Mercruiser 350 mag/bravo 3 over revving

[drolds72]

I bought a 2006 25' Azure with a 350 Mag MPI and a Bravo 3 drive. 24" pitch props. Picked it up in another state at a marina in June, and they ran it on muffs- ran perfectly. Get it home, and my lake went no wake until today. Throttle it up, and it comes right up on plane, however, if I let it, it will wind right up to 5800 rpms. Manual says 4800-5200 is spec. Tach seems to work at idle, as I have 17 hours on it between 600 and 1000 rpm. Seems like it is accurate at idle. Pulled out tach to check setting on back and its correct. Has anybody ever heard of a tach that is accurate at idle, and not at higher RPM? My plan is to verify operating rpm with another tach, and then move on to propping if need be. Any better way to do this? I find it really hard to believe that a dealer maintained boat would be allowed to run like that for 8 years. Dry weight on the boat is 5200 pounds, if that matters..

 

[thumpar]

You should be hitting the rev limiter before 5800 RPM. My Faria tach will sometimes read high and a tap will fix it. I would try another tach first.

 

[Scott Danforth]

Verify the tach with a photo tach off the crank, or with a diagnostic tach off the coil.

most of the gauges are inaccurate and merely a suggestion. the tach in your dash could be off by 10-15% which at idle is not perceivable, however at WOT could be as much as 800 RPM

 

[drolds72]

Thanks! will try to get a test tach and run it right from the coil. I don't have a shop manual for this boat yet, and was unsure if there was a rev limiter...
And after seeing what a new pair of props cost, going to verify the operation of my depthfinder as well!

 

[aerobat]

like mentioned you should hit the rev limiter on a mpi before and, assuming you have smartcraft instruments, you should get an overrev warning in the display on a MPI afaik.

 

[HT32BSX115]

Howdy,

I seem to remember reading an article about marine instruments years ago and the author indicated that it's not uncommon for a tach to be as high as 500RPM off (high) at the higher end of the scale.

Also, if the boat is properly propped, and 24p sounds about right on a 25' boat with a 2.00:1 or 2.20:1 drive, It would probably be impossible for your engine to turn that high.

It also cannot be rubber prop hubs since the Bravo III has none.

If it were the coupler, it would only do it briefly AND the rev-limiter would activate anyway.

I would get another tach.


Cheers,


Rick

 

[drolds72]

Wound up buying a diagnostic tach, and the Faria tach, was indeed reading high. The interesting part, was that the error was not linear. Idle speed read correctly and the error increased percentage wise from 5% at 2k to about 20% at WOT. I have never seen an instrument off by that much. I read somewhere else, that the tach was adjustable. There are 2 adjustment pots at the 12 oclock and 6 oclock positions. playing with the 12 one got me within 200 rpm of actual. Does anybody know what the other pot is for? I hope I did not just speed up the hour meter... Thanks for the responses- everybody was spot on!

 

[drolds72]

Just got an response from Faria about how to adjust the tachometer. On this particular 5" tach, there are 2 adjustment pots. one at the 12 oclock position, and one at the 6 o'clock position. They said the fine tune high adjust was at the 6'oclock, and the low rpm/zero adjust is at the 12'oclock.