Why Is My Generator Surging: Causes and Fixes, Easiest First

The engine speeds up, slows down, speeds up again. The lights pulse in time with it. That's surging, also called hunting. It's not the same thing as "surge watts," the brief extra power a generator supplies to start a motor.

Short answer: on a portable generator that has been stored, surging is usually a lean carburetor. Stale fuel leaves varnish that partly blocks a jet, and the governor keeps chasing the speed it can't hold. First rule out the 30-second checks (choke, fuel valve, cap vent, oil level) and normal eco-mode speed changes. Then use fresh fuel and clean the carburetor. Governor and gas-supply problems are service work.

Is it surging, or is it eco mode?

Inverter generators in eco or idle mode change speed on purpose. The engine throttles up when a fridge compressor starts and back down when it stops. Output stays steady because the inverter builds the waveform electronically. That's normal. Turn eco mode off. If the engine settles at a steady speed, nothing is wrong.

A conventional generator should hold a steady speed. Its engine speed is its output frequency. A two-pole alternator at 3,600 rpm makes 2 × 3,600 ÷ 120 = 60 Hz. As an illustration, if speed swings 10% either way, frequency swings from 54 to 66 Hz and voltage moves with it.

Real surging is a rhythmic rise and fall with a steady load connected, and it continues with eco mode off.

A load that cycles on and off, like a sump pump or fridge compressor, makes any generator dip and recover each time it starts. That's load response, not surging. An overloaded generator labors and bogs down. Unplug something, and check your load against the generator's running-watt rating.

Causes, easiest first

The 30-second checks

Choke. Fully open (off) once the engine is warm. A choke left partly on makes the mixture too rich and the engine runs rough.

Fuel valve. All the way open. A half-open valve can feed an idling engine but starve it under load.

Fuel cap vent. A blocked vent pulls a vacuum in the tank, so fuel flow falls off after a few minutes. If loosening the cap briefly steadies the engine, the vent is the problem. Clean it or replace the cap, and don't run with the cap off.

Oil level. Check it on level ground. On some units, oil near the low mark, or the generator on a slope, lets the low-oil sensor cut in and out. The engine stumbles before it shuts down.

Stale fuel and a partly clogged jet

This is the most common cause on a portable that sat since last season. Gasoline breaks down in storage and leaves gum in the carburetor's fine passages. A jet that is partly blocked still lets the engine start. It just can't supply enough fuel to hold speed. The engine slows, the governor opens the throttle, speed overshoots, the governor closes it, and the cycle repeats.

The telltale sign: surging that's worst with no load and smooths out once you add a moderate load, such as a work light or a small heater.

The fix: drain the old fuel, refill with fresh, and run it. If surging continues, clean the carburetor. The steps are in the carburetor section of generator won't start troubleshooting. Water in the fuel, or ethanol blends that have separated, cause the same symptoms and need the same drain-and-clean. Prevent it next time with the habits in gasoline storage and stabilizer.

Air filter and fuel filter

A clogged air filter throws off the fuel-air mixture and can make the engine run rough and labor. Clean a foam element or replace a paper one. Some units also have an inline fuel filter or a sediment cup. Where fitted, check that it isn't clogged, with the fuel valve closed and the engine cold.

The governor: leave it to the service shop

The governor holds engine speed against load. Conventional portables typically use a mechanical governor with linkage and a spring, and many inverter units control the throttle electronically. A bent linkage, a stretched spring, or debris binding the linkage can cause hunting. That's rarely the first cause, though, and it's the wrong place to start.

What not to do:

• Don't bend governor linkages or move springs to different holes.
• Don't adjust the governor or the speed screw on a unit whose manual reserves that for authorized service.

A governor set wrong can overspeed the engine. That damages the engine and sends high voltage and frequency to everything plugged in. Setting it correctly needs a frequency or rpm meter and the manufacturer's specification. If surging continues with fresh fuel, a clean carburetor and a clean filter, take it to a service shop.

Standby generators: gas supply

A standby unit on natural gas or propane has to receive the specified gas pressure at full load, not just at idle. An undersized gas line, gas meter or regulator can hold pressure while the engine idles and let it sag under load. The engine then hunts or bogs. A propane tank that's too small, or low in cold weather, may not vaporize fuel fast enough to keep up. See propane tank sizing for a generator.

Measuring gas pressure and resizing piping or regulators is licensed gas work. Many standby units control speed electronically, so note any controller fault codes and call the dealer.

What surging does to your electronics

A surging generator swings voltage and frequency. Motors and electronic power supplies both handle that poorly. Unplug computers, TVs, chargers and anything with a control board until the generator runs steadily. Hold off on motor loads like the fridge and furnace too. See running sensitive electronics on a generator. If the engine runs steadily but nothing gets power, that's a different problem: see generator running but no power.

If a modem, a CPAP or other electronics can't wait for the repair, run them from a charged power station instead of the generator. This one's 120 V outlets put out a pure sine wave, and its 10 ms switchover suits UPS-style duty on a modem or CPAP. Don't recharge it from the generator until the surging is fixed.

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Safety while you troubleshoot

• Outdoors only. Keep it at least 20 feet from the house with the exhaust pointed away from doors and windows, never in a garage, even for "a quick test." See carbon monoxide safety.
• Let the engine cool before touching the carburetor, the muffler area or the air filter housing.
• Fuel valve off and no ignition sources before opening a carburetor bowl or draining fuel. Catch the fuel in an approved container.
• Disconnect all loads, and switch off the generator's breakers, before working on it.

Troubleshooting means test run after test run, and every one belongs outdoors. Inside, a battery CO alarm on every level of the home keeps working through an outage and shows CO in ppm. It's a backstop, not permission to run the generator closer.

Standards and code reference

The standards behind this guide, for looking up in the edition your jurisdiction has adopted:

• NFPA 54 (National Fuel Gas Code): gas piping and supply for gas-fired equipment, including standby generators
• NFPA 58 (Liquefied Petroleum Gas Code): propane containers, regulators and their installation
• ANSI/PGMA G300: portable generator safety, including CO shutoff
• UL 2201: carbon monoxide emission rate of portable generators

Code editions and local amendments vary. Confirm the adopted edition with your AHJ, and treat manufacturer instructions as governing wherever they are more restrictive.

Where to go next

• Generator won't start: troubleshooting in order
• What is an inverter generator?
• Generator maintenance schedule
• Size it properly: generator sizing calculator

More in our generator and backup power guides.