RPM Question

[DWill3332]

Good evening. I am a new member of the forum. I recently purchased a 24' 2007 SunChaser made by SmokerCraft. It is powered by a 90 HP Mercury motor and I transport it on a tandum axle Masterryde trailer. I bought it from the Tracker Boat Dealer in Prattville, Alabama in December of last year. The boat looked brand new and I found out that the former owner only put it in the water a half-dozen times. Those folks were great to deal with and really treated us well throughout the deal. I will post pictures soon. We have had it out on Lake Oliver here in Columbus, GA several times. We live only 5 minutes from the lake so it makes it easy to get in the water quickly. I have added a Hummingbird 383c GPS/FishFinder - mainly for the GPS feature and the speedometer. With just me and the wife, it runs 23 to 24 mph without a headwind. When our grown children and thier spouses join us, I can get 20 to 21 at 5500 rpm. My question, and the real reason for joining the forum now (I have been reading it for several months now) is last Saturday, we pulled a tube for the first time. I read all the debates on "use the lifting rings or don't use the lifting rings" to pull a tube. I was determined to wait until I could buy a tow bar, but gave in and used the lifting rings to pull the tube. Now for the question - normally, at wide open throttle, my tach reads 5500 to 5700 rpm. Saturday, after we had run for a little while, I noticed that the rpms had moved up to 6500 and then to 7000. The motor did not sound any different and the speed did not increase. Any ideas why the motor would do that? Is it something that I need to get checked out by the local dealer? I appreciate your help.

[dockholiday]

Welcome

Sounds like a bad tach reading. Pretty sure you would have notice a jump of 1000 to 1500 rpm by sound, also think you would have a rev limit warning kick in if the rpm had been that high. Merc make a smart gauge which has a tach as one of its many functions. It is digital (or mine is) an there is as much as 400 rpm diff in this an the designated tach.
Not familiar with Lake Oliver, what size lake is it?
doc

[DWill3332]

I suspected the tach was the problem since I did not notice any changes in the sound of the motor or the top speed. I plan to keep an eye on it to see if I can detect any other changes.

Lake Oliver is a 2,150-acre Georgia Power hydropower impoundment on the Chattahoochee River. Oliver Dam provides electic power for most of Columbus, GA. Oliver is two dams below West Point Lake. Just below West Point is Lake Harding which is much larger than Lake Oliver. Between Harding and Oliver is Goat Rock Lake which is formed by Goat Rock Dam. Goat Rock is much smaller than Oliver somewhere about 1,000 acres.

Thanks for the response.

[dockholiday]

Okay heard of goat rock..Here is a link to keep an eye on the water levels....
http://www.srh.noaa.gov/ffc/html/rrm.php
http://lakes.southernco.com/
doc

[chill'nthemost]

My tach was jumping sporadically at the end of season last year. I was told to turn the selector on the back of the tach that is adjustable for different motors with a screwdriver round and round (I'm guessing not all tach's have this feature). For whatever reason it seems to have worked. First trip out last Sunday the tach was steady all day. I had previously checked all the wiring and connections, so I believe this if what fixed the problem. Doesn't cost anything to try it.

[DWill3332]

Thanks, I take a look at my tach and see if it has that feature. It is a Farinia (sp?) tach that came with the boat.

[DWill3332]

Good evening. We took the boat out today for a day of tubing on Lake Harding. The tach maxed out at 5500 RPMs. Before we left, I looked at the back of the tach and checked the wiring connections. Everything looked OK. The tach was reading 500 RPMs sitting in the driveway without the motor running. I tapped on it (a little precussion maintenance) and it went back to 0. So, maybe that solved my problem. We'll see.