Why Does My 4-Stroke Motocross Bike Get So Hot?

There are many reasons why your four-stroke motocross bike is getting so hot. Fortunately, I can show you how to help prevent your dirt bike from overheating.

Before you do anything, there is one thing you should check¬† that some riders choose to ignore. Coolant flows through the engine to keep your bike ‘cool’. If there’s little to no fluid, it’s just going to overheat. So, before every ride,¬† pop the cap off the radiator to check and make sure it’s full of coolant. Just by doing this you can prevent a costly engine repair. On the other hand, if you keep having to fill the radiator because the coolant level is down after every ride, then you have problems. There is a leak somewhere, and you’ll have to take stuff apart to find it. It will often be a blown gasket (base, head, or water pump), a hole in the radiator, or in a hose.

2-Stroke Piston That Overheated (Applies to 4-strokes as well)

That brings us to the next possible culprit. A worn or torn gasket will allow coolant to seep through, reducing cooling, and very possibly causing damage. The least damaging can be the water pump seal/gasket. If you’re lucky, it will just dribble out somewhere around the impeller cover and onto the ground. It can also mix with the tranny oil, causing it to corrode and eventually seize. If that is the problem, it’s best to replace it right away.

A blown head gasket can be a cheap fix, but it could also cause an engine malfunction. If you’re running your bike with a bad head gasket, it’s probably burning coolant, which will show up as white smoke out the exhaust.

A faulty base gasket is usually going to be worse. Coolant will leak into the transmission, contaminating the oil. This will eventually cause it to fail, or the piston/cylinder will overheat and seize due to the lack of coolant in the radiators. It can also happen in a matter of hours or even minutes. This is why it’s always a good idea to take the top-end of an engine off (if it’s a used bike you bought) to see if anything needs replacing, or if something was put in wrong by the previous owner (happens WAY too often, so don’t ever think it won’t happen to you!).

The other common factor of overheating is not one that everyone realizes. Water isn’t the only thing that keeps a bike cooled. Air is still needed to flow past the radiators to keep the radiator temps down. So, if there’s no air-flow, then there is little cooling. Simply put, if you’re idling or putting around a lot, your bike is going to start overheating. Not only does it get hot from little air-flow, but the little amount of gas going through the engine also causes it to get too hot. The gas actually cools the intake and exhaust valves as it passes by, so 4-stroke motocross bikes need to be ridden fairly hard to stay cool. This is one of the major reasons why valves burn up, so don’t let your bike sit and idle for very long (How To Properly Warm-up Your Dirt Bike).

There are a few ways to fix these problems. If it’s a gasket, then pretty much the only thing to do is replace it. Improper jetting will cause problems as well. Too rich and it won’t run right at all. Too lean and it will burn the piston down. Read the Jetting 101 Guide Here and it will show you how to properly jet your dirt bike.

If coolant is weeping out of the overflow tube on a hot day, a simple fix to keep the temp down could be a High-Performance Radiator Coolant. It can bring down the running temperatures by up to 50 degrees Fahrenheit. If there is nothing defective on your bike, this alone can prevent overheating problems.

Damaged Cylinder From Detonation

Another way to cool a bike down is with different gas. I wouldn’t necessarily recommend this for that sole purpose, because if your bike regularly overheats then it has bigger problems. Higher octane gas, such as race-gas, will make your bike run a little cooler. Although, doing this will require re-jetting. Higher octane gas burns at a slower rate, reducing the engine temperature. But, if you keep the previous jetting, it will be on the rich side because it’s putting more gas into the engine than it can burn. Race-gas is used by many racers to help make their bike run better (instead of nasty pump-gas), and probably cooler.

-Tom Stark

Why Is Coolant Coming Out The Overflow Tube?

Has coolant recently started peeing out the overflow tube on your dirt bike? Do you have to refill the radiator with fluid after every ride? If you’re lucky, it’s a cheap and simple fix. But if you choose to ignore the problem, it will only get worse. The two most common explanations for coolant coming out the overflow are: the bike is getting too hot, or there is too much pressure in the cooling system to hold the fluid.

Put A Cap In It!

The easiest problem to fix could be a faulty radiator cap. It may not be able to hold the pressure because it is worn or broken, allowing coolant to leak past it and out the overflow. Without spending any money, take a cap from another bike and put it on to see if that’s the problem. Ride around until the bike is warm, and if it doesn’t puke out any fluid, problem solved! If it does continue to spew coolant, then you know the radiator cap is not the problem.

Radiator Low On Fluid

Can’t Handle The Pressure?

Another common problem that will not only cause the bike to overheat, but will over-pressurize the system is a blown head gasket. It will bring engine temps up greatly, and possibly cause the coolant to boil over. Then the engine leaking compressed air into the system just blows it out the radiator because the cap cannot hold that much pressure. Before you install a new head gasket, though, you will want to check and make sure the cylinder and head are perfectly flat and not warped (usually caused by improperly torquing the bolts). If it is warped, you’ll want to take that to a machine shop and get it fixed. Putting a new gasket in with a warped cylinder/head will do you no good, so don’t ignore it!

Liquid AND Air Cooling

Radiators don’t cool the engine by themselves. They NEED air to keep cool as well, so if they’re not getting any air-flow, it’s probably going to overheat the engine. A common problem that beginner and trail-riders have is riding too slow (More on this Here). Also, if a radiator is all bent up or many of the fins are twisted out of shape, that will reduce air-flow to the radiators. You can usually tweak most of the fins back if you’re careful, but it’s best to get a radiator fixed by a professional if it is badly bent or smashed. To increase air-flow even more, you can get a Vented Front Fender, or make one yourself if you have the patience.

Check Your Propeller

A cause of overheating may be, although not as often, a broken impeller. If it’s cracked or part of it is broken off (yes, even one small piece) it won’t provide sufficient cooling to the bike. If none of the above are the problem, just pop off the water pump cover to see if the impeller is bad or if there’s corrosion. If there is damage, it’s probably going to be an expensive fix if you buy OEM, so you might as well buy a Boyesen Water Pump Kit for a little more and be done with it, for good.

If none of these are the solution to your dirt bike’s problem, then you have bigger issues. If that’s the case, check out my article Why Does My Dirt Bike Get So Hot?

-Tom Stark