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How to Diagnose and Repair a Broken Lawnmower Fuel Line

Bob Jackson
Last Updated on
by Bob Jackson

This tutorial explains how to diagnose and repair a broken lawnmower fuel line for a Gravely ZT1640 lawn mower (Model #915034) that ran briefly, quit and wouldn’t start.

The spark plug, air cleaner and fuel lines are checked until the problem is found and repaired. The mower is about 4.5 years old, has about 280 hours on the engine hour meter and is equipped with a 16 HP Briggs and Stratton Intek OHV single cylinder engine.

Gravely ZT1640 Lawn Mower
Gravely ZT1640 Lawn Mower

The problem began one morning when I backed my mower out of the garage to mow the lawn. The mower started up immediately, ran strong but quit shortly after I backed it out of the garage. I tried restarting the engine, it would fire maybe once or twice on full choke, but wouldn’t start. After checking a few basics like the fuel selector switch it wouldn’t fire at all when cranking. The engine sounded exactly like this fuel starvation sound file on the Briggs and Stratton customer support site.

Lawn Mower Engine Starts, Then Shuts Down

Every engine needs fuel, spark and air in the correct amounts to run. I always check the oil, air filter, fuel tanks and tires before mowing. When the engine won’t start, the troubleshooting steps are:

Bad Gas

Although I don’t use fuel stabilizers during the winter months, I’ve never had a problem with bad gas and have refilled both tanks a couple of times this season with newly purchased gas. The mower stays in the garage so I tended to discount the possibility of bad gas.

Air Filter

A dirty air filter can prevent a mower from running. I rechecked the air filter, it was clean. Just to be sure, I removed the air filter and cranked the engine. No joy. I replaced the air filter.

Spark Plug

The spark plug had been replaced at the beginning of this mowing season, so I doubted if the spark plug itself was bad. To check if the spark plug was being energized, I took a 12 inch strand of NM 14/2 copper wire and stripped about 1 inch from the ends. Bending one end of the wire into a U shape, I pulled off the spark plug wire and gently pressed the U end of the copper wire into the spark plug boot. I used a pair of insulated electricians plies to hold the boot and wire. The other end of the copper wire was held next to the head of the spark plug while a helper turned the starter key. I had a strong bright blue spark between the copper wire and spark plug. OK – I’ve got spark, so there doesn’t appear to be problem with the electrical system.

Aside: There are numerous commercial spark testers available that do a great job.

Fuel Pump

The ZT1640 has a vacuum driven fuel pump. To check if fuel were getting to the carburetor, I got a fire extinguisher (just in case!) and a plastic can to catch the fuel. I removed the fuel line from the carburetor expecting fuel to drain out. The carburetor and fuel line were dry! Not a single drop of gas. Now I’m getting somewhere! The problem is fuel starvation. But why?!

Holding the carburetor fuel line in the plastic can, I had a helper crank the engine. No fuel was flowing. I was also looking at the fuel filter and noticed air was bubbling up from the bottom of fuel filter. Ah ha! This told me:

  • The fuel pump was working.
  • The fuel filter empty of gas.

Therefore a fuel line must be clogged or maybe I had a bad fuel selector / shutoff valve.

Fuel Pump and Fuel Filter - Gravely ZT1640 Lawn Mower
Fuel Pump and Fuel Filter – Gravely ZT1640 Lawn Mower

Clogged Fuel Filter, Fuel Line or Fuel Shutoff Valve

I removed the metal clips that secure the fuel lines and removed the fuel filter. Blowing gently through fuel filter I found the fuel filter to be clear, so this isn’t blocking the fuel flow.

Using my portable air compressor and a blow gun with a tapered rubber nozzle, I checked the fuel lines by:

  1. Setting the fuel selector valve to the right tank.
  2. Removed the gas cap for the right gas tank.
  3. Pressed the air nozzle to the fuel filter supply line.
  4. Blew some air into the line.
  5. Repeated steps 1 to 4 for the left fuel tank.

The air whistled out of the fuel tank in good volume. Huh!? Something’s wrong here. The fuel lines aren’t blocked, but the air shouldn’t whistle; rather it should bubble up through the gasoline in the fuel tank.

Checking for a Clogged Fuel Line
Checking for a Clogged Fuel Line

I repeated the blow-through tests on the fuel lines while shining a flashlight into the fuel tanks. Definitely – the air was blowing into the top of the fuel tank, not bubbling up through the gasoline as one would expect.

Lawn Mower Broken Gas Tank Fuel Lines

I grabbed my high intensity flashlight and shined it into the left fuel tank. I could see the fuel line lying on the bottom of the tank, but it is the head or tail of the line? It was difficult to tell. I checked the right fuel tank and saw the yellow/tan fuel line running down the side of the tank from the external fuel line fitting. I touched it with a metal rod to trace it’s direction and it fell off and sunk to the bottom of the tank! Disappointing, but mystery solved – the fuel lines are broken inside the tank.

Broken Fuel Line inside the Gas Tank - Gravely Lawn Mower
Broken Fuel Line inside the Gas Tank – Gravely Lawn Mower

Here’s a closeup of the broken fuel line at the bottom of the gas tank.

Broken Fuel Line inside the Gas Tank
Broken Fuel Line inside the Gas Tank

Remove the Broken Fuel Lines

It was obvious that a special tool would be needed to fish out the broken fuel line or I’d be here all day. I went to my local auto parts store and bought a 24 inch retriever claw for about $6.

24 inch Retriever Claw
24 inch Retriever Claw

A retriever claw is used to grab items in difficult to reach places –  such as when you drop a bolt or nut when working on a car engine. This is the business end of the claw.

Retriever Claw
Retriever Claw

The retriever claw and a flashlight made quick work of extracting the broken fuel lines.

Retriever Claw and Broken Fuel Line
Retriever Claw and Broken Fuel Line

The old fuel line. The left top end broke off at the metal fitting at the gas tank sidewall after deteriorating in the gasoline and/or mower vibration. The bottom end is weighted so it sits on the bottom of the tank and has a metal screen. I didn’t see any identifying marks on the fuel line – it appeared to be vinyl that had aged to a tan color.

Broken Gas Tank Fuel Line - Gravely Lawn Mower
Broken Gas Tank Fuel Line – Gravely Lawn Mower

Overtime, the fuel line deteriorated and became brittle and cracked where it laid on the bottom curve of the gas tank. This particular crack wasn’t the cause of the fuel starvation since it was below the fuel level about 6 inches from the weighted brass end.

Fuel Line Deteroriation from Gasoline
Fuel Line Deteroriation from Gasoline

This repair is continued in How to Diagnose and Repair a Broken Lawnmower Fuel Line – Part 2.

Take care,

Bob Jackson

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Bob Jackson
Bob Jackson
Technology product manager by day and a prolific handyman in the evenings and over the weekends. Bob was the founder of the original Handyman How To website and that tradition continues on this site with excellent new handyman content into the future.
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