Well, I bought this 1973 MirroCraft 16-foot Lake Fisherman last November from people who just wanted to get rid of it. Boat, motor trailer for $500, which was originally from Wisconsin, I wanted the boat more than anything else due to the aluminum quality to restore and use it in Florida. I have a background in restoring antique and classic outboards, my favorites are the Evinrudes. So, curiosity got the best of me and wanted to see if this 1998 Mercury Force would run, and it did after cleaning up the carburetor, and then I measured the compression in both cylinders 160-lbs each ... this motor basically had no hours on it. After 3-months of work I completed the restore project and tested it this last Tuesday to see how it would run, that is when I discovered the large swing between factory specs and operating WOT speeds. All the other criteria of the setup are correct cavitation plate etc., another plausible possibility is I stripped the bottom to raw aluminum and coated it with FASCO Super Slick epoxy with Teflon to seal any possible leaks and give the bottom a smoother profile ... and it is slick. It would be hard to believe it would make that much difference. Faria is the manufacturer of the tach capable of 8000 RPM and I followed the installation and settings to the letter at 12-poles. I bought it from Great Lakes Skipper and in the processes of confirming the setting.
That sounds good, but does the motor SOUND like it's running 7k? Your ears can tell you a lot.
As you accelerate, does it reach a point where it keeps revving more without gaining much speed?
It could be a spun prop hub. I've seen that many of times. Here are a few other thoughts:
- Is the prop visibly damaged?
- Could your motor be bolted too high on the transom? (drop a hole or two)
- Could the motor be trimmed up too high? (Check the location of the stop pin)
- Could it be the wrong shaft length for the transom? I've seen that many times, too!
Just trying to help you think of some common causes of over-revving, if it's not the tach.
Back to the tachometer... Your setting of 12P should be right, but there is the possibility that it's either 8 or 20 pole. Here is the chart:
https://fariabeede.com/site_manuals/IS0086_V.pdf
Sometimes, when a Force motor has ignition issues, owners will swap parts out from a parts motor, and the settings won't be correct for the year. I've seen that, too. SHOULD be a 12, but it's an 8.
Just for kicks, try setting it at 8 pole and maybe at 20 pole and see if it seems right. Does that tach have a 20P setting? Not all do. Make sure that you are pushing in a little as you adjust the pole settings, or it can mess up the tab inside, then you just have to guess where it lands.
Finally, on to the prop... It's possible that you need to go up in pitch, but it doesn't seem likely. Average RPM change is about 200 rpm per pitch. You need to drop about 1400 RPM, so that puts you 7-8P higher, meaning you will need about a 21 pitch prop.
I have NEVER seen a 50 that will push a lake boat with a 21P prop.
The above checks are pretty quick and easy. If everything above checks out and all indications are that the motor is over-revving, I recommend that you get a 15P Mercury prop from Walmart online, West Marine, or somewhere else that will accept a clean return, and then try it and see what happens.
I hope you are able to figure it out. Those little engines aren't the strongest, but they run really well, as long as you don't let them overheat. INSTALL A TELL-TALE IF IT DOESN'T ALREADY HAVE ONE!
