Tacho reading 700 rpm more than it should....help/advice please!

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Mike k

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Last week took my 24ft Weekender for a quick blast and was totally puzzled why at 2800 rpm I was only getting 9-10 knots, as I would normally be getting about 13-15 knots First fears was a rope around the prop of a turbo failure but had no real evidence of that and not much fouling really .It was a 10m tide that day and another thought that it could just be the pull/push of the tide but that has not been such an issue before.Anyhow I have checked the prop with mirror and light and no rope issues, however before starting her up a couple of days ago I noticed that tacho guage was already showing 700 rpm and on starting shot up immediately and the sound of the engine did not justify the rpm being displayed. Engine is a 250HP Mercruiser.

How do these Tacho's work and is there an easy fix so the dial goes to zero on switching off?

Thanks for any advice

regards

Mike
 
cannot really help, just to note that yesterday I was pretty quick pressing the big red button to stop the engine and then (in order to avoid the annoying beeping sound) I turned the key rather too quickly and the rev counter stayed somewhere around 400rpm.
Was puzzled, turned the engine back on, revs hit the right #.
Turned off again, but was late on the key and the rev counter dropped to 0.
Tried again and surely enough when you turn the key to off needle stays were it is.

So could try turning the key on and see (without starting the engine) and see if revcounter drops to wherever it should be and start the engine and see what happens.

Typically rev counters take the signal from the alternator, so something's dodgy there, a connection maybe? (although that usually means the counter doesn't work at all!)

cheers

V.
 
vas said:
cannot really help, just to note that yesterday I was pretty quick pressing the big red button to stop the engine and then (in order to avoid the annoying beeping sound) I turned the key rather too quickly and the rev counter stayed somewhere around 400rpm.
Was puzzled, turned the engine back on, revs hit the right #.
Turned off again, but was late on the key and the rev counter dropped to 0.
Tried again and surely enough when you turn the key to off needle stays were it is.

So could try turning the key on and see (without starting the engine) and see if revcounter drops to wherever it should be and start the engine and see what happens.

Typically rev counters take the signal from the alternator, so something's dodgy there, a connection maybe? (although that usually means the counter doesn't work at all!)

cheers

V.
Click to expand...

thanks Vas- I will try that with the key, just wondering if something needs freeing up.
 
the tacho is supposed to stick when you turn the engine off.

it was on tick over 700 rpm ish when you turned it off so it stays at tickover.

oil / temp gauges should also stick.

its easy to adjust by turning a screw on the rear of the tacho

i suggest its fouling on you hull or on the paddle wheel and not worth doing anything with until after lift out however you could get a digital tacho .
bit of tape on the vicous damper and get an accurate reading before adjusting the tacho to match

i do mine every 2 years in order to keep the engines in sync
 
DAKA said:
the tacho is supposed to stick when you turn the engine off.

it was on tick over 700 rpm ish when you turned it off so it stays at tickover.

oil / temp gauges should also stick.

its easy to adjust by turning a screw on the rear of the tacho

i suggest its fouling on you hull or on the paddle wheel and not worth doing anything with until after lift out however you could get a digital tacho .
bit of tape on the vicous damper and get an accurate reading before adjusting the tacho to match

i do mine every 2 years in order to keep the engines in sync
Click to expand...

thanks Pete and great to hear from you, hope all is well.

Mine don't normally stick ( or I have not noticed it before- probably more like it!) ,is this a common thing then because in my car all needles go to zero on switching off. Does the adjustment screw you mention on the rear of the tacho just release the 'stuck' indicator or is it going to alter the calibration? Would appreciate your thoughts on the link between the tacho and the hull fouling/paddlewheel - because my speed is measured by a different instrument. Cheers Pete

thanks in advance
Mike
 
Are we to assume your tacho is over reading to the tune of 700rpm? Would you normally expect 9-10 knots at 2100 rpm rather than the 2800rpm your tacho was showing last week?
Try turning off your batteries, this may reset your instruments. Never come across a tacho sticking when you turn off. I think "everything" should return to nil (as per your car).
 
JT57 said:
Are we to assume your tacho is over reading to the tune of 700rpm? Would you normally expect 9-10 knots at 2100 rpm rather than the 2800rpm your tacho was showing last week?
Try turning off your batteries, this may reset your instruments. Never come across a tacho sticking when you turn off. I think "everything" should return to nil (as per your car).
Click to expand...

yes normally 9kn at 2100 RPM and thanks I will try turning off the batteries.
 
Hi Mike,

I'm very well thanks :)



As you know I have had a pair of engines just like yours ( cummins-mercruiser 300hp) except I have an upstairs too so thats 4 rev counters .

all the gauges read last known value as you switch off.

As I said above

rev counter
oil
temp

I had an issue with a rev counter about 6 months after I got the boat and I had one replaced under warranty that also holds its last know value on switch off.




There are two screws on the rear


calibration screw

you can set your revs with your throttle to say 2000 using a hand held strobe (this will be dead easy on your boat as you have made that locker hatch to access your impeller which is ideal access to your viscous damper.

once at 2000 rpm you adjust the screw until your gauge reads 2000 rpm


the second screw is to select which engine your rev counter is attached to

4 cylinder
6 cylinder (this is yours)
8 cylinder


the selector screw can move and sometimes needs putting back to the right setting.


if your boat hull or prop is fouled you cant expect usual performance

if the paddle wheel is fouled it may be saying 10 knots when you are actually doing 13

this is what you need if you really want to check

http://www.ebay.co.uk/itm/Professio...o-With-Case-/111225418023?hash=item19e58d3127
 
Last edited:
DAKA said:
Hi Mike,

I'm very well thanks :)



As you know I have had a pair of engines just like yours ( cummins-mercruiser 300hp) except I have an upstairs too so thats 4 rev counters .

all the gauges read last known value as you switch off.

As I said above

rev counter
oil
temp

I had an issue with a rev counter about 6 months after I got the boat and I had one replaced under warranty that also holds its last know value on switch off.




There are two screws on the rear


calibration screw

you can set your revs with your throttle to say 2000 using a hand held strobe (this will be dead easy on your boat as you have made that locker hatch to access your impeller which is ideal access to your viscous damper.

once at 2000 rpm you adjust the screw until your gauge reads 2000 rpm


the second screw is to select which engine your rev counter is attached to

4 cylinder
6 cylinder (this is yours)
8 cylinder


the selector screw can move and sometimes needs putting back to the right setting.


if your boat hull or prop is fouled you cant expect usual performance

if the paddle wheel is fouled it may be saying 10 knots when you are actually doing 13

this is what you need if you really want to check

http://www.ebay.co.uk/itm/Professio...o-With-Case-/111225418023?hash=item19e58d3127
Click to expand...


thanks very much Pete that's great info- many thanks for taking the time to provide the details.

Cheers

mike
 
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