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Dirt Bike Not Starting After Winter – Mechanic’s advice

Springtime is a fun time, or at least it was until you discovered your favorite toy didn’t make it through hibernation. Don’t worry about it. This is a common complaint and as a mechanic, I see it a lot at this time of the year. This is something we can fix right now and very shortly you’ll have a big smile on your face!

A dirt bike that won’t start after winter likely suffers from stale gas and carburetor contamination. Draining the old gas, draining the carburetor bowl, and refilling with fresh gas will solve the problem.

In this post, you’ll learn why your dirt bike won’t start and what you need to do to fix it right now. I’ll also share some top tips for dirt bike storage.

Dirt bike won't start

Problem With Gas, Dirt Bikes & Winter Storage

The problem most commonly associated with dirt bike winter storage is stale gas. The problem is modern gas contains ethanol, (E5 and E10) and ethanol gas begins to turn stale after about two months, but this varies depending on ambient conditions and how much exposure to the atmosphere.

In addition, ethanol attracts moisture which causes all types of carburetor issues.

Unlike modern cars and trucks, small engine kit such as dirt bikes employs an open to atmosphere gas tank. This allows the tank to breathe, which is crucial for fuel flow.

Modern cars and trucks on the other hand use sophisticated modules and valves that allow for a sealed gas tank. The advantage to a sealed gas tank is twofold, it protects the environment from harmful vented hydrocarbons and at the same time prevents moisture from entering the tank.

It’s this moisture that causes dirt bike fuel problems over the winter months.

How To Diagnose Dirt Bike Stale Gas

If your dirt bike has been sitting with gas in the tank over winter and that gas hasn’t been treated with a gas stabilizer, then you can be sure your gas is stale. To check, try removing a sample in a glass jar. Stale gas looks the color of pee. Still not sure?

Try the gas shot test. This test bypasses the fuel system and adds fresh gas direct to the cylinder.

To perform the gas shot, use the following steps:

  • Remove the spark plug
  • Add a capful of fresh gas to the cylinder
  • Refit the plug
  • Attempt to start the engine
Spark plug fitting guide

Diagnosing the results

Two results are likely:

1 Engine starts or attempts to fire – Fires may be described as an engine that doesn’t start but tries to. You’ll know an engine that fires as it will puff smoke from the tailpipe as it tries to start. Such an engine under these test conditions likely suffers from stale gas.

2 Engine makes no attempt to start – Likely an ignition system issue (lack of spark). I wrote a post about ignition system testing and you can check that out here “Dirt bike won’t start when hot”

How To Drain Dirt Bike Stale Gas

In the workshop, I use a siphon to drain the gas tank, but removing the carburetor fuel feed line works too. Using a siphon is however a ton easier, they only cost a few dollars and a good siphon will lift oil, brake fluid, coolant, etc., so you’ll find a ton of uses.

Dirt bike gas line

To drain a gas tank using a siphon, open the gas tank cap place the siphon tube into the gas tank, and pump the siphon. Easy!

Or remove the gas line to drain it manually.

To drain the tank manually you’ll need a gas line clamp and a suitable container. Gas stinks, it’s best to drain the gas tank outdoors.

The draining process is as follows:

  • Turn off fuel valve
  • Remove gas cap
  • Remove the gas line from the carburetor and place it in the container
  • Turn the gas valve on and drain the tank

Most recycling centers will accept waste gas and oil.

After the tank has been drained, you’ll need to drain the carburetor bowl and that’s what we’ll cover next.

Draining Dirt Bike Carburetor

Dirt bike carburetor bowl drain

All dirt bike carburetors employ a carburetor bowl. The bowl is a reservoir where gas and often dirt collects, Draining the bowl helps clean the fuel system of contaminated gas.

The process is as follows:

  • Turn the gas valve off
  • Remove gas tank cap
  • Gain access to the carburetor
  • Located the carburetor bowl
  • Locate the carburetor bowl drain fastener known as the bung (usually a small drain screw in the bowl base)
  • Place rag under the carb bowl and open bung
  • Open the fuel valve and add a small amount of fresh gas to the empty tank to help flush out the carburetor bowl
  • Close the bowl bung

Top Tips For Dirt Bike Winter Storage

A dirt bike that’s stored correctly will start without issue come spring. Proper storage isn’t difficult and it requires little effort or expense.

Here are my top dirt bike storage tips:

  • Change engine oil (4-stroke engine) – Removes internal engine moisture and acidic contaminants
  • Use gas stabilizer in the fuel tank – See below
  • Fill gas tank – Helps prevent condensation on inner tank walls
  • Clean and oil air filter – Prevents trapped moisture and rodents
  • Turn the gas valve off – Takes fuel pressure off the float and valve
  • Inflate tires – Helps seal and protect tire walls
  • Add oil capful engine oil to the cylinder – See below
  • Set engine to top dead center (TDC) – See below
  • Seal exhaust – Stuff tailpipe outlet with an old rag or plastic bag
  • Remove chain – Remove the chain and seal it in a plastic bag with some oil
  • Lift bike – Support chassis to lift tires clear of the ground
  • Protect metal – Spray bike thoroughly (not rotors or pads) with WD40 or use Teflon coating (guards against corrosion)
  • Rodent bait – Rodents love to eat wiring insulation
  • Store indoors – Overwintering indoors is critical
  • Breathable cover – A breathable cover keeps moisture trapping dust and dirt off the bike while allowing the bike to breathe.

Use gas stabilizer in the fuel tank

The gas stabilizer is specially formulated to repel moisture and keep ethanol gas fresh for up to twelve months. The gas stabilizer is strongly advised for your dirt bike, in fact, use it in all small engines ATVs, Outboard motors, Tractor mowers, Chainsaws, etc.

Gas stabilizer

The gas stabilizer is not a substitute for a two-stroke mix, so if your dirt bike is a two-stroke mix as usual then add the gas stabilizer. Mixing is easy, your gas stabilizer will recommend the mixing ratio.

A common ratio is 1/2 ounce stabilizer (about a tablespoon full) to every gallon of gas. Ride the bike about to help distribute the mixed gas throughout the fuel system. That’s it, your bike’s carburetor is protected from winter gumming

Add engine oil to the cylinder

I like to add a capful of engine oil to the cylinder. Oil in the cylinder helps protect the walls from corrosion. The process is as follows:

  • Remove the spark plug
  • Use a syringe or funnel to add a capful of fresh engine oil to the cylinder
  • Turn engine over to distribute oil

Set engine to top dead center (TDC)

Setting the engine to Top Dead Center closes the valves and seals the cylinder which helps protect the engine cylinder walls from moisture and prevents sticking valves.

To set the engine to Top Dead Center (TDC), follow these steps:

  • Remove spark plug
  • Place a suitable non-metallic object into the cylinder to make contact with the piston crown (a pencil works fine)
  • Using the Kickstarter or socket and ratchet, turn the engine over slowly so as to push the pencil from the engine
  • When the pencil is pushed furthers from the cylinder, the engine is set to TDC
  • Refit plug
Engine set to TDC

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