Water Pressure Check; Serial Number Location - Mercury 45 Jet 50-55 Service Manual

4. Crank engine thru at least 4 compression strokes to
obtain highest possible reading.
5. Check and record compression of each cylinder.
Variation of more than 15 psi. (103.5 kPa) between
cylinders indicates that lower compression cylinder is
in some way defective, such as worn or sticking piston
rings and/or scored piston and cylinder.
6. Compression check is important because an engine
with low or uneven compression cannot be tuned
successfully to give peak performance. It is essential,
therefore, that improper compression be corrected
before proceeding with an engine tuneup.
7. Cylinder scoring: If powerhead shows any indication of
overheating, such as discolored or scorched paint,
visually inspect cylinders for scoring or other damage
as outlined in Section 4 "Powerhead."

Water Pressure Check

NOTE: To perform these checks, a Water Pressure Gauge
Kit, P/N 91-79250A2 is recommended.
1. Water pressure at idle, in neutral, is 1-3 psi (7-21 kPa).
2. Water pressure should increase, then drop to 4-6 psi
(21-35 kPa) prior to 2500 RPM (due to poppet valve
opening.)
3. At 2500 RPM, water pressure should not exceed 12 psi
(83 kPa). Readings above 12 psi at 2500 RPM may in-
dicate a stuck poppet valve.
4. Static test (boat stationary - operate in forward gear
with a cut down " smaller diameter" propeller) at 5000
RPM or above is 7-12 psi (48-83 kPa).
CAUTION
!
A MODIFIED PROPELLER OR LOW PITCH PROPEL-
LER IS REQUIRED TO PERFORM THE ABOVE STATIC
TEST. STATIC TEST REQUIRES THE BOAT BE STA-
TIONARY IN THE WATER SECURED TO A DOCK OR
TRAILER AND RUN IN FORWARD GEAR. DO NOT USE
A FLUSHING DEVICE FOR THIS TEST.
1-22 - GENERAL INFORMATION and SPECIFICATIONS
a
a - Compression Gauge (P/N 91-29287)

Serial Number Location

The engine serial number is located on the starboard side
of the swivel bracket (as on all Mariner/Mercury models)
and also on the cylinder head (a).
a
51046
51139
90-817643R1 DECEMBER 1996