Electronic rev-counter and wasted sparks

[Boxprod]

Dear wise folk,

I'm off down a rabbit hole again, this time about having an electronic rev-counter on my '74 widecase 350. This started simply because I was trying to get more accuracy than the original small needle-waver offered, so in about 1979 I fitted an old Norton Smiths mechanical version, which was the 80mm size and considerably better - but it vanished at some point over the years of storage.

So I recently acquired a Chinese marine tacho quite cheaply, and worked out how to set it for a single cylinder (despite the 'instructions'), and that seemed fine, but after a couple of months it started reading inconsistently and variably, so that one was removed.

I then found on eBay an old Elliot Design rev-counter, which was a very reasonable price, and seems to have the right spec. As you can see from the photos, it's probably a late one in the company's life judging by the graphics.

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It is expecting a single pulse every two revolutions - it clearly says so on the back: "1p/2R". Should work a treat. But of course, I've still got the Motoplat system, which I've had refurbished and checked, and it works very well indeed. The trigger rotor, running at half crankshaft speeed, is not 'sided', but has a two trigger magnets at 180 degrees - so you can't get the rotor on the wrong way round.

Which means, I've realised, there are two pulses per revolution rather than one. So the smart new rev-counter reads double the speed it should.

I have, therefore, a number of options in the absence of any further information - the company vanished in about 2005, I think, and there's no sign of any details about their products anywhere I looked on the internet.

Option 1 is to revert to a mechanical rev-counter by buying a 4:1 from a reputable supplier rather than an eBay Indian business.

Option 2 is to contact a firm like Speedograph Richfield to see if they feel able to re-calibrate (which may not be possible, and will certainly be expensive).

Option 3 is to investigate the Elliot's insides and see if I can make sense of them without causing any damage.....

Finally, does anyone have any idea what that yellow wire might be for? It's showing a resistance of 750 ohms when put to an earth (-ve). No photo of any old Elliot instruments has a yellow wire, and it's not mentioned on the back of the case, so what on earth does it do?

All suggestions accepted with gratitude!

Pete.

[blethermaskite]

Pete, been there with this on my own 350 mk3 when I had the Motoplat ignition. in the end I was lucky enough to acquire a mechanical Smiths race rev counter (which is the same size as the stupid std rev counter) and was/is a perfectly functioning instrument for the job.
Cheers George

[Jordan]

I had an analog 3 wire type aftermarket tacho that had a switch on the back.
It was to select the number of cylinders in a car application.
I looked inside, and there were different value resistors for each selection.
So calibrating yours may be as easy as installing a correct resistor value, perhaps with a temporarily fitted variable resistor.
Your fourth wire may be for illumination.

[Boxprod]

Thanks for the suggestion, Jordan, about the yellow wire, but I'm afraid that this instrument is pre-LED and there's no hole for a bulb-holder, so it's definitely not for illumination. It's also connected somehow to the -ve (earth) side. Research needed.

On the matter of reconfiguring the signal to match the expected one pulse per two revolutions, I've dug around and finally found the following:
https://www.ebay.co.uk/itm/284440826701?mkevt=1&mkpid=0&emsid=e11400.m144671.l197928&mkcid=7&ch=osgood&euid=ff7507bdf55e467fa5b3b13fbd52b13c&bu=43093538218&exe=0&ext=0&osub=-1%7E1&crd=20251109033847&segname=11400
Looking at it carefully, it seems that even though it's originally intended for multi-cylinder applications, the chart of its switches shows that if you had an 8-cyl engine and swapped it for a 4-cyl, or a 12-cyl swapped for a 6-cyl, then you need to halve the number of pulses - and in both cases the switch pattern is the same.

s-l140.png

I reckon that should work for halving the number of pulses from 2 to 1. After all, the little black box doesn't know how many cylinders it's dealing with, just the number of pulses. Fingers crossed.

I also found a US version here: https://www.ebay.co.uk/itm/141979625888?chn=ps&_ul=GB&mkevt=1&mkcid=28&google_free_listing_action=view_item, which would do the job according to the maker's detailed description, but it's a bit bigger and buying from the USA these days can be a bit fraught.

But then I find that https://www.iconic4x4parts.co.uk/product/uiversal-rev-tachometer-rpm-converter-adapter-4-6-8-10-12-cylinders/144 may well have some stock of the first one here in the UK. I await their reply.

If it works, when it gets here, then all should be well. I'll let you know.

Pete.

[Boxprod]

A successful result!
The pulse converter came from the UK supplier, at the same cost as the one from Portugal, and it's quite small, so easily concealed.
Five dipswitches sealed in resin, but there's the familiar challenge of interpreting the "instructions". Being either brave or foolhardy, I decided to start with what I thought would be right, but it was ticking over at about 12,000rpm.
So I tried setting the dipswitches one at a time, and reckoned there wasn't much danger of frying either the converter or the instrument, undil I found the right combination. According to the "instructions", I'm converting a 12-cylinder to a 4-cylinder... All I know is that the old 350 single is ticking over at about 1,200rpm, and at 4,000rpm on the rev counter, the crankshaft is turning at 3,800-4,000 according to a cheapo friction tacho.
A happy afternoon's work.
Pete.

[Boxprod]

And a few months later, an update.
It turns out that the Elliot Design revcounter dates from the mid-90s, judging by the telephone number printed on the back - UK numbers had the initial 01 added in 1985.
According to a friend who really does understand electronic engineering (professional video cameras and the like), there's not a snowball's chance in hell of even the most robust circuitry still being in good order. At the very least, all the capacitors will be shot, meaning that however good the signal pulse, the instrument simply won't be accurate, and if the pulse is a bit erratic (50-year-old Motoplat ignition...) then the whole thing is a wasted effort.
The best known of the UK instrument repairers told me that they don't do motorcycle instruments, but they could possibly have a look at it, although they didn't think they would actually be prepared to work on it, so it was time to think again.
Back to a mechanical instrument, I decided.
Having tried an Indian knock-off - my advice: don't! - I furtled through the etherweb and discovered that Wassell sell/sold revcounters that look quite like the Smiths "big number" ones fitted to late-ish NVT-era Triumphs etc., and that they are made in Taiwan. In the US you can find them under the Emgo name.
Taiwan these days does pretty high-quality manufacturing, and I found one on fleabay for a happy discount - full price is about £65, so not too frightening.

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It's nicely made, and the needle is well damped, unlike the original 60s and 70s ones, and it looks good too. The ratio is right, at 4:1, and it fits very neatly in the suspension mount that came with the Elliot.
Fingers crossed,
Pete.

[Duccout]

Hi Pete,

Nothing is ever easy. Is that a bicycle Speedo that you are using? I have these on my bicycles, but have never considered using one on a motorcycle. Is it GPS or mechanical?

Cheers,

Colin

[Boxprod]

Hello Colin,
You're absolutely right about the bicycle speedo. It's one with a magnet on a front wheel spoke and the sensor mounted on the brake plate stay. All you have to do is run the cable up with the brake cable(s), then set the wheel circumference and Bob's your uncle. It's wonderfully accurate, and has other functions to play with, like odometers, a clock, total running time, and even maximum speed. Please don't ask!
I put one on my 50-year-old Ginetta G21 (Google it if you're a car enthusiast), and it worked a treat for 20 years or so. I've just sold the car, but the speedo was always better than the Smiths needle-waver.
All the best,
Pete.

[Duccout]

Thanks Pete, that's good to know.

Cheers,

Colin

[blethermaskite]

Hi Colin, I have been using bicycle speedometers on motorcycles for years, mainly because a number of my more obscure Italian bikes had unobtainable speedo drives or as ex racers never had speedo drives......initially all I needed was a speedometer for MOT purposes and the good old bargain basement 'Halfords' bike unit did the job fine (wire and magnet type) On my Aermacchi ex race bike (on the road) I used a second hand ( originally very expensive) GPS unit got of Ebay which was all singing and dancing with a multitude of functions (which I never used!). So they are legal to use, MOT acceptable loads of cheap ones available and pretty discreet (if needs be) too. Cheers George