1973 Mercury 850 85HP correct Tachometer?

[pman7]

Re: 1973 Mercury 850 85HP correct Tachometer?

At WOT It's only reading about 3600 when I know I'm going about 5200.

How do you "know" it's going 5200?

 

[scottrbishop]

Re: 1973 Mercury 850 85HP correct Tachometer?

I just had the boat out on the river with my marine mechanic about a week ago and he hooked up his marine tach and it read 5200. The boat hasn't changed performance wise, and it rips on to plane in a couple seconds. I don't see any way the motor could turn an extra 1600 RPM.

 

[scottrbishop]

Re: 1973 Mercury 850 85HP correct Tachometer?

The response I got from Teleflex was to try setting it to 5p or 4p and see if I could match the tach to actual RPMs. Is there a logical reason why another setting would be accurate?

 

[Chris1956]

Re: 1973 Mercury 850 85HP correct Tachometer?

Scott, Unless your Stator is bad or an incorrect model(and I do not even know if that is possible), 6P should be the setting. Sounds like the techs were grasping at straws.....

 

[scottrbishop]

Re: 1973 Mercury 850 85HP correct Tachometer?

Hmmm... The dealer said the Stator was checked and found to be good, wouldn't a bad stator also give me some engine trouble? Is there really nothing else that this could be? Should I resort to exchanging the tach for another at the store?

 

[diaric]

Re: 1973 Mercury 850 85HP correct Tachometer?

try different settings and see what you get.some tachs the settings arn't pole but position. mine needed 4 pole, but i had to put it on three(if memory is correct), position 3 was 4 pole on mine. won't hurt it on wrong setting, just doesn't read correctly

 

[ricksrster]

Re: 1973 Mercury 850 85HP correct Tachometer?

Make triply sure you are set at 6P or not between two numbers.

 

[Chris1956]

Re: 1973 Mercury 850 85HP correct Tachometer?

Scott, That motor has a battery driven ignition. The stator is just there to charge the battery. That motor also never had a voltage regulator, just a full wave rectifier. The only way to know for sure what the problem is is to try a known good tach, and compare them. Now some tachs have adjustment knobs. Maybe yours is out of calibration?

 

[scottrbishop]

Re: 1973 Mercury 850 85HP correct Tachometer?

After another day at the lake I found that if I set the dial to 4P I get exactly 5200 at WOT, what my mechanic measured it at earlier. I wonder why 4P would be reading most accurately? The installation instructions show the gray wire splitting into a Y and attaching to either end of the rectifier, would this have anything to do with it not reading correctly on 6P?

 

[diaric]

Re: 1973 Mercury 850 85HP correct Tachometer?

to get a good idea if its reading correctly, whats it say at idle, in neutral and in gear

 

[scottrbishop]

Re: 1973 Mercury 850 85HP correct Tachometer?

I'll have to double check next time I'm out, but I think Neutral was reading about 1100 and a slow troll was about 700.

 

[scottrbishop]

Re: 1973 Mercury 850 85HP correct Tachometer?

I found that the slowest in gear idle I could get was 750 and neutral was a solid 1400. I know my Merc manual says Idle is 550-600, does that tell me my tach is off? I was thinking about getting on of those tiny spark plug tachs to compare their readings. I can't use an voltometer tach on a 2 stroke right? I'm not sure what else to try.

 

[diaric]

Re: 1973 Mercury 850 85HP correct Tachometer?

its not reading correct then. at 1400, you'd get a big clunk when you put it in gear, and it wouldn't drop so much when in gear, about 100 rpm or so