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old merc problems


Marc38's Avatar
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04-01-03, 04:55 PM   #1  
Marc38
old merc problems

hello again, I have a 1970 merc 50 that is giving me some problems but as a middle class father with a daughter getting married in Jamaica this year I can't quite afford a new one. Seems to start and idle great right off the trailer and runs great at speed but when we slow down to idle to troll it loads up and smells real rich. had the carb rebuilt and someone mentioned that it may need a new set of reeds. does anybody know where I might find some for an engine that old? If memory serves me correctly from way back do they go in just like a motorcyle engine? thanks for the time and advice.........

 
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04-08-03, 04:16 PM   #2  
BoatCop
A compression test and leak down test will be the best way to determine the condition of the cylinder seals (rings and reeds). (new comp = 125 PSI, 110 is SAT) All cylinders should be within 5 PSI of each other.

From your description, it does sound like a compression problem. With lower compression, there is less complete combustion, causing a rich condition, and probably oil fouled plugs. It runs OK at high speed, because it heats up and temporarily seals better.

Most Mercury engine dealers, especially those who have been around for a while, should have replacement reed valve assemblies.

 
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05-03-07, 08:04 PM   #3  
Same thing happened to me

I have an old merc too, its about a 1987 and it had a 140 in it. It ran great at top speeds and died out when you slowed down and then was tuff to start. I did everything you can think of to make it run better, new points, plugs, condensor, new carb adjustments, even tried an electronic fule pump.

In the end, it was in fact a weak engine. I replaced it with 110 which had been rebuilt and used the same carb on it...it started up and runs perfect at the low idle speeds. However, I ran into a nightmare with the exhaust ports not lining up like the old engine. So, I agree that a compression test is a must. If you do not have good compression, no amount of fussing will fix the root cause. However, along the way I did learn that fuel pressure can cause that sort of problem. If you have good compression, and have done everthing else as far as adjusting the timing etc. Then you may want to ask a local parts store about a fuel line pressure regulator and match it up with the specs of your carb. You may also want to ask a local marine mechanic if your boat had a single fuel line going to the carb or if you had a return line to the tank which will eliminate the execessive fuel issue. Good luck, and if I had it to do all over again, I would seriously consider pulling the head and getting a valve job instead of replacing the whole motor. If the clylinders looked egg shape when I pulled the head, then I would pull the engine and have it bored out and get an engine overhaul kit. There are enough good machine shopes around that can add in special benefits like balancing your crankshaft, adding special rings, and other tricks to make your rebuild well worth their fees. All I know is now I turn the key and mine purrs like a kitten and I'm glad I got rid of that old engine.

 
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