When a marine engine whether it’s an inboard or outboard won’t 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 sequence.
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. It’s 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.