Most modern motorcycles are liquid-cooled these days. Some bikes, like Harleys and my Honda XR650L, are still air-cooled like in the old days. If you just bought your first air-cooled motorcycle, you may be concerned that standing still in traffic will cause it to overheat and damage the engine. How bad can it be?
An air-cooled motorcycle will not overheat in slow-moving traffic, provided that the air-fuel mixture and idle speed are correct, the valve clearances are within specification, and the oil is changed regularly. If it is really hot outside and you need to stand still in traffic for long periods of time, it is best to switch off the engine.
This is one of those questions where you will probably never get a clear answer. That doesn’t mean that we can’t get closer to the truth. Here’s what I’ve found out since I bought my air-cooled Honda XR650L.
Will an Air Cooler Motorcycle Overheat in Slow Traffic?
Air-cooled motorcycles have been around for ever and they were carefully designed not to overheat. Air-cooled engines have fins on the cylinder head that radiate heat into the air and away from the engine. While it is true that airflow over the fins will assist in dissipating the heat, it doesn’t have to be a a strong wind. A very slight breeze will be sufficient in most cases.
Air-cooled motors also tend to have larger oil capacities with an oil system that carries heat away from the hottest areas in the motor. Some air-cooled engines have external oil coolers with a little radiator that sits exposed to the airflow. This will help cool the engine more efficiently. Just remember that when oil gets too hot too often, it loses some of its lubricating properties. That is why it is very important to change your oil after you suspect the engine might have run a bit hotter than normal.
I’ve spoken to different mechanics and some say that even the slightest bit of air flow over the fins on the head is enough. Other riders, mostly on forums, disagree and are determined that even a few minutes of idling without moving will cause catastrophic engine failure.
It is so difficult to know for sure whether an air-cooled engine got too hot or not. If, for instance, your motor is expected to last 50 000 miles and due to getting a bit too hot in traffic, it will now only last 30 000 miles… Does that mean the engine overheated? Will you know for sure that that is what caused the engine to wear prematurely?
Either way, if it is 100 degrees Fahrenheit on a wind-still day and you are stuck in bumper-to-bumper traffic on hot, radiating asphalt, it is worth it being mindful of the possibility of the engine getting too hot.
Will a Harley Davidson Overheat in Slow Traffic?
I am mentioning Harleys separately here for a number of reasons. Firstly, Harleys are very popular in the U.S. and there are many of them in daily use. Secondly, unlike single cylinder thumpers, Harleys have air-cooled twin-cylinder engines with one cylinder in front of the other. Finally, many modern Harleys have a built-in safety system to prevent overheating at idle.
Many modern, fuel-injected, Harleys have a system known as Engine Idle Temperature Management System (EITMS). This has been available on the touring motorcycles since 2007 and can now be found on various other Harley models. This system, when enabled, automatically switch off the fuel supply to the rear cylinder when it gets too hot. The rear cylinder still moves up and down and essentially becomes an air pump to help cool the rest of the engine. Also, less explosions means less heat generated. After a while, the engine may even shut down completely by itself.
The EITMS can be disenabled by following the instructions in the owner’s manual. On carbureted Harleys, proper maintenance is critical as we shall discuss in a moment.
Evidence of Air-Cooled Engines Overheating
I read all the forum threads I could find on the topic and found many different opinions. Some riders say their bikes run just fine in hot conditions and slow bumper to bumper traffic. Others report overheating, even while moving slowly.
There were some old-timers that have done hundreds of thousands of miles on air-cooled motors for over four decades. They swear they’ve never had an air-cooled motor overheat in slow traffic. Others tell stories of catastrophic engine failure after only a couple of minutes of idling.
Regardless of who you want to believe, there are a few things that you can do to prevent your air-cooled motorcycle from overheating. In many counties outside of the U.S. it is legal to split lanes which means you never need to come to a complete stop for very long. In the U.S., California remains the only state where it is legal to split lanes. But that doesn’t mean you are destined to kill your bike in rush hour traffic. Overheating of an air-cooled engine can be prevented with proper maintenance.
How to prevent an air-cooled engine from overheating
- Fresh oil: The main purpose of oil is to lubricate the moving parts inside the engine. Oil also helps to move heat away from where it is generated, i.e. inside the cylinder head. Oil loses its ability to lubricate properly after a certain amount of heat cycles. That is why you need to change your oil regularly. The hotter a heat cycle, the more the oil degrades and loses lubricating ability. Change your oil regularly, or immediately after a ride where you suspect the engine got a bit too hot.
- Valve clearance: Proper valve clearance is critical to prevent your motorcycle from overheating. Over time, the valve clearance will reduce until there is no clearance left. If the valve gap gets too small (especially exhaust valves), the valves won’t close properly and allow the explosion in the engine to escape and heat up the exhaust manifold or even melt a piston. A friend of mine burned out his KLR650 like this.
- Idle speed: Make sure your idle is properly adjusted and not too high. The higher your idle RPM, the more explosions occur per second. More explosions, more heat.
- Air-fuel ratio: This is particularly important on carbureted motorcycles. If the air-fuel mixture is too lean, the engine will run hotter (due to too much oxygen in the combustion chamber). Some bikes, like my XR650L, are jetted too lean at the factory in order to pass emissions regulations. That’s why so many single cylinder thumper riders immediately re-jet their carbs. If you are unsure, rather run your engine a little too rich than a little too lean. In the video below I explain this in more detail.
Maintain your bike properly and change the oil at regular intervals (more frequently if you live in really hot climates) and you should be fine. I mean, BMW’s big R1200GS was air-cooled until fairly recently, and they’ve been around the world so many times without overheating. So has the trusty old R1150GS’s.
If it really gets very hot and you know the traffic is not going to start moving in the next minute or two, just switch off your engine so the internal explosions can stop for a while.
Conclusion
Motorcycles have been air-cooled since the late 1800’s and only in more recent times did many manufacturers switch over to liquid cooling. Air-cooled engines were designed to last, even in hot summer months and in traffic. If you are really worried, just fit a temperature gauge to your bike to see what difference certain speeds or conditions make. And make sure your bike is properly maintained.
If you are not overheating wearing your rider gear, then your bike will probably be fine too.
