If your motorcycle engine starts fine when it’s cold but struggles to start when it’s hot, there are several possible causes.
Problems with fueling, ignition, or compression could all make your motorcycle difficult to start when it’s hot.
This article will take you through some basic checks to help diagnose and fix the issue.
Check Fuel Supply
We need to find out if the fuel is reaching your engine. This is a quick check if you can easily access the spark plugs.
- With the engine hot and not starting, try starting the engine for a few seconds.
- Remove the plug cap and take one of the plugs out.
- The plug should be wet with fuel.
- If the plug is dry, no fuel is reaching the engine.
On bikes with easy access to the gas lines, another quick check is to disconnect the gas pipe and check if fuel flows.
On carburetor-fed engines with no fuel pump, the gas should flow without the ignition on, but engines with injectors will need the ignition switched on to prime the fuel pump.
There are several reasons why a hot engine might not get the fuel it needs.
1. Blocked gas tank vent
You might overlook this problem since it’s not specifically related to a hot engine. However, when you start your bike’s engine, fuel will flow from the tank with no problem. But as the tank empties, a blocked vent will create a vacuum inside the gas tank.
The vacuum will gradually slow the fuel flow until there is too little to run the engine.
It would be easy to mistake this for a problem associated with the engine being hot.
To test for this fault, simply open the gas-tank filler cap, and if the engine starts without a problem, the vent is blocked.
To clear the blockage, first find the hose that fits onto the nipple on the bottom of the tank. Disconnect this and push a small-gauge piece of wire up through the nipple and into the tank. The wire should reappear pretty easily at the top of the vent. If not, there could be more debris in the vent pipe.
Now check the rubber pipe that you disconnected. To see that it’s unobstructed, you can either blow air through it or take it off and run water through it. In either case, it should be completely clear.
Reconnect the pipe and go for a test ride to ensure the problem is fixed.
2. Bend in a fuel line
Gasoline turns into a vapor when heated. Fuel lines that run close to the engine or behind fairing panels could quickly get hot enough to cause vapor formation.
In a fuel line with a bend or an upward turn, the vapor could cause air bubbles to form. This, in turn, may create a vapor lock and prevent fuel from flowing.
Check the fuel lines for bends and either reroute the pipe or cut the pipe to a shorter length.
3. Air leaks
Getting the correct air/fuel mix to the spark plug is essential for an efficient, easy-starting engine. The mixture will be weak if there is a leak somewhere in the fuel system.
This may not be obvious when you start your motorcycle’s engine from cold. You will have used the choke on carburetor engines to make the mixture richer, and the ECU on injection engines will supply extra fuel when starting from cold.
Of course, when you start a hot engine, you wouldn’t use the choke, and the ECU doesn’t add fuel for a cold engine start.
If the engine starts when it’s hot by using the choke, then you may have a leak somewhere. You can also test this when the engine is running by spraying a little carb cleaner on the carb intake pipes. If the engine speed increases a little, there is a leak.
Once you have located the leak, replace the offending piece and test again. If the bike is a good few years old, replacing all the rubber boots around the intakes may be a good investment.
Your motorcycle’s ignition system contains several electronic components that can fail when they get hot.
1. Spark plugs
Your motorcycle’s spark plugs endure an extremely hostile environment. When they fire, the air/fuel mixture explodes and subjects the plug to as high as 800° Celsius temperatures.
Over time, the spark plug materials can begin to break down.
Spark plugs can be checked using a simple process:
- Remove the spark plug from the engine.
- Connect the spark plug cap.
- Hold the plug by the insulated plug.
- Touch the plug thread against a metallic piece of the engine.
- Crank the engine.
- There should be a strong, blue spark.
- If the spark is weak, either you are not making a good connection with the threads, or the plug is failing.
If you suspect your spark plugs, don’t worry; they’re not too expensive to replace.
2. Ignition coil
The ignition coil provides the high voltage required to generate the spark that ignites the fuel in your engine. A failing ignition coil may get hot, increasing its internal resistance and reducing the voltage it supplies to the spark plug.
A weaker spark will make starting a hot motorcycle engine more difficult. Testing coils involves measuring the resistance, but this is not 100% accurate and less so if your problem occurs when the engine is hot.
How to test an ignition coil is a topic for another article.
3. Engine control unit (ECU)
The ECU is a small box of electronic components, and one of the enemies of electronics is heat. Both the components and the soldered connections on the printed circuit boards can be adversely affected by heat.
Components will begin to break down with excessive exposure to heat, and solder joints can turn into “dry joints” with high resistance. In both cases, it could cause an issue with starting your motorcycle when it’s hot.
It may be possible to have your ECU tested or fit a fully functioning ECU if you have a helpful dealer. Modern motorcycles may have a built-in test facility. In most cases, if the ECU is faulty, then straight replacement is the only option.
4. Starter motor
Another electrical component that can be adversely affected by heat is your starter motor. It may not have enough power to crank the engine when it gets hot. If you can bump-start your motorcycle easily, this could indicate a failing starter motor.
Starter motors can be tested and repaired, but whether that is cheaper than simply replacing it will depend on several factors.
Poorly adjusted valve clearances
The valves in your motorcycle engine allow the fuel/air mixture in and the burnt gasses out. Valve clearances are critical and should be checked at the mileage specified in the manufacturer’s service schedule.
When the valve clearances are too small, insufficient air/fuel mixture can get into the chamber before the spark plug ignites the gas. As the engine gets hotter, the clearances could reduce further, worsening the problem.
Exhaust gasses need to be cleared from the combustion chamber entirely after ignition. Leaving the gasses behind will affect the efficiency of the subsequent combustion, and as the engine gets hot, the problem will increase.