strange Tachometer problem 96 Speedster

[wendttr]

96 speedster
both engines run fine and I can hear they are at similar RPMs when throttles are pushed together.
left tach is fine, right tach tends to drop off in RPMs after running then occasionally bounce back up.
I bought a replacement used tach which works fine on the left engine but does not work at all on the right ?
where on the engine does the signal come from ?

 

[boatmonkey82]

i think your voltage regulator is bad , that where 99% of 2 strokes and diesels get there tach signal from . it comes from the pulses off the charging system , it should be your grey wire from the regulator . they are bad for going out . go into your mpem ' big grey box' and swap tach leads and see if the problem moves , if it does then its your reg if not then its your wiring or tach . or doesthis motor feed the tach signal from the mpem itself ? hell i cant remember , its late and im on my pain meds:hurray:

 

[seadoosnipe]

Coil?...

i think your voltage regulator is bad , that where 99% of 2 strokes and diesels get there tach signal from . it comes from the pulses off the charging system , it should be your grey wire from the regulator . they are bad for going out . go into your mpem ' big grey box' and swap tach leads and see if the problem moves , if it does then its your reg if not then its your wiring or tach . or doesthis motor feed the tach signal from the mpem itself ? hell i cant remember , its late and im on my pain meds:hurray:

I think this will be a first time I will not be able to agree with you. Of course, your on pain meds so at least, you have an excuse:rofl:

The signal for your tach is sent by your MPEM, which receives it's signal from the coil. Every time the engine makes a revolution, it sparks. The faster it sparks, the faster the crank is moving in RPM. The MPEM takes that information can converts it to shaft RPM. But, on the 787, that spark happens every 180* verses 360*. It's called a wasted spark ignition system.

You can follow the grey wire back from the tach to your mpem, check the plug connection and even, test the wire for continuity. But, personally, unless I just had the time to kill, I would not worry about it. Your ears can tell when the engines are running equally. There is a pitch that is "unison" in sound. So easy to tell when they are not running in harmony.........

 

[boatmonkey82]

sorry i am thinking outboards . :stupid: . yea when your hair and eye balls start vibrating in perfect harmony with the twins screaming at 7 k then your in the sweet spot .

 

[bradm]

try this

If its the same as my 95 speedster then there in one connector supplying 12 volts for each tach and then a single grey wire with the signal for each tach. Try swapping the 12V supply wires to other tach. See if the problem changes. Then try switching the grey signal wires see what happens. Its a very easy fix if its just the 12 volts. And if its the signal wire then I think it would be worth running a new wire right from the MPEM and see if that fixes it.