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. [expander_maker id=”1″ more=”Read more” less=”Read less”]
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 ½ 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 ignition system. 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 don’t 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.