Re: 78' 80 horse merc trouble
You came to the right place. Welcome to iboats.com, Ashcroft.<br />2 cycle outboards are finicky things. They can be very reliable. After all, your engine is just a young-pup at age 12 years of age.
<br />If you maintain it properly, you can get 12 more no sweat.<br />The symptoms you described can be anything from one cylinder not firing, to fuel starvation, to a broken ring.<br />To diagnose the problem you are having, you will need the diagnostic tools.<br />The first and most important tool you'll require is the shop manual. Most public libraries have reference-only shop manuals for just about every outboard in broad use today. So while yours is on order, make a trip to the librart with a pocketfull of dimes (copy machine).<br />1.Get the spark plug type and gap. <br />2.Compression specs.<br />3.the voltages and test procedures for your stator,trigger and coils.<br /><br />Other diagnostic tools are;<br />
Spark testor; These are about 8 buks at the auto parts store, but one can be made using a wire with an aligator clip on one end, a hose clamp large enough for the threads on an old spark plug, and an old working plug.<br /><br />
A good volt/ohm meter. The 'bible' will state the you'll "need a mercury ignition tester" or some such rot, but a good volt/ohm meter capable of 600VAC/DC, and ohms reading capability will do fine for go/no-go testing.<br /><br />
Compression gauge; This is the same one you would use on an auto, and they are only about 10-15 buks.<br /><br />First, pull the plugs, one at a time, and note condition. It should be as if painted flat black.<br />Sure, this is carbon,and on a 4cycle, a sign of oil in the combustion chqamber, but this is 2 cycle, and that's a healthy look, provided it doesn't look like "tar" or black putty.<br />If it looks brand new, it indicates trouble, (unless it
is new), but after one hour of operation, it should have residue on it. If not,it could mean water in the cylinder from a blown bhead gasket or exaust mainfold or cracked block--has steam-cleaned it..Ungood.<br /><br />But while each plug is out, check the compression on that cylinder. The number must be within 10% of all the other cylinders.<br />If you have low compression, this is your problem.<br />compression is used to draw vacuum on the intake stroke, exaust on the exaust stroke, and these 'push/pull" pulses of compression operate the fuel pump. Without good compression, no good fuel pump.<br />If the compression is strong, and ignition checks good, and fuel is clean and fresh (less than 4 months old in summer heat), clean filter, carbs, no air leaks in the fuel line, tank vent open and allowing air to replace fuel burnt, and the exaust manifold don't have a glob of carbon plugging the exaust (or a wasp nest,don't laugh, it happened to me), and the plugs are not fouled, and the engine isn't overheating, it should run fine.<br /><br />BTW, 500RPM is near peak for this engine, meaning if it is running at close to maximum RPMs and you aren't moving as you should, you may have a spun hub in the prop.<br />I don't remember, but I think your engine should not rev over 5800RPMs at full throttle.
