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1978 mercury 50hp starting problem

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Biketim

New member
HELP- i have a 1978 mercury 50hp (500) engine that i am having a little trouble with. the engine will crank and sometimes start up but when it does start up the engine will rev really high and shut off. i have rebuilt the carbs replaced the spark plugs and check compresstion (compresstion is good). i have been told by a few people that it could be a governor problem or a vacuum leak. and i am not to sure if that would be the problem or not. and im not to sure how to find the vacuum leak and or adjusting the governor. its a duel carb system too. any help would be better then none thanks for the help.
 
...when it does start up the engine will rev really high and shut off.
Click to expand...
When starting the throttle should be at about 1/3. Have you looked at the fuel pump? It won't pump fuel if the crankcase is leaking pressure...no pressure pulse will keep the pump from working.
 
There is no governor on one of these motors.---------------Sounds like you need to look at the carburetors again.------A motor that starts and stalls clearly has a fuel supply problem.-----------Is this a problem that started suddenly ?----------Did it run OK last year ??
 
I just repaied a Merc 200 (20 hp two cylinder--half of your 50) and it did the same thing. The problem with it was bad coils--both of them.

Question: Does your motor have 4 coils, or a distributor?

Jeff
 
It has 4 coils and the issue started last year. I just got it running last night. I think it was just not getting the fuel from the tank once it got warmed up it worked great. Thanks for the help guys. What do you think 4 new coils would run me
 
Coils for my '77 Merc 500 were about $60 each. Some Merc coils are $25 each, big suprise, not mine! I see complete sets of used coils on Ebay from time to time for good prices.

I had a bad plug wire, guess what happens what that occurs .... that's righ, the coil goes into melt down! The previous owner must have had issues, I had 1 new coil and it was connected to a plug wire with a solid core. The solid wire broke inside the plug cap and caused a meltdown. Electericity will find its way to ground somehow, even if thru the side of the coil to the block. I made an entire new set of plug wires when I got my new coil.
 
Good info. Thanks.

Lots of used coils on Ebay, but be sure to get the blue or black ones. The others (red, green, orange, and yellow) are different.

Jeff
 
I don't think that you can go by color, my OEMs are black, my replacement is orange. Be careful though, as Jeff said, they are not all the same.
 
Orange and black coils do not interchange. The ones with the pigtale in the back of the coil (red, orange, yellow and green) are wired differently internally than the coils without the pigtale (black and blue).

Jeff
 
My replacement coil is orange and has no pigtail, it was a direct replacement and has been in use for an entire season. The part number was confirmed by a Merc dealer, it is identical in every way except for the color and a slight difference in part number. The original number was obsolete. Oddly enough this was the only coil that had been replaced from the original, and the previous replaced coil was orange too.
 
Weird! Long as it does NOT have the pigtail in back, then it will work for your installation.

Strange that it's orange. Never seen one like that.

Jeff
 
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