1984 Mercury inline 6 missing at half throttle

[garretj]

Hello-

I have an 84 inline 6 mercury that I am currently troubleshooting. The motor starts and idles fairly well, at about half throttle there seems to be some 'missing' performance. After half throttle the missing pretty much stops but the motor does not have the top end that it should. It runs fairly smooth at wot but again not the speed that it should, maybe 25 mph. If anyone has any suggestions I would love to hear and I can easily provide photos or videos if necessary.

I've done some minor work on it by replacing all the spark plugs, blowing out fuel lines, tried the decarb method with seafoam, but with no real luck. Thanks for the help.

 

[Chris1956]

Take a compression test, and a spark test. If she is not firing on all 6 cylinders, that is likely the issue. After that the carbs are suspect.

 

[garretj]

Thanks Chris. I did do a compression test and all of the compression was equal on the top 5 cylinders (couldn't access the bottom one for the test). I will check to make sure all of them are getting spark. Thanks!

 

[garretj]

I checked the spark in all the cylinders and appears to be good. I guess I am thinking the carbs need rebuilding.

 

[garretj]

I had it out today on the lake and it was running pretty terrible at the beginning. It has been running pretty inconsistently recently, sometime getting up to around 25 mph and other times not getting past 15. Turned out this morning I had wrapped some weeds around my prop that seemed to slow it down significantly. I removed them and it ran a bit better after that, but still very sluggish. Another issue that seemed to spring up over night is an oil leak. I found some on my driveway the day I used the "Dunc's Decarb" method, didn't think too much of it, but again this morning it was leaking into the water.

Would love to get some advice or suggestions on where to go from here.

 

[Chris1956]

Unless you have a large boat, I would expect better speed than 25MPH out of that motor. Any idea what the RPM is? What size and type of boat, and what pitch prop?

An oil leak could be a misfiring cylinder, bad fuel pump or carb issue. I guess I would recommend you rebuild the fuel pump to eliminate that as a cause. Carbs are probably next to be rebuilt, as is a link and synch. If still no-go, maybe high-speed stator winding or switchboxes are suspect.

 

[garretj]

So checking spark again it appears that the top cylinder (#1) is getting no spark! I know that the power pack? was replaced on this motor not that long ago, but other than that I don't know much about this problem. All other cylinders, including 2 and 3 that are on the same power/coil pack are getting great spark.

 

[garretj]

Been a while since the last post, but thought I would keep this thread open so I don't have to rewrite the issues I've had with the motor. I redid the floor and seats in my 16’ boat so I naturally wanted to see if I could fix the engine issues I was having to make it an all-around nice ride. I took it to a mechanic in my area and got some bad news. He said spark and compression on all cylinders was good (apparently I didn’t know what I was doing when testing spark a few years ago) and that the timing was good (not exactly sure what this is..sorry). He said it’s ‘puddling fuel’ and that cylinder #2 and #3 have washed plugs. He seems to think it something internal (reed valves or gasket) and the job requires a complete tear-down. Too much money for my blood. I’ve done some research on the reed valves and everyone on this forum seems to think these should not be failing. The standard advice seems to be rebuild carbs, link and sync, or rebuild fuel pump. Do any of the symptoms of this motor sound like they could be solved by these fixes?

Thanks!

 

[Chris1956]

Reeds are pretty robust, and if damaged, ruin the idle. You say your motor idles, so not likely reeds. Puddling fuel is a new one on me. Check fuel pump diaghram, as if it leaks, it will leak raw fuel into middle cylinders,


Those motors do leak water into the cylinders. If the spark plugs of a cylinder or two are real clean after running it for a while, suspect water. If the clean plugs are the bottom one or two, expect crank seals are leaking. Any other plugs show clean, suspect exhaust baffle or exhaust inner cover.