1_Pull_Wonda
Cadet
- Joined
- Jun 20, 2008
- Messages
- 11
To clarify the compression testing technique for a vintage 2stroke with pull start.
1.) Unscrew all spark plugs from engine
2.) Ground ignition system: Insert spark plugs back into their boots. Clip alligator lead/wire to the metal at the sparking end of the spark plug. Clip the other end of the wire lead to anywhere on the block. Do this for each spark plug.
3.) Open throttle all the way.
4.) Insert compression tester into spark plug hole.
5.) Pull starter rope 3 times and measure reading. Repeat step 4 and 5 for each cylinder
6.) Make sure there is no more than a 10% variance for cylinder to cylinder.
Is it necessary to ground the spark plug/ignition system on a 2 stroke, pull start engine ?
Is it necessary to open the throttle all the way for a 2 stroke, pull start engine?
The compression tester needle should "jump up" on each complete stroke of the piston. And, when testing the next cylinder, should "jump up" in the same manner. Example....
Top piston:
1st Stroke = 25 lbs
2nd Stroke = 45 lbs
3rd Stroke = 65 lbs
4th stroke = 80 lbs
5th stroke = 90 lbs
6th stroke = 95 lbs
Bottom Piston
1st Stroke = 25 lbs
2nd Stroke = 45 lbs
3rd Stroke = 65 lbs
4th stroke = 80 lbs
5th stroke = 90 lbs
6th stroke = 95 lbs
Is there any difference to pulling the starter rope only 3 times or puling the rope as many times as it takes to "top out" the compression tester needle ?
Performing the compression test with engine cold/room temperature this doesn't give a true compression reading (assuming you have an accurate compression tester) because, the internal components (piston, piston ring and cylinder) HAVE NOT expanded under heat. The test will still reveal problems a like blown head gasket or cracked power head block. As well as possibly revealing scored/pitted cylinder walls, cracked worn rings or major carbon/oil build up as long as this damage is severe enough to result in a significantly different compression gauge result....more than 10% difference ?
Performing the compression test with engine at normal operating temperature (or as close to normal operating temperature as possible) gives a truer compression reading (assuming you have an accurate compression tester) because the internal components (piston, piston ring and cylinder) HAVE expanded under heat. The test will reveal problems a like blown head gasket or cracked power head block as well. The test will also reveal scored/pitted cylinder walls, cracked/worn rings or major carbon build up as long as this damage is severe enough to result in a significantly different compression gauge result....more than 10% difference ?
The idea of adding a little 2 cycle oil to the compression chamber poses several problems and a possible a false reading. 1st.) The oil would have to be added with engine tipped horizontal, spark plug holes facing upward and inserting the compression tester. 2nd.) The engine would have to be tipped down/vertical and tested quickly before the oil would have chance to puddle at the bottom of the combustion chamber. The process would need to be repeated for both cylinders. This type of test would reveal a major problem with engine such as a blown head gasket or cracked power head but, it WOULD NOT reveal damage such as scored/pitted cylinder walls nor worn/cracked piston rings nor major carbon build up. The oil would create a temporary seal between the piston and cylinder wall hiding these defects.
Okay, did I miss anything ? Agree ? Disagree ? Why ? Comments ?
1.) Unscrew all spark plugs from engine
2.) Ground ignition system: Insert spark plugs back into their boots. Clip alligator lead/wire to the metal at the sparking end of the spark plug. Clip the other end of the wire lead to anywhere on the block. Do this for each spark plug.
3.) Open throttle all the way.
4.) Insert compression tester into spark plug hole.
5.) Pull starter rope 3 times and measure reading. Repeat step 4 and 5 for each cylinder
6.) Make sure there is no more than a 10% variance for cylinder to cylinder.
Is it necessary to ground the spark plug/ignition system on a 2 stroke, pull start engine ?
Is it necessary to open the throttle all the way for a 2 stroke, pull start engine?
The compression tester needle should "jump up" on each complete stroke of the piston. And, when testing the next cylinder, should "jump up" in the same manner. Example....
Top piston:
1st Stroke = 25 lbs
2nd Stroke = 45 lbs
3rd Stroke = 65 lbs
4th stroke = 80 lbs
5th stroke = 90 lbs
6th stroke = 95 lbs
Bottom Piston
1st Stroke = 25 lbs
2nd Stroke = 45 lbs
3rd Stroke = 65 lbs
4th stroke = 80 lbs
5th stroke = 90 lbs
6th stroke = 95 lbs
Is there any difference to pulling the starter rope only 3 times or puling the rope as many times as it takes to "top out" the compression tester needle ?
Performing the compression test with engine cold/room temperature this doesn't give a true compression reading (assuming you have an accurate compression tester) because, the internal components (piston, piston ring and cylinder) HAVE NOT expanded under heat. The test will still reveal problems a like blown head gasket or cracked power head block. As well as possibly revealing scored/pitted cylinder walls, cracked worn rings or major carbon/oil build up as long as this damage is severe enough to result in a significantly different compression gauge result....more than 10% difference ?
Performing the compression test with engine at normal operating temperature (or as close to normal operating temperature as possible) gives a truer compression reading (assuming you have an accurate compression tester) because the internal components (piston, piston ring and cylinder) HAVE expanded under heat. The test will reveal problems a like blown head gasket or cracked power head block as well. The test will also reveal scored/pitted cylinder walls, cracked/worn rings or major carbon build up as long as this damage is severe enough to result in a significantly different compression gauge result....more than 10% difference ?
The idea of adding a little 2 cycle oil to the compression chamber poses several problems and a possible a false reading. 1st.) The oil would have to be added with engine tipped horizontal, spark plug holes facing upward and inserting the compression tester. 2nd.) The engine would have to be tipped down/vertical and tested quickly before the oil would have chance to puddle at the bottom of the combustion chamber. The process would need to be repeated for both cylinders. This type of test would reveal a major problem with engine such as a blown head gasket or cracked power head but, it WOULD NOT reveal damage such as scored/pitted cylinder walls nor worn/cracked piston rings nor major carbon build up. The oil would create a temporary seal between the piston and cylinder wall hiding these defects.
Okay, did I miss anything ? Agree ? Disagree ? Why ? Comments ?
