If you are trying to start a dirt bike with low compression, it may be time for a new engine. Low compression can happen as a result of old age or from running the engine without oil. If the problem is only on one cylinder, then replacing that cylinder head should solve your issue.
If problems persist and any parts have been replaced, then there might be an underlying problem such as worn piston rings or valves. It will need professional help to diagnose and fix.
The question is will this dirt bike start if you swap the crank, rods, and pistons from the good engine?
Yes, dirt bikes with high mileage often start fine even though running compression is low or nonexistent. You may have to do a fuel system cleaning and carburetor overhaul first but your bike should run again. If you rebuild the carbs yourself this restoration will cost much less painlessly than having a shop do the work.
How to Fix Low Compression Problem?
Symptoms of Low Compression 2 Stroke
The symptoms of this problem in 2 strokes are as follows:
- Low power
- Problem in starting
- Spark plug fouling
- Doesn’t idle well
4 Stroke Low Compression Symptoms
The symptoms of this problem in 4 strokes are as follows:
- Competitors in power
- Hard to start
- Running rough
- Kick starter is easy to kick over
During examining this problem on your dirt bike, you’ll notice a variety of indications and symptoms that suggest the engine is wearing out and needs to be serviced.
It doesn’t matter if it’s a two-stroke or four-stroke engine. Both types of engines exhibit similar symptoms, although they differ in some ways from two-stroke motorcycles.
Low Compression Test
Here are some key points to elaborate on how to compression test will conduct:
- A simple trick to do a compression test is to raise the dirt bike off the ground and remove the spark plugs.
- Measure how much “t” is left on your compression tester then attach it to the first cylinder’s spark plug hole.
- Insert the handle fully into its protective rubber boot as far as it will go, pump hard three times rapidly, hold firmly with both hands on the tester handle, then pull quickly.
If there are low compression levels you will feel or hear air escaping. This will need to be repaired for your dirt bike to run again.
If you want to buy new pistons and rods for your Yamaha YZ125 dirt bike it is recommended that you use the same type of parts. In other words, if you have a cast-iron cylinder sleeve in your dirt bike, buy a used set of pistons and rods that also have a cast-iron sleeve.
Most parts manufacturers sell piston kits with matching brand rod bearings. If your dirt bike has high mileage it is best to change the rings and oil pump while you are replacing the pistons and rods.
The Yamaha YZ125 has a displacement of 124cc and the compression ratio is 9.5:1. The operating range for the fuel-air mixture is between 11:1 and 13:1, more than 10:1, and there may be difficulty in starting and/or poor performance until corrected (see below).
When the engine misses at high speeds, you should check the spark plug to make sure it is not fouled. If it is fouled replace it. If the problem persists, you may have debris in your gas tank or carburetors that are causing this issue. Clean out your gas tank and fuel lines.
If none of these options remedy the situation, you may need to check your compression.
Check to Plug in the Cylinder
If your dirt bike will not start and it has low compression, look at things like a wrong spark plug in the cylinder (a few sizes difference can cause issues), valve clearances, timing that is off, or valves that are stuck open or closed by debris. You can also do a compression test.
If your dirt bike has low compression, you can remove the spark plug from the side of the cylinder that is misfiring and if air bubbles come out then you know that there is a hole in that cylinder. You can choose to rebuild these cylinders or find a replacement engine for sale.
By Sludge in Oil Pan
Sludge in the oil pan or crankcase usually indicates that you have worn piston rings, worn cylinder bores, worn valve guides, and valves seals are damaged. But it could also indicate a serious problem with internal bearings because sludge can build up on bearings as well. If your dirt bike has low compression or is smoking, you may need to replace the bearings in your dirt bike.
Low Compression by Debris
If your engine has low compression it could be a stuck open or closed valve by debris, broken ring(s), bad piston, stuck open or closed intake, or exhaust valves that are not seating because of leaks from damaged seals.
You can check for broken rings by removing the spark plug and putting the compression tester on the spark plug hole. If you get a reading, there is no broken ring.
Worn Ring Piston by Gas
If your dirt bike engine has low compression it means that rings are worn which allows too much gas to pass by them or valves are leaking. You should rebuild these parts and replace the piston and rings.
Dirt bike compression is not always best measured with a gauge, etc., on the dirt bike itself because that would need the removal of pistons from cylinders and then reinstallation. A more accurate reading can be taken from a service manual for this particular dirt bike model.
Note: If your Yamaha YZ125 has low compression you will need to rebuild the engine.
If your dirt bike has low compression in one cylinder or more it is likely that you need to replace the piston rings. You can check for broken rings by removing the spark plug and putting the compression tester on the spark plug hole. If you get a reading, there is no broken ring.
2 Stroke Low Compression – After Rebuild
If you have a dirt bike sitting for a few years due to low compression what would you need to do?
1. Check the air filter
2. Clean carbs
3. New spark plug
4. Test battery and charging system
Each of these steps is important to ensure that the bike does not start at all.
1. Check the air filter to make sure it’s clean, any dust or debris can cause serious issues
2. Clean your carbs completely and replace fuel lines if they are old and cracked
3. New spark plug will help because having a bad spark will cause the engine to not even turn over
4. Test your battery for power and charging system.
The Procedure for Starting a Dirt Bike
1. Check compression with a sign of RPMs when the throttle opens halfway
2. Remove the spark plug and check the condition
3. If compression is low, replace the spark plug to ensure proper heat range
4. Ensure there is no debris in carbs; clean air filter; replace fuel lines if needed
5. Remove carburetor bowls and perform rebuild (if the bike hasn’t been used for a while)
6. Reinstall carbs and check for proper operation
7. Check for spark plug condition; replace if needed
8. Open the throttle to start (after checking the air filter, fuel lines/filter, etc.
If you are trying to start a dirt bike that has low compression, it may be time for a new engine. Low compression can happen as a result of old age or from running the engine without oil. If the problem is only on one cylinder, then replacing that cylinder head should solve your issue.