Start A Flooded 4 Stroke Dirt Bike (Mechanics hack)


Flooding the engine is easy to do. Sure it’s a pain, but it’s an easy problem to solve, and before long, you’ll have her humming.

To fix a flooded bike, follow these five steps:

  1. Allow bike sit for ten minutes
  2. Choke off
  3. Ignition on
  4. Throttle held wide open
  5. Crank the engine

By the end of this post, you’ll learn two ways to fix your flooded bike. You’ll know why it happens and how you can prevent it from happening.

What Is Flooding

When fuel enters the combustion chamber and for various reasons doesn’t ignite, the result is a nonstarting engine, and this condition is known as flooding.

The fuel, instead of combusting, saturates the cylinder and soaks the plug-in gas. The plug, being an electrical component, struggles to operate when soaked in fluid. The situation is quickly compounded by continuous attempts to start the engine.

Symptoms of flooding include:

  • Non starting
  • Strong of raw gas
  • Gas in the air box
  • Gas leaking from the carburetor
  • Gas leaking from the exhaust
  • Black smoke on start-up

In very severe cases of flooding, the cylinder may fill with gas, causing the piston to lock inside the cylinder. It’s a condition known as hydro locking. Symptoms of hydro locking include a kick start that won’t move, or if when cranked by battery power, the engine won’t start, clicks.

How To Dry The Cylinder Quickly

The easy way to dry out a bike is first to allow the fuel time to evaporate. Ideally, you could leave the bike sitting for an hour, and the problem would mostly be repaired.

For most, this isn’t a real option. You want action more immediately. That said, letting the bike sit for even just 10 minutes will help start the motor.

  1. Allow some time for evaporation
  2. Turn choke off – Choke adds more gas for cold starts, but we have plenty of gas already. Some bikes may have Auto choke. If that’s the case, you can’t turn it off, instead jump to the plug removal & crank method.
  3. Turn ignition “On” as we’re expecting the engine to start
  4. Open the throttle fully, but slowly and hold it wide open. This increases airflow through the engine and helps evaporate the raw gas. Removing the air filter helps too improve airflow even more.
  5. Hit the start button, holding the throttle open. The engine won’t fire immediately but will after a few cranks.

If it doesn’t catch and run, go ahead and jump to the next method, Defcon 2.

Plug Removal & Crank Method

This method is reserved for those that need immediate action, or the previous method didn’t work. This is simple and is guaranteed to dry out the cylinder. It’s more work than the previous method, but not much.

You’ll need a plug wrench and a shop towel.

Step 1

Go ahead and remove the plug, wrap the plug wire in a shop towel and position it away from the cylinder hole. IT’S IMPORTANT TO GET THIS CORRECT. SPARK AND GAS COULD IGNITE, CAUSING DAMAGE AND INJURY.

Alternatively, unplug the primary coil wires if easily located.

Step 2

Remove the plug, examine, clean, and gap. The plug will likely be soaked in gas. Clean it with a shop towel and wire brush.

Check the plug gap. It’s a simple process of measuring the plug electrode gap with the gauge and adjusting it with pliers if needed. An average gap spec is about .028 – .032 in (.7 – .8 mm).

Step 3

Crank over the engine, with the plug, removed. This should be done in a well-ventilated area as raw gas will spray into the atmosphere from the plug hole. Be sure to locate the covered plug wire as much as possible, away from the plughole.

8 to 10 full cranks should do the job. If your bike is fuel injected, hold the throttle wide open, this prevents pulsing of the injector(s).

Step 4

Refit your cleaned and gaped plug or fit a new one. Tightening an old plug – tighten 1/4 turn after it seats. New plug – tighten 1/2 turn after it seats.

Step 5

Plug wire on, crank over the motor without choke first. If it fails to start, apply 1/2 choke and try again.

If the bike fails to start, you may have an ignition system fault, and the flooding is merely a symptom.

Why The Engine Floods

The reasons engine floods are wide and varied from operator error to mechanical. In this section, we’ll try and list all the most common ones.

  • Operator error – applying choke when engine is hot
  • Laying the bike on it’s side – may flood the engine
  • Water soaked electrics – storing bike out side can cause flooding indirectly
  • Dirty air filter – chokes off air and floods engine with gas
  • Bad gas – old gas won’t ignite and instead floods the plug
  • Bad plug – a dirty or poorly gaped plug will struggle to ignite gas
  • Engine modifications – changes to how the engine breaths
  • Carburettor faulty/too rich – carb supplying too much gas
  • Carburetor float bowl sticking – floods engine with gas
  • Carburettor float needle leaks – floods engine with gas
  • Faulty petcock – floods engine with gas
  • Bad coil – prevents good spark
  • Bad CDI box – causes misfiring and flooding
  • Bad CKP sensor – causes misfiring
  • Bad stator – causes misfiring

How To Prevent Flooding

You already know flooding is very often caused by an underlying problem. The flooding is a symptom, not the root cause.

The best way to prevent flooding is good maintenance, and storage of your bike comes first. How your bike is stored will have the largest effect on performance and reliability. Both directly affect, as you know, the chances of flooding.

Storing your bike

Bikes, while weatherproof, aren’t impervious to moisture, and moisture is the killer of all machinery. Maybe your bike won the lottery of life and lives in a dry state. I am so jelly.

But for most, we have to deal with rain and cold and moisture. A bike should be stored indoors if possible and always covered using a breathable cover. Plastic will only serve to lock moisture in.

The bike should be stored dry and clean with a full tank of treated gas. Gas goes stale and needs to be treated to keep it fresh. A gas stabilizer will help gas keep its Zing for up to one year.

Spraying the electrics with WD40 will help keep moisture out. In fact, coat the whole bike (brakes excluded) if you’re storing long term.

A full service at the end of every season and a good routine maintenance schedule will all but eliminate flooding:

  • Oil and filter (screen clean) every 300 miles or 3 months
  • Air filter clean and oil every 100 miles or 1 month
  • Carburetor bowl drained monthly and filter yearly
  • Carburetor mix adjustments as needed
  • Plug clean & gap every 3 months and changed yearly
  • Spark arrestor cleaned twice year
  • Coolant strength checked yearly and changed every 2 years
  • Valve lash checked and adjusted yearly

You may also find the following posts useful:

How often should I start a motorcycle?

Bike won’t start, no click?

Can I ride with choke on?

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John Cunningham

John Cunningham is a certified mechanic and writer on ATVFixed.com. I’ve been a mechanic for over twenty-five years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of ATV ownership, from maintenance & repair to troubleshooting.

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