Dual-fuel generators are a great idea — gasoline when you have it, clean-storing propane when you do not. But I get a steady stream of them where the machine runs perfectly on gas and flatly refuses to run on LP. The good news is that a generator that won’t run on propane almost never has an engine problem. It is a fuel-delivery issue in the propane side, and once you understand how the LP system feeds the engine, the fix is usually quick.
Let me walk the propane path from tank to engine, in the order things go wrong.
The 30-second answer: If your dual-fuel generator won’t run on propane but runs fine on gas, the LP fuel is not reaching the engine properly. Confirm the tank is not low and the tank valve is open all the way (open it slowly so the excess-flow safety does not trip), purge the air out of the LP hose, and follow the propane start sequence in your manual — the demand regulator needs the engine cranking to draw gas. In cold weather, propane vaporizes poorly and the tank can frost, which starves it. A failed regulator diaphragm is the less-common cause.
How the propane side actually feeds the engine
On LP, fuel does not flow on its own the way gasoline gravity-feeds a carb. Liquid propane in the tank boils into vapor, travels through the hose to a demand regulator, and that regulator only opens and releases gas to the engine when the engine is turning and pulling a vacuum. So the propane start is a little different from gas: you need the tank open, the line full of gas (not air), and the engine cranking to make the regulator deliver. Miss any of those and it will crank all day without firing.
What I check first, in order
- The tank and its valve. A low tank, or a valve not opened fully, is the most common cause. Open the valve slowly and all the way — snapping it open makes the built-in excess-flow safety think there is a leak and it chokes the flow to a trickle. If that trips, close the valve, wait, and reopen slowly.
- Air in the hose. A fresh hookup is full of air. It can take a bit of cranking to purge that air and get gas to the regulator. Follow the priming steps in the manual.
- The fuel selector. Make sure the changeover is fully in the LP/propane position, not halfway between gas and propane.
- The start sequence. Propane starting often uses the choke or a primer differently than gas. Use the LP procedure the manual lays out.
The cold-weather problem
Propane has to boil from liquid to vapor to feed the engine, and cold air makes that hard. On a cold morning the tank can frost over, the pressure drops, and the generator starves or will not start on LP at all — even though everything is hooked up right. A bigger tank has more surface area and vaporizes better, so a 20-pound tank beats a one-pound cylinder in the cold. Keeping the tank out of the wind and reasonably warm helps. This catches a lot of people who assume the generator is broken when it is just physics.
When it is the regulator
If the tank is full and open, the line is purged, the selector is right, and it still will not run on LP, suspect the demand regulator. The diaphragm can fail or stiffen with age, and then it will not open on engine vacuum. Regulators are replaceable, and on a dual-fuel machine it is the one propane-specific part that actually wears. Confirm the simple stuff first — the regulator is rarely the problem on a machine that ran fine on LP until recently.
Frequently asked questions
Why does my generator run on gas but not propane?
Because the propane side feeds differently. Almost always it is a low tank, a valve not fully open, air still in the hose, or the wrong start sequence — not an engine fault. Work the tank-to-regulator path.
Do I have to open the propane tank valve a certain way?
Yes — open it slowly and fully. Opening it fast trips the excess-flow safety inside the valve, which restricts the gas to a trickle and makes the generator starve. If it trips, close, wait, and reopen slowly.
Why won’t my generator start on propane when it’s cold out?
Propane vaporizes poorly in the cold and the tank can frost, dropping pressure. Use a larger tank for more vaporizing surface, keep it out of the wind, and expect harder LP starting in cold weather.
The propane fix in one breath
- A dual-fuel generator that won’t run on propane (but runs on gas) has an LP delivery problem, not an engine problem.
- Open the tank valve slowly and fully, purge air from the hose, set the selector to LP, and use the manual’s propane start sequence — the regulator needs cranking to deliver.
- Cold weather starves propane; use a bigger tank and keep it out of the wind.
- A failed demand-regulator diaphragm is the less-common cause — check the easy stuff first.
| Part / consumable | DIY price | When I use it | Time |
|---|---|---|---|
| LP hose & regulator assembly | $18–45 | Cracked hose or failed regulator | 20 min |
| Demand regulator (only) | $15–35 | Diaphragm will not open on vacuum | 20 min |
| Larger LP tank (20 lb) | $30–60 | Cold-weather vaporizing | — |
| Thread sealant (gas-rated) | $4–8 | Any LP fitting you open | — |
Pro service vs DIY math.
A shop charges $75–150 to diagnose a fuel problem, and on propane it is usually a free fix — a valve opened correctly, a purged line, or the right start procedure. A regulator swap is a modest part and an easy job. The only reason to bring in help is if you smell gas at a fitting you cannot seal; a leak in an LP system is one thing not to live with.
Safety — quick reminders.
Propane is heavier than air and pools low — if you smell gas, shut the tank valve and do not start the engine until it is sealed and cleared. Check fittings with soapy water for bubbles, never a flame. And never run a generator inside a garage or shed, even with the door open — carbon monoxide is invisible and deadly. Full set in generator safety.
If it also cranks hard when you switch back to gasoline, pair this with the won’t-start diagnosis for the gas side.