1995 60 hp mercury running erratic at 5K

Hello all, brand new to this forum and need some help. I have a 1995 Mercury 60 HP and just recently is has started running very erratic between 5 and 6 thousand rpm. It's as though you are accelerating and decelerating very quickly manually with the throttle but ONLY between 5 to 6 K. It runs great under 5K and at WOT. I have already installed a new fuel/water separator filter and that did not help. Any thoughts or opinions? I thought that maybe the ethanol gas was playing some tricks on me but does not seem it would only affect a certain rpm range. Maybe ignition timing is not advancing??? What's a good way to check it? Really would appreciate any and all ideas. Thanks

WOT is close to 68k. Per the tach. Most say this is way too high but it has indicated this for many years with no trouble at all. I even checked with the manufacturer of the pontoon and the motor has the correct pitch propeller on it for use with this boat.

WOT for that motor is 5500 rpms, so if your tach is working fine you are way over-revving.

Now have to call BS on the mfg saying that the pitch is correct. Unless they came out with a marine engineer, weighed your boat/motor/gear/passenger/junk combination and then tested there is no way you can look on a chart or even guess that just because "Bob" can use a 17 pitch (or whatever) that "Steve" can use the same pitch.

It just doesn't work like that. Way too many factors. What the Mfg can say is that perhaps you should start with XX pitch and then let the tach tell you how to adjust.

If you are getting 6800 with say a 15 pitch (as an example) and your target is 5500 (max) you need to reduce by 1300 rpms or about 7" of pitch - so if you were running a 15 and getting 6800, a 22 pitch should take you down to about 5400 rpms.

There is no way that you would run the same pitch on your rig, if you routinely ran with 5 adults on board as Bob would only carrying himself. No "crystal ball" can determine that.

Hey, manufacturers BS us all the time, right? Just saying. I did get a Tracker pontoon dealer parts guy to look up what propeller came with the boat and motor and his specs indicated what is on the engine now. BTW, the boat is a 1995 year model Sun Tracker 21 foot pontoon and the motor is a Mercury Tracker Pro Series 60. The prop numbers are 48 42738 A10 8P, is that an 8 pitch prop? The engine has performed well over the years with the tach reading the same as it does now. If the tach is inaccurate, guess that means that the erratic running is happening at a lesser rpm too. Anyway, the main concern now is to get the engine running good again. Thanks

i would get the tach fixed but you aint gonna do that from what i can see from your comments...but you actually dont know the rpm range for the failure...having said that i would give the motor a good dose of sea foam....get the motor running on a hose..kill it..disconnect the fuel line and put the line in a can of seafoam...start the motor and let it suck the seafoam in until it dies..let it set overnight..hook the hose back up and start the motor and run it til the smoke clears..then take it out and run it...make sure you have new good gas...the merc dealers sell a cleaner and a treatment that i use..a little expensive but it goes a long way on the recommended mix..after you do the seafoam bit i recommend you use these or some equivalent as a maintenance item..
if this improves or removes the problem you need to schedule a carb pull and clean...the motor is 20 years old..
you cannot always make a decision based on a miss at a certain rpm range...it could be anything from plugs to whatever...you have different load requirements based on the rig at different rpm's....plus a problem could be there at other rpm's and its not bad enough for you to know it...

i omitted a key step...the first thing to do is to check compression and spark...spark should jump 7/16 of an inch with a good snap...spark testers are cheap and sold at any good auto parts store...i would also throw a set of new plugs in it while i was doing this..