When a marine engine whether itís an inboard or outboard wonít start there is a procedure for testing
to find the problem. These test procedures will work with most gas powered engines.
A few tools needed for testing the marine engine includes hand tools that would include hand wrenches,
screwdrivers, and a socket set with misc. extensions. There is also a need for some special tools a test
light or a volt/ohm meter, a compression gauge, a remote start, and a spark tester. A rule of thumb for
testing engines is that you need compression, spark, and fuel to run, and should be tested in that
Compression is rather easy test to be made, the spark plugs need to be removed and the plug wires
grounded. The easy way to ground the plugs is to install the plugs into the wires after removing from
engine and set aside away from working area so as to be grounded at engine. For an inboard type engine
the coil wire may be shorted to block to keep the spark at bay. Now the compression tester can be
installed in one cylinder and the engine spun over by key or by the remote start attached to the starter
solenoid. Warning, be careful of moving parts such as belts, flywheel, or prop. Do not
allow engine to start, remove all spark plugs before testing. Compression for most engines will be around
115-125 pounds. There are some low compression outboard engines that run in the 80 pound area. The best
way to test is to look for consistency between cylinders. Itís normal for compression to very 5-10 pounds
between cylinders. Any more than about 10 pounds indicates a problem with piston/cylinder, head gasket,
or valves. Test all cylinders and look for inconsistencies. See service manual for repairs.
So you have good compression, next step is to check for spark. Warning, when checking
spark all spark plugs are to be removed, and watch for moving parts as stated above when spinning over
engine. A spark tester will need to be attached to all plug leads and tool grounded at block for this
test. Turn key at helm to spin engine or turn key to the on position and use a remote start to spin
engine and look for a Ĺ inch blue spark to jump at tester. A yellow or a small spark gap indicates a
weak ignition. A four cycle engine with a distributor the spark should be tested at the coil and at the
plugs. No spark, spark missing on one or more cylinders or intermittent means there is problems with the
A basic tune-up would be good place to start. With older engines this may involve replacing points,
condenser, rotor, cap, plugs and plug wires. Make sure to use original oem plugs that were installed
from factory. If it came with Champions reinstall Champions, came with NGK, replace with NGK. Use the
service manual to get the proper specs. When changing the dwell (setting of the points) it will change the
timing, so always set points first, and then check timing. With a lot of outboards and electronic
ignition inboards, plugs and plug wires pretty much consists of a tune up. If there is still a spark
problem consult a service manual.
So your spark is good, now it is time to inspect your fuel system. I suggest starting at the fuel tank
and work towards the engine. Check screen and/or anti-siphon valve at tank for being plugged with debris.
Check fuel line for cracks or damage. Check to make sure vent on tank is working properly and not plugged.
Check fuel water separating filter for water. If you donít have this filter inline I would suggest trying
another fuel tank. I like using a 6 gallon outboard tank with proper fittings for the outboard engine and
hooked directly to the fuel pump for an inboard engine. Inline fuel water separating filter kits are
available for most engines if water is found to be the problem. If all looks good so far I would check
the filters at the carburetor. Warning, leaking fuel and gas fumes are a fire and
explosion hazard, be careful! Now you have only a few choices left for the problem either the fuel pump
or the carburetor. If the engine has been sitting for a year or longer I would have the
carburetor/carburetors cleaned. Fuel pumps do go bad but not that often, with the last resort rebuild or
replace the fuel pump. Carburetor cleaning and adjusting can be a tricky thing especially for the not so
mechanically talented people. So go as far as you can, consult the service manual as needed, and when you
find yourself getting over your head, it will be time to seek the professionals.
Hopefully some of this information will be helpful for troubleshooting the problems on your boat
engine whether its an outboard or inboard engine.
Blue Water Marine 8690 S. Oceana Drive, Montague Mi. 49437