Tach and top speed questions

I just bought my first boat and after being out a few times have some questions. The boat is an '88 Bass Tracker Pro-17 with an '88 (I believe) 35hp Mercury. The previous owner removed the oil tank (not sure if all the injection stuff was taken off though) so it is all pre-mix. The prop is a ~10'' diameter 15 pitch. The top speed with just me and gear is 25 mph and with 2 people it struggles to get 22 mph. These speeds were verified with my GPS. The prop has some knicks and gouges on the leading edge of the blades. Am i losing some mph? Or does that speed sound right with what I have? I plan on getting a replacement prop soon and wonder what to expect for top speed or if I should change the pitch. The other question is about the tach. It jumps around at WOT and I cannot get a reading (which i would like to have to determine my new prop pitch). Sometimes it will max out the tach and I know it is not spinning 8,000 rpm. The back of the tach has a selector that reads 4 6 8 A B and it is currently set to 6. Is this the correct setting for the motor? I spun the selector around after reading that doing that may solve the jumpy tach issue by cleaning the contacts. While running the batt is getting between 12.5-13.0 volts. Is there anything else to check? Regulator? Rectifier? I am fairly familiar with 2 strokes (I have a quad and snowbobile), just not the electronics/ignitions on them. I consider myself pretty mechanically inclined as well and hope to learn as much about outboards as I can now that I have one.

Thanks
Brian

Are those voltage readings at idle or close to it? If they are at higher RPMs then you have a problem. First thing to check would be the rectifier. It can affect charging and the tach. I don't think that motor has a regulator. If you need the instructions I think I can dig them out for you. BTW, moving that switch on the back of the tach like you did often clears up erratic operating tachs.

Those readings are throughout the rpm range. I measured it at low to mid throttle with a multimeter (at Batt) while running in the driveway and when WOT in the water the dash gauge reads about the same. Is there a way to test the rectifier? Or is it just by measuring the voltage when running? What should the voltage be normally when running? With the tach, I tried to switch it to the 4 position while out on the water today and it seemed to shift the rpm range up. At the 6 position, where it was originally, it seems to read high then start jumping when i get near WOT. I should have tried the other positions while out today. Would wrong setting cause the tach to jump around? I am trying to find out what the A and B settings are for on the tach but all i can find are numbers based on pulses or cyls.

You almost certainly have an engine electrical problem. Best to get it fixed now before it does more damage.

If your motor is a 2 cylinder then the tach should be set on 4. If 3 cyl then set it on 6. It is based on pulses. While running at WOT you should be seeing 14+ volts. So it sure looks like you have a bad rectifier. For some reason I can't post pictures right now. (may be an IE9 problem) Anyway go here http://www.crappie.com/crappie/mecha...ch-smoked.html and go down to the 6th post for instructions for testing the rectifier.

Sounds like a bad tach. As far as your speed that sounds about right for the motor you have.

Thanks Guys. When I got off work I cleaned the contacts on the rectifier and flipped the tach switch to 8 just to try it out. It didn't jump around at all and based on the sound it seemed correct. It read 5-600 rpm (as opposed to 1000 on the 6 setting) at idle and when reved it started to scream as it reached 6000 (before part throttle would occasionally jump around and max out the tach). The tach moved smoothly throughout the whole range. Not sure if it is the correct setting on the tach, but it seemed like it. I was reading and it sounded like different tachs are labeled differently. Some have ''p'' after the number (i.e. 4P) and the setting depends on how the tach is labeled (pulses or poles?). When I set the tach to 4 yesterday the whole rpm range shifted up and instead of jumping around it buried the needle at wot. I checked the batt again while the motor was running and it was a consistant 12.89V from idle to part throttle. I followed that link and will test my rectifier and probably pick up a replacement. I will probably order a new prop with week as well when I get my extra TM prop.