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1974 85 hp mercury wont start

M

milkman

New member
I recently put this motor i got from a friend on a boat and installation went fine. After i got everything hooked up i tried to start it and it wont start. It is getting spark to the plugs and there is fuel in the float bowls. I dont know where to look next. Any help would be great. thanks
 
Here is what I would do.
Nr.1 Check your compression on all cylinders if above 100 PSI good.
Nr.2 Check to see if your choke is working.
Nr.3 Mix a spray bottle with fuel/oil and give the carbs a good dose and crank her up.
Hopefully she will fire up and stay running,if she fires up and dies,it might be time to clean out the carbs,check the fuel pump,and check your fuel lines all the way back to your tank including you pump bulb.Any leaks air or blockage will cause her to die out or run crappy.Since you say you have good spark these are the things I would check first.Check back let us know,you will find plenty of good advise here to get you up and running.
 
I checked the compression and it from top to bottom 1-105 2-95 3-85 4-110
I was told that the compression would be higher when it warmed up. Is that true?
 
cylinder 3 is not looking good at all (n) 25 psi difference not good.

did you have all the sparkplugs out when you did it?
 
I would have been expecting 110+ cold all the way through at least.At the very least you are looking at a honing and ring replacement.
 
it is not the high amount of psi anywhere around 100 or above is good but you want them to be within 10% of each other.
 
well for starters i need to do a compression test myself. i got the engine from a buddy and those are the numbers that he gave me. i did get a spray bottle with gas oil mix and spray it into carbs, and it kept hitting but wouldntstart fully. i also got a small back fire that started a fire that was put out quickly without any damage to anything.
 
That's true and it also depends on the instrument used,when I had a buddy test mine with his "Merc marine compression tester" he was getting a reading of 120 and I was getting 90 on my old mercedes benze tester,no I don't own a mercedes benze,I wish:D,however, I would like to see over the 100 cranking cold on this size of motor but I think he has a stuck ring or two on #3,carbon build up,we hope,so a honeing and new rings will do,we hope.I rebuilt my beautiful merc800/1969 and I have 125 across the board on my old "merc" tester,and all I did with the cyls was a hone and new rings. But while I was in there I replaced all the bearings as well,which I thought was a good idea at the time.
 
i also had a man tell me to take marvel mystery oil and put into each cylinder and let it sit for a day and then crank it over several times to get it into the rings. he said he has had compression come back up with doing it. i didnt figure it would hurt so i did it.
 
You have no choice but split the case I would suggest while doing that do the bearings as well.On #4 at best you're looking at broken rings at worst a rebore and oversize piston.And replace rings and hone the others.
 
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