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Low compression on your dirt bike is one of those things that is relatively inevitable. Whether your engine is a two-stroke or a four-stroke, its compression will eventually run low and will require your attention.
In a two-stroke dirt bike engine, the likeliest cause for low compression is worn pistons or rings. A four-stroke engine also has piston rings that need to be replaced regularly. However, the latter can also experience worn intake or exhaust valves, leading to low engine compression.
As is the case with most dirt bike questions, there is a lot of detail that goes into these common problems, and the more you understand about the machines, the more capable you will be at solving issues before they happen.
How to Tell If My Dirt Bike Has Low Compression
Low engine compression is the number one reason for dirt bikes working perfectly one day and struggling the next. Fortunately, there are many symptoms that indicate that your engine is in need of attention. In the case of a two-stroke dirt bike, you may notice any of the following signs that indicate your engine compression is low.
- It becomes difficult to start
- The kick-starter is too easy to kick over
- Your dirt bike feels lower power than usual
- Your spark plug is fouling
- Your dirt bike won’t idle well
If you have a four-stroke dirt bike, many of the symptoms will be similar to the two-stroke variety, but with a few notable differences. You can expect to see any of the above, but with the following added to the list.
- Dirt bike backfires or pops when decelerating
- Complete loss of power
- Runs very roughly
The only benefit of low engine compression is that it is easy to identify, thanks to these extremely obvious symptoms. Once you find your dirt bike showing any of the above signs of low compression, it’s time to get it tested.
How to Test Your Engine Compression?
Completing a compression test is reasonably straightforward, but there are a few variables to be aware of before starting. There are actually two ways to test an engine, warm or cold. The concept behind a compression test on a warm engine is that it will give you more accurate results because the machine will be closer to its average operating temperature.
While this has the benefit of accuracy due to the thermal expansion of the piston, cylinder, and rings being more representative of your engine when it’s running, it can be challenging to replicate this when testing. There are a few steps that need to be completed to run the test, and your engine will inevitably cool down as you get prepared, so in the pursuit of accuracy, you may end up with more variable results.
Choosing to perform a cold compression test is much simpler. However, it is important to be aware that the resulting compression values that you get from your test on a cold engine will be lower than a warm one. While mechanics will go back and forth over which choice is better, repeatability is the key to successful testing, and a cold compression test is infinitely easier to duplicate.
Step-By-Step Two Stroke Compression Test
- The first step is the simplest one, and that is to remove the seat from your bike. Note that some bikes will give you sufficient access to the engine that you won’t need to remove the seat, but consult your owner’s manual to determine if yours falls into that category.
- Remove both the fuel tank and the radiator shrouds from your bike. If your bike has a petcock equipped, turn it off before removing the fuel line. For safety and cleanliness, catch any fuel draining from the line with a rag.
- Next, you will need to remove the spark plug cap. However, before you remove the plug itself, use compressed air to blow dust and debris out of the plug cavity so that it can’t get inside your engine. After doing so, remove your spark plug.
- At this point, you should install your compression tester into the spark plug hole and ensure that all of your fittings are tight to reduce the possibility of getting false readings.
- You will then hold the throttle wide open and kick the bike over five times as quickly and as hard as you can. Then, confirm the number recorded on the compression tester.
- Once you’ve recorded the value somewhere, reset the testing gauge, and repeat your test three to five times to ensure that your results are accurate.
Testing compression on a two-stroke dirt bike is not a very complicated process. As long as you follow these steps and confirm the details of your exact make and model of bike in your owner’s manual, you should be able to get consistent readings of your dirt bike’s compression.
It is worth noting that a four-stroke engine will also involve a compression relief system and will have a specific allowable compression range. Consult your owner’s manual to find this information, and if your test indicates low compression in your four-stroke engine, you should complete a leak down test to determine what components need replacing.
What Level of Compression Should My Dirt Bike Have?
Now that you have a recorded compression value, you will need to compare it to your dirt bike’s recommended minimum compression. As mentioned above, your owner’s manual will have a minimum PSI specification, but there are commonly upheld two-stroke PSI values that you can use as a guideline. Be aware that these numbers are not absolute for every dirt bike.
- 50cc engine – 120 PSI minimum
- 65cc engine – 120 PSI minimum
- 85cc engine – 130 PSI minimum
- 125cc engine – 140 PSI minimum
- 250cc engine – 170 PSI minimum
- 500cc engine – 140 PSI minimum
Along with various makes and models possessing individual minimum requirements, where you live may also make a difference. Suppose you run identical tests on a single dirt bike at sea level and again at a higher altitude. In that case, you will receive different values as the difference in air pressure will provide alternate conditions.
How to Fix Low Compression?
With all the information you’ve gleaned from your dirt bike’s compression test, you will be ready to fix the actual problem. In most two-stroke engines, the primary reason for low compression will be a worn top-end, which may require replacing the piston or perhaps just the piston rings. However, if your cylinder has worn out of specifications or has physical scratches on it, you may need to rehone or replate it.
If you have a four-stroke engine suffering from low compression, you may need to replace the valves, seals, and potentially a new timing chain to ensure its reliability. Whether this is a task for you or your local mechanic depends on your mechanical aptitude and familiarity with top-end rebuilds.
Don’t Let Compression Get You Down
While the signs of low engine compression can be dramatic and alarming if you are not familiar with it, taking the time to understand your dirt bike’s engine will pay off in the long run. Even if you aren’t mechanically inclined, identifying problems with your machine before they get serious can save you time and money by preventing them from getting worse.
At the end of the day, knowing more about your bike will make maintaining it more straightforward and ensure that you spend less time worrying about what might be wrong and more time back out on the track!