol sets
06
Common repair categories
The repair follows the test result. These are the services this symptom most often leads to.
07
What to send when you book
- Your suburb and how we reach the generator
- Make, model and kVA from the data plate (a photo is ideal)
- Fuel type: diesel, petrol or gas
- Whether it surges at no load, light load or full load
- A short video with sound, taken from a safe distance
08
Related faults and services
Not sure which symptom fits? Try the generator symptom finder.
Safety before you check anything
- Exhaust contains carbon monoxide, which you cannot see or smell. Run a portable generator outdoors only, away from doors, windows and vents, and never run any generator in a house, garage or other enclosed space.
- A standby generator set to automatic can start at any moment, when the mains fails or on its exercise timer. Keep the canopy closed and stay clear of fans, belts and the exhaust.
- Generator output, the changeover and the control panel can be live, and some parts hold stored electrical energy after shutdown. Leave panels, covers and wiring closed for us to open.
- The engine, exhaust and coolant stay hot after running. Never open a radiator or coolant cap on a hot engine.
- Do not refuel a running or hot generator. If you smell gas at a gas-fuelled set, do not operate switches; leave the area and contact your gas supplier.
Carbon monoxide guidance: US Consumer Product Safety Commission: Generators and engine-driven tools (carbon monoxide).
Send the details on WhatsApp
This opens WhatsApp with a short message already typed: brand, model, capacity, symptoms and suburb. Fill in what you know and send it. Blank lines are fine.
- The data-plate photo is added inside WhatsApp after the chat opens: tap the attachment or camera icon and choose the photo. A link cannot attach the photo for you.
- Take the photo only when the generator is switched off and has cooled down. Do not reach over or into a running or hot generator, and keep away from the exhaust.
- A standby generator set to automatic can start by itself when the mains supply fails. If you are not sure how it is set, photograph the plate from a distance or send a photo of the whole set instead.
Step 9: book a diagnosis visit.
Related Pages
Fuel, speed and output
Common Questions
Questions about surging
Why does my generator surge up and down?
The engine cannot hold its speed. The usual causes are air in the fuel, a blocked filter, a weak lift pump, a governor or actuator fault, or on petrol sets a carburettor gummed up by old fuel.
Why does it surge more with no load?
At light load a small change in fuel makes a big change in speed, so any unsteadiness in the fuel supply or the governor shows up most. Tell us whether it surges at no load, light load or full load.
Can surging damage my appliances?
It can. Speed swings carry through to voltage and frequency, which sensitive electronics, inverters and motors do not like. Switch off sensitive equipment until the surging is fixed.
Is surging the same as unstable voltage?
Not quite. In surging you hear the engine speed change. If the engine sounds steady and only the voltage swings, the cause is usually the AVR or wiring.
Can old petrol make a generator surge?
Yes. Petrol that has stood for months leaves gum in the small carburettor jets, so the engine runs lean and hunts. Cleaning the carburettor and using fresh fuel usually cures it.
My generator started surging after a diesel delivery. Why?
Water or dirt in the new fuel can restrict the filters and starve the engine. We check the filters and the fuel before anything else.
Still unsure what the fault is? Call or WhatsApp 082 057 4605 for onsite diagnosis.
Service Areas
Onsite visits across Gauteng
We come to your generator anywhere in Gauteng, 24/7. Choose your region for local suburbs, call-out details and the faults we see most there.
What to send before we attend
These details help us plan the visit and bring the right parts. Send what you can on WhatsApp; if you can’t find the model number, we still book the visit.
- Location
- The suburb or site, so the visit can be booked in the right area.
- Make and model
- The generator brand and model, if they are known.
- kVA
- The size printed on the data plate.
- Fuel type
- Diesel, petrol, LPG, natural gas, or dual-fuel.
- Controller model
- The module name on the panel, when it is visible.
- Fault or alarm
- What the set does, and any text on the screen.
- Photos
- The generator, the data plate, and the controller or alarm. Send these on WhatsApp.
Book a Visit
Tell us how it surges
At no load, light load or full load? A short video with sound, taken from a safe distance, helps us bring the right parts.
- Call
- 082 057 4605
- Message us on WhatsApp
What happens next
- We reply with questions about the generator, or to book a visit.
- A technician comes to you. For a breakdown we arrive within 1 hour.
- The fault is diagnosed on site and you get a written quote. No work starts until you accept it.
Charlie, lead technician: Degree in Mechanical Engineering, Degree in Electrical Engineering, Diesel Plant Fitter. Generator specialists since 2000.