Your engine feels sluggish and bogs down. You check the bottle style filter and see it’s only 1/2 full. What’s going on?
The top 8 reasons an ATV fuel filter isn’t filling include:
1 Low fuel level
2 Petcock not fully open
3 Fuel filter blocked
4 Filter incorrectly fitted
5 Porous gas lines
6 Gas tank blockage
7 Fuel pump fault
8 Air locked carburetor
In this post, you’ll learn about the most common reason your gas filter isn’t filling and what you can do about it.
Fuel System Types
Most ATV fuel systems are pretty simple, but other more modern bikes may employ a more sophisticated fuel-injected computer-controlled setup. The most basic system will use a mechanical slide carburetor with gravity feed fuel.
A gravity feed system requires the gas tank to be positioned higher than the carburetor. This is a simple solution and works well until your gas tank runs low, and you climb steep hills.
The addition of a fuel pump solves this problem, and most modern ATVs will have one fitted. Common types include electric, mechanical and vacuum-driven pumps. A common setup is a vacuum drive pump positioned upstream. It draws the gas from the tank through the lines, filter, and pump it to the carburetor fuel bowl.
1 Low Fuel Level
A low gas level can lead to the filter not filling up especially traveling on uneven ground. Gravity feed systems suffer greatly from this condition and can cause the bike to stall or hesitate.
2 Petcock
Your bike is fitted with a fuel tap, which needs to turn to the “On” position. A faulty valve could prevent a sufficient flow of gas from filling the filter. Removing the fuel line upstream of the petcock will allow you to check the flow.
3 Fuel Filter
A blockage in the filter will obviously obstruct flow and prevent the filter filling. If your filter looks dirty or old, go ahead and change it out. This is an easy fix. Your ATV may have a second filter fitted. It’s not uncommon to have a reusable mesh filter integral to the carburetor. Check your carburetor maker and do some research. A restriction of the carb integrated filter may also cause a restriction further upstream in the second filter.
4 Filter Direction
Most inline filters are directional, and if they are, they’ll have an arrow that points to the carburetor (direction of flow). Directional filters may have a simple integral one-way valve to prevent gas drain back. Fitting it the wrong way will cause a fuel restriction.
An incorrect filter size can also cause a half-filled filter. The volume of the filter is too great for the flow rate of the pump. Check the OEM specs of your filter.
5 Porous Gas Lines
Old gas lines become perished and porous, and a fuel pump may draw in air through the porous lines, which will reduce its ability to fill the filter. Check over all your gas lines, bend them, and look for any sign of cracking. Check your line clamps too, a vacuum leak at a clamp will have the same effect.
6 Gas Tank Blockage
A blockage in the gas tank could be something as simple as leaf foliage, the gas drains through a very small orifice and is easily blocked. You can eliminate as a possible cause by removing the gas tank line and checking flow.
7 Bad Gas Cap
A gas tank needs to breathe. Gas that leaves the tank needs to be replaced with air. If it’s not, the gas will stop flowing. That’s why gas caps are vented, but they can block, or a MacGyver style cap may have been fitted. The easiest way to quickly eliminate this as the problem is to go ahead and remove the gas cap, run the engine and check your filter.
8 Fuel Pump Fault
Your fuel pump moves gas from the tank to the carburetor bowl. It may be positioned close to the gas tank or close to the engine. There are two common types electric and vacuum-driven. An electric pump is reliable, but brake pads do wear out. The pump usually activates with the ignition on, and that should be enough to fill the filter.
Some pumps may only initiate a start when the bike is cranked over. Some pumps are fitted with a check valve to prevent extended cranks associated with fuel drain back. Electric pumps can be heard operating and are usually positioned close to the gas tank.
Troubleshooting electric gas pump tips
- Locate pump and listen with ignition on and on crank
- Check power supply connections, fuse and relay
- Check ground connection
- Hot wire pump – run power and ground to test
While your pump may be running, it may still be faulty. Only a fuel pressure test kit will confirm head pressure.
You’ll find all the tools I recommend here on the ATV tools page.
Vacuum Fuel Pump
A mechanical vacuum pump is a far less complex animal. It employs a diaphragm and scavenges an engine vacuum to drive it. A typical pump will have a gas line from the gas tank and a gas line out to the carburetor fuel bowl. These pumps are usually fitted closer to the engine.
A third pipe (from the crankcase) is the vacuum supply line that runs the pump.
Troubleshooting a vacuum pump
- Locate pump – usually bolted to the engine
- Check vacuum supply pipe – commonly perished
- Check lines for kinks
Removing the pump outlet pipe while cranking the engine should cause fuel to flow. Like the electric pump, your vacuum pump may show signs of life but still be faulty. Only a pressure test will confirm this.
Carburetors come in two main flavors, mechanical slide and CV (Constant Velocity) – vacuum actuated. The mechanical slide carburetor is the basic type, and it’s been around forever.
Fuel Injected Closed Loop Fuel System
The latest, more sophisticated kit uses a fuel-injected system and some emission-compliant kit may employ a closed-loop automotive-style fuel system. This system offers huge advantages in terms of reliability, emissions, and self-diagnoses.
An oxygen sensor in the exhaust senses the fuel mix ratio, and the onboard computer makes the necessary fuel adjustments known as fuel trim.
It will typically employ:
- Fuel pump – supplies the gas
- Fuel filter – filters gas filter
- Fuel regulator – controls pressure and often integral to the pump
- Fuel pressure sensors – sends fuel pressure readings to ECU
- Fuel rail – pressurized fuel rail reservoir
- Fuel injectors – supply gas to the intake manifold
- Temperature sensor – supply’s ECU with engine readings
- Air flow meter – supply’s ECU with air volume readings
- TPS – Throttle position sensor
- MAP – engine load sensor
- O2 sensor – measures the burnt fuel in the exhaust system and sends info to the ECU
- ECU – makes all the calculations and controls the fuel system
If a fuel-injected bike isn’t filling the fuel filter, it’s likely it’s an electric fuel pump issue. But the other great advantage of these bikes is their self-diagnosis abilities. Check the ECU for fault codes.
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John Cunningham is a technical writer here at ATVfixed.com. He’s a Red Seal Qualified Service Technician with over twenty-five years experience. He’s worked on all types of mechanical equipment, from cars and trucks to ATVs and Dirt bikes.