tacho reading 35% low

[mark robson]

We have a 1969 Mk 2 car with a HC engine and overdrive ( well it came from the factory as a HC anyway )

The whole car drives beautifully, the engine revs nicely and makes lots of lovely noise.

But the tacho doesn't spin up the dial the way it should.
I have done some 'research' and in 4th gear with overdrive off at 1,000 rpm the speedo indicates 40 km/h.
I don't think overdrive is stuck on and it's the wrong ratio anyway.
The book says this should be 1350 rpm. It's the same in all the gears. 2nd gear should be reading 2600 revs at 40 km/h
and it reads 1900 when I change gear to 3rd.
While I haven't calibrated the speedo scientifically, it 'feels' like it's reading reasonably accurately.
The traffic does not leave us behind.

So I thought maybe it's a 6 cylinder tacho from a C, but the face definitely says 4 Cyl Negative earth.
It seems to be reading about 35% under all the time. At idle the tacho points to about 800/900 revs but
at these low revs it's hard to tell if the ration is the same there.

Anyone got any ideas apart from buy a new tacho?

Where is the inductive loop wire pictured in the manual?

PS whilst I have extensive DIY experience ( my most recent project was replacing the engine in an F )
I am a newbie to the B.

[Peter Cresswell]

Hi Mark
Firstly the inductive loop is on the outside of the rear casing of the tacho.

Most Smiths tachos have a potentiometer that allows you to adjust the reading. I am not sure if that applies to the MGB instrument, but Caerbont Instuments (see http://www.caigauge.com/) might be able to help. Also JDO Instruments (http://www.jdo1.com/). The problem you then have is calibrating it. One way is to put the car on a rolling road, another is to borrow another MGB tacho and connect the two into the car and adjust yours until they are the same, or send it to Caerbont or JDO to get it calibrated.

Does anyone else on the forum know for sure if the induction loop MGB tacho has a potentiometer to adjust the reading?