Mercury Tach settings

[~Nickolas~]

Hey folks,
I have an older Mercury 115 { 1980 } on the back of a 16' Tri Hull and was doing some wiring and noticed the settings on the back of the Tach. It goes from 1-6 and it is set on number 1. It appears to be correct when out in the water and how it idles and full throttle. Just wondering if anyone knows what the numbers stand for on this particular year/model. I see a lot of posts about 12 pole for these motors so what does 1 stand for or 4 for that matter?

Thanks

 

[yamatech43]

Re: Mercury Tach settings

These are the number of pulses PER REVOLUTION of motor....don't worry about it and it's correct for a two-stroke.

 

[Chris1956]

Re: Mercury Tach settings

I would think 6p is the correct setting for that Merc. You might try it and see if you believe the readings.

You can alos check on the manufacturer's web site for the info.

 

[~Nickolas~]

Re: Mercury Tach settings

At idle it seems to be right. It has a 19 pitch 3 blade aluminum prop on it and at full throttle it is about 5800 rpm. I seriously doubt the previous owners did anything to it as they didn't know anything about boats. There were about 6 of them that fished from it in the river as in all foreigners and were not into boats but just fishing. What website to check are you speaking of to check into? These are the original gauges in the boat as I see a 1979 stamped on the back. They have an M on the face plate. Only thing is when I'm cruising at 4000 rpm it doesn't seem like it is that high but maybe I am wrong. Anyway that's why I was asking about the numbers on the back as to what exactly they stood for...

 

[Silvertip]

Re: Mercury Tach settings

Those number on newer tachs indeed correspond to the number of pulses from the charging system with each engine revolution. I've seen tachs however where the numbers are geneic meaning #1 might equate to 6 pulse, 2 to 5 pulse, 3 to 4 cylinder, 4 to 6 cylinder and 5 to 8 cylinder.

 

[~Nickolas~]

Re: Mercury Tach settings

I was wrong on the Tach numbers... I went over today to do a few things and looked at the back of the Tach again and it has 1-4 and set on number 1. It is a vintage Medallion Tach with a 1979 date stamped on the back of it. They must use whatever is lying around at the time of manufacture because I noticed the speedo also has the numbers around the fitting where the vacuum hose slips on. No need for numbers when you have a vacuum operated speedo that works off pitot pressure. Anyway thought I would add a note for correct info as I thought it was 1-6 but turns out to be 1-4...

 

[ronsealdeath]

Re: Mercury Tach settings

If the tach is not quite set to the right number of poles/pulses etc.... then it may still appear correct at idle but not at higher RPMs. I found this out with an auto tach that I replaced the resistors on. Needless to say it gave me a plausible idle of 900 RPM but a WOT of 3200 RPM! Not correct I can assure you.
Buy a cheap laser tacho thing from ebay (?10 ish in the UK) and test it at idle and 2000 rpm in the bucket. Try each setting and see which is actually reflecting RPM of the flywheel.

 

[Frank Acampora]

Re: Mercury Tach settings

The tach setting depends upon the type ignition, the type tach, and where the signal is derived. For example: If the engine has a distributor ignition and/or the signal pulses are taken from the CD box, then your engine would have six power pulses per revolution. If there are multiple CD boxes and signal is taken from one box, then it would be less than six--probably three. NOTE that unless the tach is marked "pulses" these do not correspond to the numbers on the tach switch. If the signal is taken from the alternator stator or A/C terminal on the rectifier, setting would depend upon whether or not the stator was 12 or 20 poles. Merc did make both at various times and some tachs were designed to work on both systems while most were dedicated to one type alternator.

Without knowing all the variables and your specific engine, it is impossible to say which tach setting is correct. However, if it appears correct at idle and when you get it on the water it also appears close (somewhere near 5000 at wide open throttle) then most likely the setting is correct.

You probably will get a more accurate answer over on the Merc forum.