Have fitted a pair of Facet Cube fuel pumps to the Twini.
Now I find that I have interference with the tacho. Tacho fluctuates at constant revs and does not respond correctly as the revs change.
The tacho is a Smiths one with a single wire from the coil that plugs into the rear of tacho (no loop at rear of the tacho).
Unplug the power to the pump and tacho acts normal.
There is no relay in the system as the pump apparently only draws a maximum of 3 amps.
Anyone had experience with this and a simple fix?

No direct experience with this specific problem, but it sounds like a simple thing to fix. I'd guess that the fuel pumps are causing a ripple in the power supply to the tacho, which is causing it to read wonky.

That's easily fixed -- simply place an electrolytic capacitor (say, 22uF) across the power supply close to the tacho (positive side to +12V, negative to earth). This will 'absorb' the ripple and dampen it down.

_________________
Simon

The adventures of an owner builder in the Tallarook Ranges

I had similar thoughts but with a bigger cap on the pump and a choke on the supply side.

Or, fit a better quality pump.

_________________

I'm intested in this..Its a twini..and you have it running off the coil. Is it 1 coil or 2. I'm thinking you have changed something, not just the fuel pump or it the the tacho also..Intested as the feed for the tacho come from the coil and live feed from the ign

_________________
Stop Licking the Dog...I Don't Care Who Started It

One of those "radio noise suppressors" you can buy should help. If you go the single electrolytic type make sure you have a good high voltage rating (like 25V) to handle any transients.

Otherwise the normal method is to try and damp the noise at the source, try a standard points ignition condensor across the power supply of the pumps to try and clamp any spikes.

cheers

Jacob

_________________
'72 Clubman Van - 1022cc, 295 head, 731 cam - Daily Driver
'69 Morris 1100 S - Dinged by a bus, in shed under repair
'64 Morris 1100 - Early 1100, long term project

Two coils - one for each engine.
The tacho with the problem is for the rear engine. The power for the fuel pump comes from the coil + side. The front engine has the other style tacho and does not have the problem.

why are you taking the the rear fuel pump feed/Power from the front Coil it should be from the IGN, as in the main power. Sencer wire should be from rear coil/motor

_________________
Stop Licking the Dog...I Don't Care Who Started It

Sorry for the confussion.
The rear pump power comes from the rear coil.
The tacho sensor wire comes from the ignition switch that supplies power to the rear coil (and now rear fuel pump).

Oooh, I think your problem might start there. Try taking the rear pump power feed from a high-current source like the starter solenoid back there, the voltage ripple from the pump is probably giving your coil a hard time too.

_________________
Simon

The adventures of an owner builder in the Tallarook Ranges

That will need a relay too or it will run 24/7.

I'd just run them off a relay, easy to fit, the coils act to damp and it isolates other systems too.

_________________
Dean Hutton's first car was a Mini.

Oops, yes.. good point

_________________
Simon

The adventures of an owner builder in the Tallarook Ranges

Thanks to all for the ideas.
I will try the "high current source" and condensor ideas today.
I was trying to avoide the relay option, but I suspect that is the answer.