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Rough Midrange performance

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unclebob

Contributing Member
I have a 60 HP Mercury, 1994 model with low hours. I purchased it about 2 yrs. ago and the overall performance is good. However, it has a rough spot between idle and 2000 rpm. It seems like an engine miss but the spark is good on all cylinders thru this range, (checked with an adjustable spark gap tester, also the plugs look ok but have been changed also). I have rebuilt and cleaned all carbs. The engine idles perfectly at 750 rpm and the top end is great. Boat is aluminum 17 ft and tops at 5200 rpm at 34 mph. The engine timing is set at 8 degrees at idle and 22 degrees at WOT. The carbs are in-sync with each other and with spark advance per the manual specs. An alternate prop has been used with no effect on this issue. Compression is 110 psi on all three cylinders. I have run several cans of Sea Foam thru engine and I run only 90 octane ethanol free gas. I don't know if the check valves located in the crankcase could be causing this issue. Thought I would seek opinions before I go any further. :confused:
 
Jeff,
Thank you for your reply. I did not mention this in my original post but I have already tried this. My optimum setting is 15/8 turns out for all three carbs. I can troll with the engine at idle for extended periods without any issues. When I accelerate from idle the engine does not stumble it just seems to have a miss. My original gut feeling was that it was a carburetor problem or a prop vibration but I can't seem to make it go away. I have connected a timing light to each plug wire while accelerating but I cannot detect any skips. Also, I have checked the fuel filter and the water separator for any contamination. The primer bulb stays hard during the run time. The floats are all set level and once on plane the motor is strong and responsive without any issue. The reason that I mention the check valves is that I read this in a troubleshooting section of a Mercury outboard manual on-line. There are 4 check valves. Two are located at the top and bottom of the crankcase. I believe these are for recirculating oil from the bottom to top. There are two more located in front of the reed valves, I think these are needed for low end RPM performance but I am not sure. This information is from an exploded view of a Mercury manual. I have previously owned a 120 HP Johnson that had a stopped up check valve which caused it to spit at idle. I was the original owner and put up with for several years before I fixed it as a guess.
 
Guyjg,
Last boating season while trying to fix this same issue, I purchased a new primer bulb and hose without any improvement. I have not checked the fuel pump pressure while motor running. I assumed that if there was an output problem it would worsen at higher RPM's. Thanks for the advice.
 
I have fixed my problem with the rough acceleration. I rebuilt the carbs, the fuel pump, replaced the check valves, (both the two in the reed valve ass'y and the recirculation valve at the bottom), and replaced the fuel filter. The engine now idles best at a different idle mixture screw adjustment, (1 7/16 turns out. I am not quite sure why as I did not change the float. I think my root cause was the fuel pump but I am glad I did the other things as well.
 
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