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85 mercury 75 hp troubles

N

nickhaulotte

Regular Contributor
Hi all, Buddy of mine just bought a aluminum boat with a 75 hp mercury on it. I am not used to mercury but have rebuilt a force motor. The issue we are having with this is. rpms seem high by the sound of motor (no tach) while at a idle. When the motor is trimed down all the way and go into gear it dies out. You can trim the motor up a bit (1/4 up) and try agian and it slips into gear without dying and the rpms will go down to where they should be in fwd gear at idle 700-800 (roughly). I think the cavitation plate is too low. Should be at bottom of hull and up 2" lower. Would this be causing the dying? Also, there is a bolt hole next to the top cyl. spark plug what is this for. T-stat? Looks as tho something was there at one point but has broken off in the head. I can stick a srewdriver in there and feal that its not true. rough and not square with block. any answer to these questions will be greatly appreciated thanks. also i think this is a thunderbolt ignition. Is this right? I was planing on doing a link and sync to this motor but would like to get the answers for the questions first so i can have a plan of attack Nick
 
...there is a bolt hole next to the top cyl. spark plug what is this for....
Click to expand...

A thermal switch for overheat alarm; wire rotted off. Is there a beep when the ignition is first turned on. Did you check the compression?
 
did not check compression on this motor didnt have one and buddy wanted it (lol). There is not a beep when the igntion is turned on. it does look like this "bolt" broke off in the block. Maybe torqed too hard? Did not see any wiring lead for this maybe someone took it out. it does not feel there is a down cyl or anything but will check compression if he will let me ( kinda hard headed). Thanks
 
they were new when we picked motor up will look at those further when i can. The bolt hole that im talking about is and the right side of plug. Is that where these items # 35 and 45 go to? I know how these schematics are sometimes in not showing excactly where the peices go. I really appreciate it. Probably might not be till this weekend till i can look at the plugs.
 
The pictures posted do not appear to be from an 1985 --75 HP model.-----------------An exact serial # will get you the exact ansers !!
 
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