Engine Acceleration Is Poor; Troubleshooting With Vacuum Gauge - MerCruiser 305 CID (5.0L) Service Manual

SERVICE MANUAL NUMBER 24

Engine Acceleration Is Poor

Idle mixture screws improperly adjusted
Incorrect ignition timing
Incorrect distributor or amplifier advance
curve
Accelerator pump
Distributor cap or rotor
Vacuum leak
Spark plugs
Float adjustment
Dirty carburetor
Low compression

Troubleshooting with Vacuum Gauge

Steady reading between 15-21 inches at
idle rpm
Extremely low reading, but steady at idle
rpm
Fluctuates between high and low at idle
rpm
Fluctuates 4 or 5 inches very slowly at idle
rpm
Fluctuates rapidly at idle, steadies as rpm
is increased
Continuously fluctuates between low and
normal reading at regular intervals at idle
rpm
Index
90-861327--1 OCTOBER 1999
Cause
Also refer to "Poor Boat Performance"
Reading
Special Information
Refer to SECTION 4B
Check for stream of raw fuel from acceler-
ator pump discharge nozzle, when open-
ing throttle with engine shut off
Dirty or cracked
Intake manifold or carburetor base
Fouled, burned; wrong heat range;
cracked porcelain
Cause
Normal
Vacuum leak, incorrect timing, underpow-
ered boat, faulty boat bottom
Blown head gasket between two adjacent
cylinders
Carburetor needs adjustment, spark plug
gap too narrow, valves are sticking
Valve guides are worn
Burned or leaking valve
TROUBLESHOOTING
Page 1C-17