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.
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.
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.