Then unhook it. Won’t be the first or last time a bad tach causes drive ability issues.

With Some improper reading tachs, if you rotate that dial back and forth a bunch of times you can get lucky and contacts clean themselves enough tach starts reading correctly. Note: even if this does work dies not mean will fix running issue if it is the tach causing one.

I tried routing the plug wires differently away from the main loom and the on board tacho now reads good !

I have cause to think that someone has fitted a car map sensor , I am near sea level , I have checked the live harness wire to the sensor 5.1 v , the harness ground is good. (0.12v between harness ground to engine ground) at atmospheric pressure the signal wire shows 2.31v to harness ground and 2.44v to engine earth.
At 20 inches Hg the signal wire has 0.78v to engine ground and 0.65v to harness ground.
am i correct in thinking this is too low ?
what voltage should it show at sea level sensor out of the engine ? do car MAP sensors use a lower voltage ?

Read at the Battery 12.8 running, but read 14.5 at the ALT?

A MAP is a MAP, it reads barometric air pressure. You should have a 1 BAR (1 atmosphere) sensor

MAP should be 1 to 2 V idle and 4 to 5 V at WOT

Good digging OP. What a mess your just bought boat was. Sucks. Too bad.

Yup, going along with AllDodge here atmospheric map voltage on a 1 bar map sensor should be well into the 4 volt range nearing 5.

If your tach was showing incorrect reading from plug wire interference you could have too much rfi going on too, not just wire closeness issue.

No sorry i worded that badly , now it reads 14.44 v at the battery and at the alternator . I think the battery was low and or bad connections before.

I guess my sensor is defective then ? as it reads 2.31v engine off at 40m above sea level

This should be the Original Equipment part. (MAP sensor) AcDelco # 213-1545

should there be a vacuum pipe on the regulator on the cool fuel assy , if so where should it go? I cant see any unused tappings in the intake manifold , this engine is a second hand engine that has never run properly. It could have miss matching parts

Shot in dark here... ignition timing?

Yes, the 1/8 rubber line goes under the plenum and there should be a Tee fitting to connect it

Yes i have checked the base timimg is at 8 degrees .