Sudden loss of power 1984 Merc 115

[GLV502]

My 115 hp Merc 6 cyl lost power the other day and I have not been able to diagnose the problem. #3 cylinder seems to be the one most effected. All cylinders have the same compression, spark seems to be ok under power also. Is there a chance that carb #2 has something in the main jet? When you put your hand across the throat it does not seem to draw air like the others. Any suggestions?

 

[thinksun]

Re: Sudden loss of power 1984 Merc 115

#3 goes south easy. Did you have the updated mains - #76's from 74?

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[URL=http://www.filehive.com/i.php?i=0726/bf_1.jpg] [/URL]

 

[GLV502]

Re: Sudden loss of power 1984 Merc 115

The carbs have never been touched. How difficult is it to change the jets?

 

[thinksun]

Re: Sudden loss of power 1984 Merc 115

The carbs have never been touched. How difficult is it to change the jets?

Easy to get at, need to use a good screwdriver because you don't want to strip the head. 10 min job max, remove cowling, 4 bolts for front plate, each carb has a brass plug that exposes recessed jet.

4 kits on ebay at $5 each, one kit is all you need.

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&ih=013&sspagename=STRK%3AMEWA%3AIT&viewitem=&item=230043963269&rd=1,1

 

[Chris1956]

Re: Sudden loss of power 1984 Merc 115

GLV, Have you checked the compression on each cylinder. Rejetting a carb for a cylinder that has no compression makes no sense.

BTW, why do you want to put in new main jets?

 

[thinksun]

Re: Sudden loss of power 1984 Merc 115

To clarify my point about the jets is these thing like to eat #3 pistons. You may need new piston but you should go with the larger jets to be safe -if or when engine is all good.

 

[daveswaves]

Re: Sudden loss of power 1984 Merc 115

Changing the jets to "fix" piston meltdown is not going to do it. Drop the timing to 21 degrees max instead of the 23 marked on the motor. Prop it to run 5800 wot then run all day at 5000 if you want. Mid grade gas is a good idea. Keep the carbs clean, use a fuel filter, if your carbs have a filter screen in them remove it. Merc had a bulletin about doing that to eliminate clogging. Carb kits are a good idea if they have never been done.
HOWEVER it sounds like you have pooched #3. no amount of carb work or timing will bring it back to life, gotta replace piston and rings. Not a job for the faint of heart on this engine as the entire crankshaft with all 6 pistons comes out at the same time. You will likely see other piston tops melted a bit if #3 is gone. You will need special ring compressors to get it back together too. Unless you are very adept at overhauling this particular engine you will be further ahead to purchase an overhauled block, they are available on ebay. My 2 cents.