Tachometers not reading the same

[Fireman431]

2001 Mercruiser 7.4L MPI

Replaced the tachs last week. Both engine run great. Both tachs read 650 @ idle. Up to about 2000 RPM, they both read the same. After that, the port tach starts reading higher than the stbd. At 3300 on the stbd (which I'm pretty sure is dead-on), the port is at 4000.

I know there's a fine tuning screw on the back, but that seems a little more than fine tuning. Plus, they both are running fine until 2000 RPM.

Some things I've noted:
1) Both are on the proper pulse setting (double checked)
2) Both are brand new
3) Both are same style/type/brand
4) Engines are tuned and run great
5) Sync gauge stays even as long as throttles are in-line

Running the boat at speed to adjust the tuning screw while the dash is out isn't an option. Any ideas?

Thanks in advance.

 

[RickJ6956]

Re: Tachometers not reading the same

How do they behave at WOT? Does the port tach settle down to match the starboard? What happens if you swap them?

 

[chuckz]

Re: Tachometers not reading the same

First, swap the tachs and see if the problem tracks the with the tach or if the problem stays with the engine. If the problem switches sides when you swap the tachs, one of the tachs needs repair or calibration. If the problem stays on the same side, you need to start checking what's going on with the engine.

 

[Fireman431]

Re: Tachometers not reading the same

Thanks. I'll try the swap first. Or burn the boat to the waterline.

 

[RickJ6956]

Re: Tachometers not reading the same

Or burn the boat to the waterline.

Why, do you need the practice putting it out? Er ... you're a fireman, right?

 

[chuckz]

Re: Tachometers not reading the same

If the problem ends up being the tach, temporarily wire them in parallel and adjust the "bad" one to match the "good" one. Then return them to normal wiring. It's not a true calibration because both tachs may be wrong, but they will agree.