Firman T07573 Tri-Fuel 9400W Engine Surging on Natural Gas

What’s Happening

When your Firman T07573 Tri-Fuel 9400W surges on natural gas, the engine’s RPM climbs and falls repeatedly—sometimes dramatically. This isn’t a minor annoyance; it stresses the engine, wastes fuel, and can damage connected equipment like generators or pumps. Unlike a carburetor-only gasoline engine, tri-fuel units have additional complexity: a fuel selector, a demand regulator, an air-fuel mixer, and a diaphragm that must respond instantly to load changes. Any weak link in this chain can cause surging.

At-a-Glance: Most Likely Causes

Diagnostic Walkthrough

Work through these steps in order. Start with the cheapest and easiest checks; if those don’t resolve the issue, move to more involved diagnostics.

1. Check your NG supply pressure. Natural gas pressure varies depending on whether you’re using a bottle regulator, a house line, or a bulk tank. If the pressure is bouncing—especially if it’s dropping under load—the regulator or supply line is the culprit. Use a low-pressure gauge (0–10 inches of water column is typical for NG appliances) connected to the test port on your demand regulator. Steady pressure should hold within ±0.5 inches of water column. If it fluctuates wildly, tighten all fittings on the supply line, check for kinks, and inspect the regulator for damage.
2. Inspect all NG fuel line connections. Loose fittings, cracked tubing, or corroded connectors are common culprits. Tighten every connection hand-tight plus a quarter turn with a wrench. Look for any hissing or smell of mercaptan (the rotten-egg odor added to natural gas). If you detect a leak, stop immediately and do not operate the engine. Repair or replace the line before proceeding.
3. Check the air-fuel mixer for visible cracks or leaks. The mixer is the component that blends air with NG before it enters the intake. Inspect the rubber diaphragm housing (usually a plastic or aluminum chamber with a rubber membrane inside) for cracks, discoloration, or fuel residue. If the diaphragm is stiff, cracked, or leaking, it cannot respond to load changes and the engine will surge. This is a common wear item on tri-fuel units.
4. Test for air leaks between the mixer and intake manifold. Any air entering downstream of the mixer throws off the fuel-air ratio. With the engine off, spray soapy water around the connection points where the mixer attaches to the intake. Start the engine and look for bubbles in the soapy water—bubbles indicate an air leak. Tighten the clamps or bolts; if leaking persists, the gasket may be damaged and need replacement.
5. Verify the demand regulator is clean and moving freely. The demand regulator senses engine load and adjusts NG flow accordingly. If it’s clogged with dirt or debris, it cannot respond quickly, and the engine surges. Locate the regulator (consult your manual for exact position). Look for any dirt, water, or corrosion around the inlet and outlet ports. If you see contamination, the regulator may need cleaning or replacement.
6. Switch to gasoline or propane and observe. If the engine runs smoothly on gasoline or propane but surges on NG, the problem is specific to the NG fuel system (supply pressure, mixer, or regulator). If the engine surges on all fuels, the issue is in the engine itself (governor, carburetor, or ignition) and is beyond the scope of NG-specific troubleshooting.
7. Check the governor linkage for binding or wear. The governor automatically adjusts the throttle to maintain steady RPM. If the governor linkage is stiff, corroded, or misaligned, it hunts—overshooting and undershooting the target RPM. Inspect the governor arm and all pivot points for rust, bent metal, or loose bolts. Clean and lubricate with light machine oil. If any part is bent, it must be replaced.
8. Load test under a steady, moderate load. Surging is often more noticeable at no load or very light load. Connect a modest electrical load (e.g., a 2–3 kW space heater or work lights) to the generator and observe RPM stability. If surging decreases significantly under load, the governor is hunting at light load—a tuning issue that may require professional adjustment. If surging persists or worsens, the mixer or regulator is likely failing.

Parts You May Need

• Air-fuel mixer diaphragm or complete mixer assembly
• Demand regulator (NG-specific)
• Natural gas fuel line (reinforced rubber or braided stainless)
• Low-pressure gauge (0–10 inches water column)
• Gasket or O-ring kit for mixer-to-intake connection
• Hose clamps (stainless steel, various sizes)
• Carburetor rebuild kit (if gasoline surging is also present)

When to Call a Pro

Stop troubleshooting and contact a certified small-engine technician if:

• You detect a natural gas leak (smell of mercaptan, hissing, or soapy-water bubbles) and cannot locate the source.
• The demand regulator is visibly damaged, leaking fuel, or does not respond to engine load changes.
• The air-fuel mixer diaphragm is cracked, stiff, or leaking fuel into the intake.
• NG supply pressure fluctuates more than ±1 inch of water column despite tightening all fittings.
• The engine surges on all three fuels (gasoline, propane, and NG), suggesting a governor or ignition problem.
• You have replaced the mixer or regulator and surging persists.

Frequently Asked Questions

Why does my engine surge only on natural gas and not on gasoline or propane?

Natural gas has a different energy density and flow characteristics than gasoline or propane. The NG fuel system (mixer, regulator, and supply line) must be precisely matched to the engine’s needs. If any component is worn, clogged, or misaligned, it cannot respond fast enough to load changes, and the engine hunts for the right RPM. Gasoline and propane systems may work fine because they use different carburetor circuits and regulators.

Can a stiff diaphragm in the air-fuel mixer be cleaned instead of replaced?

Sometimes, yes. If the diaphragm is stiff due to fuel varnish or light corrosion, soaking it in carburetor cleaner for 30 minutes and gently flexing it may restore function. However, if the diaphragm is cracked, torn, or permanently hardened, replacement is the only reliable fix. A failed diaphragm cannot hold pressure or respond to load changes, and attempting to clean it risks further damage.

How do I know if my NG supply pressure is the problem?

Use a low-pressure gauge (available at most hardware stores or online) to measure pressure at the demand regulator’s test port. Steady NG pressure should hold within ±0.5 inches of water column. If pressure drops when you increase the engine load, the supply regulator or line is failing. If pressure jumps erratically, the regulator is hunting. Document the pressure readings and show them to a technician if you need professional help.

Is governor hunting the same as engine surging?

Governor hunting is a specific type of surging caused by the governor overshooting and undershooting the target RPM. It’s most noticeable at no load or very light load. Under a steady, moderate load, hunting usually decreases or disappears. If surging persists under load, the problem is fuel delivery (mixer, regulator, or supply pressure), not the governor.

Disclaimer

This article provides general troubleshooting guidance for the Firman T07573 Tri-Fuel 9400W and similar small engines. Always consult your model-specific owner’s manual and follow all manufacturer safety instructions before servicing fuel systems or internal engine components. Natural gas systems require careful handling; if you are unsure about any step, contact a qualified technician. US Motor Power and its contributors assume no liability for damage, injury, or improper repairs resulting from the use of this information.

Source: Information adapted from official manufacturer documentation (reference). Always consult your generator owner’s manual for model-specific procedures.