and/or Poor Maneuverability
A lot can be said about a good mechanic, but the one thing he will tell you is that he
follows a set pattern almost every time he approaches a trouble shooting situation.
Experience and training help, but performing the same test over and over again on
different motors, can give you more knowledge than you will ever get out of a book. It's
just that the results should be predictable, within the confines of each boats limits, and
that it's the unexpected result that tips you off to what's wrong.
With that in mind, if you ever really want to test four cycle, gas engines for running
problems, get a vacuum gauge, hook it to your motor, and learn what the results are, when
it's running correctly. This way you'll know when it's not.
Troubleshooting With Vacuum
Steady reading between 15-21 inches at idle RPM = Normal
Extremely low reading, but steady at idle RPM = Vacuum leak;
incorrect timing; under powered boat; faulty boat bottom
Fluctuates between high and low at idle RPM = Blown head gasket between
two adjacent cylinders
Fluctuates 4 or 5 inches very slowly at idle RPM = Carburetor needs
adjustment; spark plug gap too narrow; valves are sticking
Fluctuates rapidly at idle, steadies as RPM is increased = Valve
guides are worn
Continuously fluctuates between low and normal reading at regular
intervals at idle RPM = Burned or leaking valve
Readings drop to Zero as engine RPM is increased = Engine overload,
clogged or restricted exhaust system
(see complete vac test with example .gifs
First, determine which engine system is causing the problem. To make an engine run,
basic components fuel, spark (ignition), and compression are required. If all three
components are present, the engine should run. If any one of the three are missing, weak,
or arriving at the wrong time the engine will not run.
Determine if there is fuel present by looking down the carburetor venturi while actuating
throttle. There should be a stream of fuel coming out of the accelerator pump nozzles if
the carburetor has fuel. Check ignition system operation. Remove coil wire from tower on
distributor cap. Hold coil wire near ground and check for spark while cranking engine
over. Repeat procedure with spark plug wires. If there is spark at the spark plug wires,
remove the spark plugs and make sure they are correct type and heat range, and not fouled
Run a compression check
on engine, to make sure the engine is mechanically sound.
Not all problems are engine related, and probably many engine problems are exaggerated by
other related items. The list below covers the often twisted paths that trouble shooting
will take you. To start, look at the list below, and cross off the items that definitely
"Are Not " the problem, and the list will seem much shorter.
Bow too low?
A. Improper drive unit trim angle
B. Improper weight distribution
C. Boat is under powered
D. Permanent or power hook in boat bottom
E. False bottom full of water
F. Improperly adjusted trim tabs (after planes)
G. Dirty boat bottom (marine growth)
Bow too high?
Improper drive unit trim angle
Propeller pitch too great
Dirty boat bottom (marine growth)
Poor running engine
Improper weight distribution
Rocker in boat bottom
False bottom full of water
Improperly adjusted trim tabs (after planes)
Drive unit installed too high on transom
Dirty or rough boat bottom
Damaged propeller; pitch too small; diameter too small
Keel located too close to propeller or too deep in the water
Water pickup or thru hull fittings located too ,close to propeller
Hook in boat bottom
Propeller plugged up with weeds
RPM Too High?
Damaged; pitch too low; diameter too small;
propeller hub slipping.
Water pickup or thru hull fittings mounted too close to
keel located too close to propeller and/or too deep in
the water (ventilation).
Drive installed too high on transom; wrong gear ratio.
Unit trimmed out too far.
Engine coupler slipping
RPM Too Low?
Damaged; pitch too great; diameter too great.
Dirty or damaged bottom;
permanent or power hook in bottom;
false bottom full of water.
Drive installed too low on transom;
wrong gear ratio.
Unit trimmed in too far.
Engine Cranks Over But Will Not Start Or Is Hard To Start
Moisture on ignition components
Distributor cap or spark plug wires arcing
Battery, electrical connections, damaged wiring
Faulty Ignition switch (check for 12 volts at coil)
Shift interrupter switch (Some sterndrive Models Only)
Shorted tachometer (disconnect and retry)
Ignition timing ( check if distributor is loose)
Spark plugs Fouled, burned, cracked porcelain
Spark plug wires faulty insulation, broken wires
Cracked or dirty distributor cap
Faulty ignition components
(Check components and wires, to and from)
Engine synchronizer (if equipped) hooked up series on purple ignition wire (dual
Synchronizers must be hooked up directly, coil terminal.
Low Voltage at coil when cranking (check battery and connections)
Engine Will Not Crank Over/Starter Inoperative?
Remote control lever not in neutral position
Battery charge low; damaged wiring; loose electrical connections
Circuit breaker tripped, Blown fuse (20 amp dash, 50 amp motor)
Ignition switch Slave solenoid
Faulty neutral start safety switch, open circuit
Engine Hydro-locked or seized
Mechanical engine malfunction (flywheel jammed, transmission seized)