"Tony, If his battery isn'
"Tony, If his battery isn't 100% full then the reading from the comp. test will be lower than expected. Were all the plugs out while testing?
from a mercury manual:
When analyzing the results of a compression check, generally the actual amount of pressure measured during a compression check is not quite as important as the variation from cylinder-to-cylinder on the same motor. For multi-cylinder powerheads the rule of thumb is that variations in pressure up to about 15 psi (103 kPa) or 15% (although on these motors since most compression should be above 100 psi/103 KPa, 15% would be MORE of a variance than 15 psi/103 kPa) may be considered normal (when comparing the lowest compression reading to the highest during a given test). Since there are no other cylinders to compare with on single cylinder powerheads you will generally compare the reading to the one that you took and recorded when it was new (you did do a compression test on it when it was new, didn't you?). Again, generally speaking, a drop of about 15 psi (103 kPa) from the normal compression pressure you established when it was new is cause for concern.
Ok, for the point of arguments sake let's say you bought the engine used or never checked compression the first season or so, assuming it wasn't something you needed to worry about. You're not alone, many of us have done that. But now that you're reading this it is your chance to take the data and note it for future.
Mercury does not publish compression specifications for some of their motors, but we've listed the normal compression specs for as many as we could locate here. As a rule of thumb on MOST Mercury motors, 90-120 psi (621-856 kPa) is at the lower limit of acceptable compression (at least for mid-range and larger motors). Published compression specifications for these models are as follows:
4/5 hp motors Minimum: 90 psi (621 kPa)
6/8 and 9.9/10/15 hp motors Normal 115-120 psi (787-856), Minimum: 100 psi (685 kPa)
20/20 Jet/25 hp motors Minimum: about 120 psi (856 kPa)
75/65 Jet/90 hp motors Minimum: about 120 psi (856 kPa)
80 Jet/115/125 hp motors Minimum: about 120 psi (856 kPa)
75/90/115 hp Optimax motors Normal 90-110 psi (621-758)
135-200 hp (2.5L) motors Normal 110-130 psi (759-896)
200-250 hp (3.0L) motors Normal 90-110 psi (621-758)
You might be interested to note that, a quick check of the specifications from other manufacturers shows that a range of about 78 psi (556 kPa) to 153 psi (1079 kPa) is not surprising for many 2-stroke motors (again generally the smaller hp motors having lower ranges and larger motors having higher ranges).
If a specification is not available for your motor, put the most weight on a comparison of the readings from the other cylinders on the same motor (or readings when the motor was new or even from the last tune-up).
When taking readings during the compression check, repeat the procedure a few times for each cylinder, recording the highest reading for that cylinder. Then, for all multi-cylinder motors, the compression reading on the lowest cylinder should be within about 15 psi (103 kPa) of the highest reading. If not, consider 15% to be the absolute limit. If the reading in the lowest cylinder is less than 85% of the reading in the highest cylinder, it's time to find and remedy the cause.
If the powerhead has been in storage for an extended period, the piston rings may have relaxed. This will often lead to initially low and misleading readings. Always run an engine to normal operating temperature to ensure that the readings are accurate.
If you've never removed the spark plugs from this cylinder head before, break each one loose and retighten them before starting the motor in order to make sure they will not seize in the head once it is warmed. Better yet, remove each one and coat the threads very lightly with some fresh anti-seize compound.
Prepare the engine for a compression test as follows:
1. Run the engine until it reaches operating temperature. The engine is at operating temperature a few minutes after the powerhead becomes warm to the touch and the stream of water exiting the cooling indicator becomes warm. If the test is performed on a cold engine, the readings will be considerably lower than normal, even if the engine is in perfect mechanical condition.
2. Label and disconnect the spark plug wires. Always grasp the molded cap and pull it loose with a twisting motion to prevent damage to the connection.
3. Clean all dirt and foreign material from around the spark plugs, and then remove all the plugs. Keep them in order by cylinder for later evaluation.
On many Mercury motors you can disable the ignition system by leaving the safety lanyard disconnected but still use the starter motor to turn the motor. This is handy for things like compression tests or distributing fogging oil. To be certain use a spark plug gap tester on one lead and crank the motor using the keyswitch. If no spark is present, you're good to go, if not, you'll have to ground the spark plug leads to the cylinder head.
4. Ground the spark plug leads to the engine to render the ignition system inoperative while performing the compression check.
Grounding the spark plug leads not only protects the ignition system from potential damage that may be caused by the excessive load placed on operating the system with the wires disconnected, but more importantly, protects you from the dangers of arcing. The ignition system operates at extremely high voltage and could cause serious shocks. Also, keep in mind that you're cranking an engine with open spark plug ports which could allow any remaining fuel vapors to escape become ignited by arcing current.
5. Insert a compression gauge into the No. 1, top, spark plug opening.
6. Move the throttle to the wide open position in order to make sure the throttle plates are not restricting air flow. If necessary you may have to spin the propeller shaft slowly by hand while advancing the throttle in order to get the shifter into gear.
7. Crank the engine with the starter through at least 4-5 complete strokes with the throttle at the wide-open position, to obtain the highest possible reading. Record the reading.
On electric start motors, it is very important to use a freshly charged cranking battery as a weakened battery will cause a slower than normal cranking speed, reducing the compression reading.
8. Repeat the test and record the compression for each cylinder.
9. A variation between cylinders is far more important than the actual readings. A variation of more than about 15 psi (103 kPa), between cylinders indicates the lower compression cylinder is defective. Not all engines will exhibit the same compression readings. In fact, two identical engines may not have the same compression. Generally, the rule of thumb is that the lowest cylinder should be within 15% of the highest (difference between the two readings).
10. If compression is low in one or more cylinders, the problem may be worn, broken, or sticking piston rings, scored pistons or worn cylinders."
