78 merc 115 troubleshootin

smike26

Seaman Apprentice
Joined
Feb 12, 2003
Messages
35
I'm suspicious of an ignition problem with my 115hp 78 Merc. What are some tell-tale signs that I'm not firing on all cylinders? When I adjust mixture on all three carbs I get noticable results. After a day on the lake I pull the plugs and they all look the same. Motor starts and runs fine but is just a little slow for a 16ft Glastron with just me and a cooler. You all have helped me so much before, so thanks in advance. Here some other facts:<br />WOT is 5500rpm, planes and gets out of hole fine.<br />Prop is 19 pitch aluminum, slightly dented.<br />Motor sits in water at the correct level from what I've read.<br />I decarb often.<br />Compression, if I remember correctly, was even on all 6 last year at 90psi. I did it cold, please inform me of the correct procedure.<br />Bottom of boat is smooth and clean.<br />No hydrofoils.<br />Have been through the whole range of manual trim settings.<br />16ft Glastron, top speed only 35mph on gps.
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
28,078
Re: 78 merc 115 troubleshootin

Smike, Your RPM is good, but your speed is way low. It should be 42+MPH for that RPM. Either your Tach is incorrect (maybe set improperly) or your prop stinks. Buy or borrow another prop and try it. Make sure the tach is set to the correct setting (12 pole alternator)
 

Clams Canino

Commander
Joined
Jan 10, 2004
Messages
2,179
Re: 78 merc 115 troubleshootin

Agreed. Sounds like its hitting on all 6.<br /><br />-W
 

tashasdaddy

Honorary Moderator Emeritus
Joined
Nov 11, 2005
Messages
51,019
Re: 78 merc 115 troubleshootin

dented how much, this could be the problem.
 
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