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1977 Mercury 500 50 hp

F

Fuddyduddy

Regular Contributor
Recently purchased an older boat outfitted with an older 50 hp engine. Took the old girl for a test run and she would run smooth but only do about 5 mph. Took her out and ran it in a barrel to clean the carbs and took her out again. Same story. Cleaned out the gas tank and got a new hose and bulb and changed the plugs. Marine Mechanic found the throttle linkage to be messed up so he worked on that and we took it out again. A bit hard to start and it ran hard but would tend to lose power sporadically to about 80 percent. Like someone would pull the top plug wire and then place it back on, This seemed to go away as the motor warmed up. I suspect a plug wire but I am real ignorant about Mercury outboards. I only know that bad things can and do happen when you own a boat. Especially an old one. Again this is a 50 HP Merc 500 1977 vintage #4707239 (that serial number doesn't seem to register on the search engine here) Thanks, Mike
 
Check spark with a spark tester,it should jump 7/16" bright blue snap,or hold it in your hand and if you can get up after,it's not strong enough :D ,just looking at the plug isn't really a good indication. Check compression,100psi+. New to you,pull the carbs and clean,blow through with air. BOAT= Break Out Another Thousand :D
Here's the link to you motor parts http://www.marineengine.com/parts/mercury-outboard-parts/500-50/4357640-thru-5531629-usa
 
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I did take the plug out of the top 2 cylinders just shortly after a cold start. The top plug was "wet". The motor sounds decent just running on 3 cylinders, but again after a warm up it runs great with no fluctuation in power. It takes about 10 to 15 minutes to warm it up to that point. I've had plug wires on autos give me fits but that was long ago. (and I have been "bit" more than once. Don't forget those lessons) Would a plug wire from an auto parts store suffice to see if it was just a plugwire acting up? Don't like to throw money at it but that would be inexpensive. I'll get a tester.Thanks, Mike
 
Don't use auto wires--they're RF resistant wires and don't work right. Look on eBay for what you need.

Jeff
 
I have been told these "screw into" the coils so maybe not even considering auto wires now. What the hell happened to simple.-Thanks, Mike
 
I have been looking. Better look here. Ebay has been good for some things but I can probably do as well here.Thanks, Mike
 
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Yes they were and the carbs were cleaned a bit while running also. Pretty much determined to be a plug wire or related. Thanks, Mike
 
I cleaned off all the electrical contact points I could get to and found a loose wire on the rectifier (right above the switch box). I tried to tighten gently and found that the bolt rotated with the nut I was trying to tighten. I know that the plug wire is good but the motor ran poorly when I started it and after a 5 minute warm up it quit like it just ran out f gas. It wouldn't start after that until I tilted the motor up a bit. Then it started right away. Weird but not if there is a short causing my problems. Any ideas? I have a new rectifier coming.-Thanks, Mike
 
I would start looking for shorts,if that mercury tilt switch is there,disconnect it.Take pics,then remove the harness and look it over,These harnesses were noted for the poor quality.If that all checks out,check the coils are the correct ones for the motor,long shot.
Then start at the tank,look for blockage at pick-up,check primer bulb,if it's old change it for the fun of it,have a look at the quick connection at the engine,pull the fuel pump check the diaphragm for pin holes,
 
Marine mechanic is coming over monday or Tuesday. This is an area I haven't looked at so it seems logical to me.I have another coil coming also so there is a chance to be able to test a few theories at least. Thanks for this. Mike
 
Primer bulb, hose and connectors are new. Tank has been cleaned and fresh regular unleaded gas at 50: to 1 2 cyc. oil. Plugs are new carbs are clean , battery and connections are good. Had my plug wire checked and it was good. May try starting it in the dark to see what may show up. I will try to wrap my head around most of what I understand this weekend and get back after it early next week. Thanks, Mike
 
haventaclue said:
I would start looking for shorts,if that mercury tilt switch is there,disconnect it.Take pics,then remove the harness and look it over,These harnesses were noted for the poor quality.If that all checks out,check the coils are the correct ones for the motor,long shot.
Then start at the tank,look for blockage at pick-up,check primer bulb,if it's old change it for the fun of it,have a look at the quick connection at the engine,pull the fuel pump check the diaphragm for pin holes,
Click to expand...

Took the boat out again and ran it and it was evident that there was a short in the motor. It showed up with a hard left turn at full throttle. Tilt switch connection was real funky but not the problem. I got to looking at the wiring harness and found just as you said. The insulation was literally crumbling on all of the wires. I ordered another from Marine Engines today. Thank you for your time and help. The harness was not exceptionally hard to remove. So I am hoping that will get me back out on the water before winter makes it impossible. Thanks again, Mike
 
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