Troubleshooting an Engine That Only Runs on Choke
An engine that only runs on choke is a common complaint, especially among dirt bike and ATV owners.
If you find it’s happening to your motorcycle or quad, read on to learn what causes it and how to troubleshoot it.
Why a Motorcycle or ATV Only Runs on Choke
An internal combustion engine needs a precise mixture of fuel and air to run properly.
When the engine is cold, it needs slightly more fuel in the fuel/air mixture for it to start, which is what the choke does. The choke increases the amount of fuel in the mixture, creating a richer mixture. This allows the engine to function at its best until it has warmed up enough to run on its normal operating fuel mixture. At that point the choke is turned off and the engine runs with a leaner fuel/air mixture.
If a motorcycle or ATV only runs with the choke on, it’s because the richer “choke on” mixture is actually closer to the engine’s normal operating fuel mixture than the leaner “choke off” mixture. So when the choke is turned off, the engine gets too little fuel and too much air for it to run and it stalls.
Why a Motorcycle or ATV Engine Runs Too Lean
If you haven’t altered the mixture screw on your carburetor, then why is the engine suddenly running too lean? Either the jets are clogged and you’ll have to clean the carburetor or there’s a leak allowing excess air into the carb.
A pilot jet and a main jet inside the carburetor control the amount of fuel entering the carb. These jets have incredibly fine tolerances, and if they get obstructed or blocked, the amount of fuel entering the carb will be reduced and the effect on the performance of the carburetor and the engine will be dramatic.
Tiny debris particles from dirty fuel can easily block a jet, but the most common cause of clogged jets is stale fuel sitting in the carb when the bike or ATV is inactive. When fuel breaks down, it leaves behind a gummy residue, almost like a varnish that builds up on everything it touches, including the pilot and main jets.
A split in the air intake boot or a vacuum line, or a loose clamp holding the intake boot to the carb or engine will allow additional air to flow into the carburetor. This excess of air overwhelms the amount of fuel in the carb and creates an overly lean mixture.
Troubleshooting Carburetor Problem
Forget about pouring a fuel additive into your gas tank to clean your carburetor. Additives won’t clear jets blocked by dirt, and aren’t strong enough to remove old fuel build-up. You’re going to have to strip the carb.
Actually this is a lot simpler than people think. To remove the carburetor, you’ll have to disconnect the throttle cable, choke cable, air intake, fuel line, a couple of mounting bolts and that’s it. Once the carb is off the machine, it’s best to clean the carburetor to minimize the chances of dirt getting inside it.
Remove the pilot jet, the main jet, the needle jet and the jet needle from the carburetor, and clean them thoroughly with carb cleaner. Otherwise, replace the jets with new ones. A carburetor rebuild kit will include all the jets needed to overhaul the carb, as well as all the O-rings, gaskets and washers. Don’t forget to also clean all the channels running through the carburetor with carb cleaner and compressed air before reinstalling the jets.
Kawasaki Dirt Bike Carburetor Rebuild Kit
Honda ATV Carburetor Rebuild Kit
Troubleshooting Air Leaks
To repair an air leak, you obviously have to find it first.
Start by making sure the clamp holding the air intake boot to the carburetor is tight, and the clamp or bolts mounting the intake boot to the cylinder block are also tight. If everything is secure, inspect the intake boot for splits or damage.
If you still haven’t found the leak, start the motor and spray carb cleaner around the air intake boot, the area where the boot connects to the carb, and the area where the boot connects to the engine. Also, spray any vacuum lines if your carb has them. The carb cleaner will be drawn in at the point of the leak, making it easy to pinpoint.
Hopefully tightening a clamp will fix the leak, but if the problem is a split boot or vacuum hose, the part will need to be replaced.
Keeping the Carburetor in Good Condition
To prevent carburetors from getting clogged, never leave them full of fuel if the motor isn’t going to be running for a while.
Set the petcock to OFF, and either run the motor until all the fuel has been used by the engine, or drain the carburetor by removing the drain screw from the bottom of the carb’s float bowl.