Generator Won't Start After Sitting? Here's the Most Likely Fix

The #1 cause: varnished fuel

If your generator hasn't run in 6+ months and now won't start, it's almost certainly the carburetor. Gasoline turns to varnish over time - the volatile components evaporate and the heavy components polymerize into a sticky residue that clogs jets, gums up needle valves, and seizes float assemblies.

We see this every spring. Snowbirds come back to their rigs in November or April, hit the start button, and nothing happens. Sometimes the engine cranks but won't fire (no fuel reaching the cylinder). Sometimes the engine fires but dies after 5 seconds (clogged jet). Either way, carb work is the answer.

The fix is cleaning and rebuilding the carburetor - jets cleaned, float adjusted, needle valve replaced, fuel filter changed, fresh stabilized fuel in the tank. Plan on $385-$685 for the service depending on which gen-set.

How to tell if it's fuel or something else

Quick diagnostic flow you can run before calling us. Step 1: check the gas tank - do you have fuel? Sounds dumb, sometimes it's the answer. Step 2: hit the prime button (most Onan units have one). If the priming pump works, fuel is reaching the carb. If it doesn't, fuel pump or line issue.

Step 3: try to start. If the engine cranks fast but doesn't fire, it's a fuel or spark issue. If the engine cranks slowly, it's a battery issue (gen-set start-battery is often shared with the chassis). Step 4: pull the spark plug and look. If it's wet with fuel, fuel is reaching but not igniting (spark issue). If it's dry, fuel isn't reaching (carb issue).

This flow takes 10 minutes and tells us before we send a tech whether it's a fuel problem (most likely), an ignition problem (less common), or an electrical problem (sometimes). Gives us a head start.

Other common causes (in order of probability)

After varnished fuel, here's the rest of the list. Bad fuel pump (varnished diaphragm or stuck check valve) - 10% of cases. Bad spark plug or weak coil - 8% of cases. Voltage regulator failure (engine runs but no AC output) - 5% of cases. Starting circuit issue (relay, solenoid, wiring) - 5% of cases. Major mechanical failure (head gasket, ring wear, valve seat) - 2% of cases.

The major mechanical stuff is rare on gen-sets under 1,500 hours. If you've been keeping up with annual service and your rig is under 1,500 generator hours, the failure is almost certainly fuel-related, not mechanical.

Why annual service prevents 90% of this

Annual service does five things: drains the carburetor float bowl (no varnish forms), changes the oil and filter (engine wear stays low), replaces the spark plug (consistent ignition), changes the fuel filter (no fuel-line restriction), and changes the air filter (consistent intake). All five take about 90 minutes total.

We charge $245-$385 for full annual service. Snowbird customers get it done in November before storage so the gen-set is ready when they come back. Year-round residents schedule it at their convenience.

The alternative is what we do post-failure - which costs $585-$985 for the carb work plus the cost of any other damage from running on varnished fuel. Annual service is cheaper insurance.

Fuel stabilizer - does it actually work?

Yes, but you have to use it right. Stabilizer (Sta-Bil, Sea Foam, Star Tron, etc.) extends fuel life from about 30 days untreated to 6-12 months treated. The trick is you have to add stabilizer to fresh fuel and run the engine long enough to circulate stabilized fuel through the entire fuel system - including the carburetor.

Don't add stabilizer to old fuel and call it good. The carburetor is still full of un-stabilized fuel that will varnish. Run the engine for at least 15 minutes after adding stabilizer to a fresh tank.

For long-term storage (6+ months), still drain the carburetor float bowl after running. Most Onan units have a drain plug or a fuel cutoff that lets you do this. With stabilized fuel in the tank and a dry carb, the gen-set will start fine in spring.

When it's worth rebuilding vs. replacing

Onan QG and Cummins gen-sets are well-supported and worth rebuilding. Parts are everywhere, the engineering is good, and a properly-rebuilt unit gives you another 10-15 years.

Older Onan models (the gas 4000 and 6500 from 20+ years ago) get harder. Parts are getting expensive, and the labor for major work is significant. We'll usually recommend replacement instead of major work on those units.

Generac and Champion RV gen-sets are usually replacement candidates after major failure - parts support is hit-or-miss and rebuild parts are sometimes more than a new unit costs.

Cost expectations for generator work

Annual service: $245-$385. Carburetor cleaning and rebuild: $385-$685. Fuel pump replacement: $385-$685. Voltage regulator replacement: $385-$585. Major service (rings, valves at 1,500 hours): $1,450-$2,850. Full rebuild on Onan: $2,250-$3,500. Replacement of failed gen-set with new Onan QG: $4,500-$7,500 depending on size.

Our visit fee is $145, applied against work. If you decline, the fee covers the trip and diagnostic. We give you the diagnostic and a written quote before any work starts. - Marc

Got questions about your rig? Text a photo to (833) 465-8787 - one of us will take a look and tell you straight. - Marc