Low or no compression is something most dirt bike riders have had experience with. In fact, most motorcycle riders have had to deal with low or no compression at some point in their life. Diagnosing the exact reason can be frustrating if it happens to you and fixing it even more so. So what are the possible reasons why dirt bikes would have low or no compression?
There are many different reasons why dirt bikes have low or no compression. The possible causes include worn valves or valves that were not adjusted properly, a blown head gasket, a damaged piston, and worn or broken piston rings. In more serve cases, the causes include rings that are worn out, cylinder walls that are scoured, pistons that are damaged, and a damaged head gasket.
In this article, we will discuss the symptoms of low compression, what causes it, how to test for low compression, and how to fix it.
Before we start, It is essential to know the difference between low compression and no compression. In a nutshell, low compression will take you a few hours to fix, whereas no compression might take you a few days to fix.
If you send it to a bike mechanic, low compression will cost you between $700 to $1k, whereas no compression could cost you between $2.5k to $4k. With low compression, your bike will still run, but you will notice a significant performance loss. With no compression, your bike will probably not be able to start at all.
What does low compression mean?
An engine works by compressing air and fuel together in a cylinder. The piston pushes up and compresses the air-fuel mixture, causing the spark plug to ignite the mixture and cause combustion, which pushes the piston back down again.
The more compression you can get, the greater the combustion, the more power you can generate. If there are any leaks in any of your cylinders, you lose compression. Losing compression will cause you the bike to lose power, and ultimately get to the point where the bike might not be able to start.
The symptoms of low compression
The symptoms of low compression can be several things. Your dirt bike will struggle to start, the bike will misfire randomly, the misfire will become more and more frequent as your bike loses compression, and finally, your bike will feel sluggish as it struggles to build power.
If your dirt bike is showing any of the symptoms I mentioned above, you may be dealing with low compression. You want to get this sorted before it leads to no compression, as we discussed above, the difference between low and no compression can end up being very expensive and time-consuming.
How to test for low compression
First, you want to get yourself a compression tester, you can find them in most stores, and they are inexpensive. Before you start, it is better to do the test while the engine is warm as a cold engine won’t give you an accurate reading when testing.
You want to start by removing the spark plugs from their cylinders and thread your testing gauge into the socket where you removed the spark plug. It is advised that you place a very small amount of grease around the O-ring of the tester before you thread it into the socket, this will reduce friction and create a better seal.
Now that you have removed all the spark plugs and attached the compression tester, it is time to do some testing. You want to hold the throttle of your dirt bike fully open (don’t worry, you have removed all the spark plugs.) You must have the throttle open to allow air into the engine. With the throttle open, you want to kickstart the dirt bike. You want to keep kicking until the needle on your gauge stops moving.
Once the gauge needle stops moving, check the psi on the gauge and compare it to what the manual of your bike says the recommended psi is. In general, it should be between 130 and 140 psi. Repeat this step for all the cylinders.
Causes of low compression
There are a few different reasons why your dirt bike is losing compression. Remember that some sections might not apply to two-stroke dirt bikes.
The first and easiest fix would be worn valves or valves that were not appropriately adjusted. Dirt bikes are often pushed to their limit and have to endure wet, muddy, and also sandy conditions. This all puts extra strain on all mechanical parts of the engine, including the valves. Damaged, misadjusted or, worn valves can all lead to a blown head gasket and should be sorted before that happens.
The second cause could be a blown head gasket. A blown head gasket can be caused when the dirt bike overheats, so it is good practice not to over-rev the bike. Make sure your air filters are clean and always do routine maintenance.
The third reason could be damaged, worn, or broken piston rings. There are a few signs of damaged pistons and piston rings. You will notice that your bike is consuming oil at increasingly higher rates or that your bike is losing a considerable amount of power when you accelerate. Your exhaust is producing grey smoke, which is a sign that your dirt bike is burning oil, hence the heavier oil consumption.
The fourth and hardest to repair reason could be that your rings are worn out, your cylinder walls are scoured, your pistons are damaged and/or you have a damaged head gasket. This will require a complete motor overhaul. The importance of routine maintenance cannot be stressed enough. A small mechanical problem can eventually lead to a break down of other components.
Solutions for low compression
Whether you attempt to solve the issue on your own or by taking it to the shop, low compression can be costly and time-consuming so please make sure that if you are experiencing problems, you are 100% sure that it is the compression.
Start by draining the fuel, remove the spark plugs, disconnect any pipes from the engine, and then remove the engine from the dirt bike. Check your cylinders, valve, pistons, and head gasket for any issues. Once you find the problem, all you have to do is replace the damaged parts and put everything back together again.
If you have little or no experience, it is advised that you take your bike to a professional as they know what to look for. Tinkering with a dirt bike engine, if you don’t know what to look out for, could cause damage to components that were previously working just fine.
I hope that you never have to face a situation where your dirt bike has low or no compression, but unfortunately that might not be the case and if you regularly use your dirt bike, probably is inevitable.
That being said, at least you now know. what the possible reasons for the low or no compression could be and should make it easier to diagnose and fix, so you don’t miss out on any fun with your dirt bike.
Happy dirt biking!