Erratic tachometer

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ts
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Erratic tachometer

Postby ts » Sun Sep 10, 2017 12:17 am

I've recently put back together an R80/7 (with some engine modifications), and noticed that the tachometer didn't work as it should. If cruising at low revs (e.g. 2.5 kRPMs), and opening the throttle, the tach needle instantly jumped up to 3.5-4 kRPMs , much sooner than the increase in engine RPMS. It was also moving around erratically when cruising at a steady engine RPM. Now, a few days later, it seems to steadily indicate 50-70 % higher revs than expected when cruising, but if pulling the clutch and waiting at red lights, it seems to settle down to a normal idle RPM.

How do those tachometers die?
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'73 R75/5, '78 R80/7, '83 R80RT

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Re: Erratic tachometer

Postby windmill john » Sun Sep 10, 2017 7:56 am

You'll have to wait for the other guys to give a more accurate answer. All I can tell you is if the tacho was totally erratic, all over the place, that normally points to a bad earth. Check the earth connector near the front coil; actually check all your earths.
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Re: Erratic tachometer

Postby Tony the Skin » Sun Sep 10, 2017 4:40 pm

Is it a cable driven tach or electric?
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ts
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Re: Erratic tachometer

Postby ts » Sun Sep 10, 2017 5:02 pm

Sorry, I forgot to mention it's the electronic version.

This morning I replaced the instrument console with one from another boxer, which apparently had all the leads of the connector to the console in the right place, in that all the dash telltale lights etc were working as they should. The erratic RPM indication continued, showing that it's the feed to the tachometer that is disturbed.

Some of the modifications is the use of an ignition amplifier (Velleman kit). This was working OK before the bike was parked up a couple of years ago, with normal tacho readings. I mention this as it is connected to the coils. But my gut feeling is that the amplifier either should work, or not?

Next I'll check the earth leads that are in the neighbourhood of the coils.
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'73 R75/5, '78 R80/7, '83 R80RT

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Galactic Greyhound
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Re: Erratic tachometer

Postby Galactic Greyhound » Sun Sep 10, 2017 7:44 pm

The signal feed to the Tachometer is taken from the 'earthy' side (i.e. what would have been the Contact Breaker side) of the 2nd 6v HT Coil.
This will likely still be the same situation when the Velleman Ignition kit was added and the Contact Breaker used to trigger the Velleman Ignition Module rather than the HT coils directly.

The 2nd 6v HT Coil will therefore now be connected to the Velleman Ignition Module rather than directly to the Contact Breaker.

Where the wire goes from the 2nd 6v HT Coil to the Velleman Ignition Module (coil terminal) there will be a second Black wire on the Coil going to the Instrument Cluster Connector pin #10 - this is the Tacho signal wire.

You should check that the signal wire has not come off the 2nd HT coil and also that the Instrument Cluster Connector is securely connected and without corrosion on pin #10 - Contact Cleaner might be useful here.

The earth to the Instrument Cluster is a Brown wire on the connector pin #12 and this should be checked/cleaned as well. This earth comes from the Brake Fluid Switch earth commoning (two Brown wires) one of which goes back to the main Frame Earth. The earth wires on this switch should therefore be examined as well as the main Frame Earth.

You will see the above in a Haynes Wiring Diagram for your 1978 R80/7.
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Re: Erratic tachometer

Postby barryh » Sun Sep 10, 2017 8:17 pm

I've been using a Velleman kit for years and no problems with the tacho. The black tacho impulse wire from terminal 1 of the front coil stays as stock and should not have been touched.

One thing you might check is that the original condenser has been removed as advised by Velleman. To do that on a bike with bean can points presents a particular problem so it's removal may have been omitted. As the bean can electrical connection block is integral with the condenser lead you have to sacrifice a perfectly good condenser by cutting the wire off flush at the block.

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Re: Erratic tachometer

Postby ts » Sun Sep 10, 2017 8:47 pm

Thanks for further comments with tips.

Regarding the condenser, the reason I refitted the Velleman amplifier was that the condenser seemed to have packed up (very erratic firing + +); this disappeared with the amplifier, and with the condenser removed from the bike.

Another thing to investigate is that the running has again become problematic, runs fine at approx. full advance RPMs and above (but difficult to tell since I don't get an accurate tach reading!), but lots of popping and backfiring at lower revs. Will look into the timing and operation of the advance weights.
ts
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'73 R75/5, '78 R80/7, '83 R80RT

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Re: Erratic tachometer

Postby ts » Sun Sep 17, 2017 7:59 pm

So, after going through every earth connection involved in the tach/ coil/ brake failure etc circuitry, to no avail, I continued with investigation the more recently developed rough running. It turned out the ignition timing was way too retarded. That should normally be easy to see with a strobe lamp, but the timing marks were all over the place - which I had taken as a worn timing chain tensioner. That was until I replaced the advance unit springs. Much easier than replacing the tensioner!

To set the timing properly, I had to enlarge the adjustment slots in the back plate by 7-8 mm. Wrong points?

With most in front of the camshaft sprocket in order, the engine started easily, ran properly, - and the tacho was working as it should.

It seems I've got the magic smoke balance set up properly for now . . . .
ts
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Re: Erratic tachometer

Postby Galactic Greyhound » Sun Sep 17, 2017 8:39 pm

..............With most in front of the camshaft sprocket in order, the engine started easily, ran properly, - and the tacho was working as it should.

It seems I've got the magic smoke balance set up properly for now . . . .

Happy Days are here again! :smile:
Ced.

R1100RT 1996.
Sent by Boson Quantum Transmission from the Starship 'Galahad'.
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