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50 hp mercury

M

M Vance

New member
I got a mid 70's 50 hp mercury outboard
put a hose to it it runs good,
get it out to the lake and it won't go very fast
1\2 mph at the most, any ideas as to the problam
 
Cooties in the carb, fuel pump on the way out or cooties/blockage/leak in the fuel system (bad tank pick-up, plugged vent, bad/leaky fuel line, clogged filter).

With a clog/extra air etc somewhere in the fuel delivery system a motor will run fine on a hose or in a tank when it's not "working very hard" - once you ask it to "start pushing" and it tries to suck more fuel, the problem manifests itself as poor idle, sputtering, stalling or limited rpms - it just can't get enough gas for what you are asking it to do...
 
It might not be kicking on all 4-cylinders. Put some pre-mix in a spray bottle and carefully spritz some into the top carb. See if the engine slows down, speeds up, or no change. Do the same with the 2nd carb.

Then, unplug the fuel line and run it until it runs out of fuel. See if it speeds up a bunch as it's starting to starve. If so, you may have a fuel pump diaphragm leaking excess fuel into the engine, causing an overrich condition on one or 2 cylinders.

Next test, if you have a timing light, put it on each cylinder, starting from the top and work your way down. For each plug wire you should see a good, steady flash. It'll be readily apparent if it's weak or thready.

If you don't have a timing lite, go to your local auto supply store and ask them for a "firing indicator". It has a neon bulb in it that will pick up spark energy when it's placed alongside the spark plug wire. A bit harder to see but, out of direct sunlight or in the evening when it's darker out, very visible.

If all wires show good spark, then pull one wire off its spark plug, stick it in a spare spark plug and lay it against the block so it's grounded. Start the engine and see if it runs worse with the wire disconnected. If it does, it means the cyl you disabled is pulling some load. If there's no change in running when you pull a spark plug boot, you've got some kind of problem in that cylinder.

They all sound good on a flusher, when there's no backpressure or load on the engine. If you have a plastic water barrel handy (or pick up one cheap on Craigslist), stick the motor in the "bucket" and fill water to at least halfway up the exhaust housing. You should be able to tell a big difference in how it's running, in the bucket.

BTW don't run it much if one or the other carb isn't fueling, that means those 2 cylinders aren't getting oil.

Last thing, we need to know the serial # to make sure what type of ign and carbs you have. It's on the metal plate riveted to the transom bracket.

Or at least whether you have (1) big ign coil and a distributor, or (4) small coils on the side of the motor. Even that will help as there are different tests to be done, depending on what you have.

Last thing, back to basics, no engine will run well if one or more cyl's have bad compression. So that's another thing to check, with all the plugs out and then you jumper +12V from the battery to the yellow wire on the starter solenoid.

HTH.......ed
 
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