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 Inläggsrubrik: Guide How to Check Your 2 Stroke Engine for Compression
InläggPostat: tor 17 okt 2013, 22:50 

Blev medlem: sön 31 okt 2010, 10:09
Inlägg: 2332
Ort: Luleå
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A two stroke motor operates on four elements: air, fuel, compression and spark. Obviously, without air, fuel and spark, the motor would not have be able to perform combustion. Compression is created by a piston as it travels upward through the cylinder and squeezes air and fuel into a dense mixture that will ignite with great force. A compression loss can be caused by a faulty seal between cylinder, piston or piston rings, creating a loss of power or ability to operate. Luckily, checking the motor's compression is a simple task.

Remove all of the spark plugs from the motor, and ground the spark plugs against the engine case.

Screw a compression tester into the spark plug hole on the first cylinder to be tested.

Twist the throttle completely open and kick start the motor to move the piston(s). Kick the motor over a few times and look at the compression tester's gauge. A motor must have a minimum compression rating of at least 100 to 125 pounds per square inch to operate properly.

Open the tester's pressure release valve momentarily to clear the gauge, and unscrew the tester from the spark plug hole. Repeat as necessary for the any remaining cylinders.

Performing the Compression Test
For the following steps and assuming everything above has been covered, hold the throttle wide open with the engine stop / kill button depressed, or have the engine run switch turned to OFF.
Note: If you're working with an electric start motorcycle, you'll likely need to leave the engine run switch turned to "ON" so as to be able to use the starter for turning the engine over, just be sure to disable the ignition system by disconnecting the primary lead from the ignition coil (small single wire or weather proof electrical connector) and Waterproof the Electrical Connections at reassembly.
Picture of a compression gauge at 210 psi
Next, with the throttle held wide open, Kick the engine over, (or by using the electric start as can be found on some models) turn the engine over until the needle on the gauge of the compression tester peaks at it's maximum pressure and record this reading for future reference.

The ideal pressures for an average 2 stroke dirt bike at sea level that is not heavily modified should be in the vicinity of the numbers below.

A 50cc, 60cc, 80cc, 100cc, 125cc 200cc or 500cc should be able to squeeze out a reading of 120-190 PSI.
A 250cc should be in the neighborhood of 170-240 PSI.
If It's a 2 Stroke You're Working On and It's Low on Compression,
Here's Everything You Need to Know to Rebuild the Top End Yourself
If you're unable to reach a satisfactory compression reading, or there's a 20% difference (or more) from the reading you obtained after breaking in a new top end, this may be an indication that the top end is in need of being rebuilt.

If it's determined that the top end IS in need of being rebuilt, you should get a service manual specific to the dirt bike, and / or refer to our article on rebuilding the top end on a 2 stroke, then disassemble the top end to check the condition and clearances of parts as detailed in the factory service manual, replacing parts that are worn beyond the service limits stated, or have a clear indication of wear, as well as at a minimum, always replace the piston ring(s) upon disassembly no matter of their physical appearance or measurement.

- See more at: http://www.your-adrenaline-fix.com/comp ... cIogB.dpuf


A Two Stroke Engine must have compression above and below the piston rings. We all know about taking engine compression readings from the spark plug hole but a lot of people seem to forget that a two stroke engine must have a sealed bottom end that will hold both compression and vacuum.

Vacuum Tester InstructionsTo test a two-stroke crankcase all you have to do is plug the intake and exhaust, then pressurize the crankcase, through a fitting in the sparkplug hole, to whatever pressure the manufacturer specifies (usually six to fifteen pounds per square inch). Then see if it will hold it for five or six minutes. For a vacuum test apply five to seven inches of mercury (HG) and see if it holds for five or six minutes.

When the piston in a two-stroke comes down it compresses the air/fuel charge in the crankcase and pushes it, through the ports in the sides of the cylinder, into the combustion chamber. As the piston rises a vacuum is created under the piston. That vacuum sucks the air/fuel mixture through the carburetor and into the crankcase.

Some guys will use this test after a crankshaft rebuild to make sure the bottom end is sealed right. Call me bad but I never do. I just have never had trouble with crankcase sealing. I always use new seals and I make sure the case sealant is spread evenly and completely on the crankcase edge. I used to use Yamabond #4 but they don't make it anymore so now I use ThreeBond #1194 Liquid Gasket. It seems to work good too. If a center crankcase gasket is called for, instead of sealant, I always use a new gasket.

A word of caution here, if the manufacturer calls for a liquid sealant DO NOT change and use a gasket. Likewise, if a gasket is called for DO NOT discard the gasket and use only liquid sealant. This would cause side clearance problems with the gearbox main and counter shafts not to mention the crankshaft.

The test can be useful for troubleshooting too. If there is a leak it can cause hard starting and poor running. Old seals can become hard and leak. This can cause difficult starting but once started the seal warms up it will work OK. The rest of the day the engine will start fine but if it sits over night, the seal will harden up again. If it leaks on the Magneto side it could be sucking air, leaning out the fuel mixture. This can cause a number of problems ranging from hard starting, poor idle, poor running to overheating the engine and possibly causing a piston seizure. In addition to all these problems, if the seal on the Power-Take-Off side (Clutch side) is leaking, transmission oil can get into the engine causing lots of smoke while running.

The cure for all this is new crankshaft seals. I don't think I have ever seen a leak from the center seams of the crankcase. If there is a hole in the crankcase careful inspection should reveal it. If there is a hole you will need to get new crankcases. I've seen people try to weld holes in crankcases but it almost never works. The heat from the welding warps the crankcases. The one time I did see it work the crankcase required a lot of careful grinding to get the shifter drum to turn. I never did find out how long that engine lasted either.

Just to make things more fun there have been manufacturers in the past who have mounted their seals on the PTO side inboard of the main crankshaft bearing. This is good for main bearing lubrication because the bearing is running in oil. It's bad because to change the seal you have to split the crankcases and pull the bearing. Of course most of the engines that are designed this way are race engines and the seal is easily replaced when you have the crank out for regular maintenance. You do follow Factory maintenance intervals don't you? Good news is most of the newer two-stroke engine designs don't do this any more.

The only problem I have with vacuum testing is that it takes as much time to run the test as it does to just put in new seals. I might feel differently if I got lots of newer bikes in but it always seems that by the time I get the engine it's ten or fifteen years old. Why waste time and money (My time, your money) on a test when it needs new seals anyway? I use the vacuum/pressure gun from my vacuum tester kit all the time... to bleed hydraulic brakes! If you want to run a crankcase compression test that is just fine. I just don't think it is necessary.

Maybe I'm just not a good vacuum tester. I guess I just don't suck good enough!

Here is a PDF file of the above instructions.

"Bonne metoden"
Ta bort tändstiftet och kolla att det "blåser" ur tändstiftshålet när du försöker starta med startmotor eller kicken
- Orkar du inte hålla kvar tummen i tåndstiftshålet när du drar runt elstarten är kompressionen OK
- Fjuttar det lite så är det nog kolvringsbyte som behövs
- Ingen fjutt alls kolv byte är nog det minsta som behövs

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