Slightly twitchy rev counter...

Batteries, Alternator, Wiring, Lighting, etc. (Please discuss ignition problems in the Engine category)
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Slightly twitchy rev counter...

#1 Post by JimB » Thu Oct 02, 2014 3:10 pm

Hi all,

When the PI's engine is nice and hot after a long run, the idle is slightly erratic... Whilst there are all sorts of possible causes, with ignition being a prime suspect, I've noticed that the rev counter occasionally 'dips' by about 100rpm. It's like a momentary twitch! I read somewhere ages ago that since the rev counter is triggered by the low voltage/tension ignition circuit, I wonder what the cause could be?

At all other times the rev counter behaves as it should.

What do you reckon folks?

Thanks :wink:

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Alan Chatterton
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Re: Slightly twitchy rev counter...

#2 Post by Alan Chatterton » Thu Oct 02, 2014 4:41 pm

Dodgy rev counter, ignition issues mean they either work or they don't!

Eric's needs a friendly little tap to make it work....... All part of the charm!
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Mike Stevens
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Re: Slightly twitchy rev counter...

#3 Post by Mike Stevens » Fri Oct 03, 2014 8:54 pm

The tachos work by either a current or voltage pulse, depending on age (and I can't remember which is which, late - voltage I think). It is possible that a poor points setting could cause an erratic effect.

Many years ago, I had a fault in one stromberg that made its needle valve to stick shut. That gave a very erratic tacho reading, so it's not just limited to ignition problems!

Also, as Alan says, it could easily be caused by a faulty tacho!

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Steve G
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Re: Slightly twitchy rev counter...

#4 Post by Steve G » Sat Oct 04, 2014 8:29 pm

I'm not sure if it's the same problem but when the one in my car played up I stripped it apart and re-tightened some of the internal screws. Some of the screws form electrical connections between the chassis of the instrument and the terminals where the wiring joins. Over the years I think the spring washers has compressed a bit and allowed a bit of corrosion to start.

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