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Tachometer tribulation

Posted: Tue Feb 11, 2020 9:36 pm
by Paul_Linden
I bought a digital tachometer and fitted it to the Square Four to get an idea of engine speeds as I run it in. However, it’s out by 4,000 RPM. The reason why, I believe, is that the tacho is for a single cylinder engine, and as it runs off the feed to the ignition coil on a four cylinder engine it reads too high - a fast tick over shows as nearly 4,000 RPM when obviously it’s more like 1,000.
Is there an easy way to fit a tacho to a square four? Maybe the easy way is just to renumber the dial! :D
Here’s a picture of the bike, the tacho is on the handlebar by the mirror. I plan to change the tank badges.
3D2ACD09-A5BC-404E-BB01-EC3590CC31F1.png

Re: Tachometer tribulation

Posted: Wed Feb 12, 2020 5:25 am
by Ian.Taylor
I tried 3 tachs that run off the ignition wire and not one read anything close to the actual rpm on my 53 VHA, VF31 or my 36 ES2..Idle would be close as it reads about 700 rpm ,once underway forget it.
I even wired it up to my 77 Suzuki GT 750 and it was also way off.
Conclusion..waste of money.

Re: Tachometer tribulation

Posted: Wed Feb 12, 2020 4:23 pm
by allan.walker
I have three of these-

https://www.ebay.co.uk/itm/Inductive-Ho ... 2906129245?

One is fitted to a Rotax-engined Armstrong, one on a Bullet and the other on a Square Four.

They run on a button cell and are triggered by wrapping a flylead around one of the spark plug leads. They are adjustable for no. of cylinders, 2 or 4 stroke etc.

All show the same reading when fitted to the same bike and seem to be accurate. (I tried one on my Commando, which has a standard tacho which I know over-reads. Using a GPS speed check against rpm, the Norton tacho reads 4200 rpm at 60 mph, when the gearing on the bike suggests it should be showing 3800. When I put the Ebay tacho on the Norton, it showed 3800 at 60 mph.)

Not 'classic' but it's cheap enough and does the job.

Re: Tachometer tribulation

Posted: Wed Feb 12, 2020 4:30 pm
by Paul_Linden
Thanks Allan, that’s what I need. Kind of wish I hadn’t wired up the other one now.

Re: Tachometer tribulation

Posted: Thu Feb 13, 2020 5:03 am
by Ian.Taylor
Allan I had 2 of those . The seller sent another when I told him it was defective..The replacement doesn't read accurately either.

Re: Tachometer tribulation

Posted: Thu Feb 13, 2020 8:50 am
by allan.walker
They are Chinese after all!
Seriously, though, there are bound to be dodgy ones. At least when buying through Ebay you can usually get the problem sorted.

Re: Tachometer tribulation

Posted: Tue Feb 18, 2020 3:39 pm
by Jake slaminko
I have a long ago modified coupling gear cover to mount a smiths tach drive to be driven off the end of the cam if you have any interest, also have the drive and tach. Would need an extension pressed into the end of the cam to mate with the drive.

Re: Tachometer tribulation

Posted: Tue Feb 18, 2020 8:41 pm
by Paul_Linden
That’s interesting Jake, I had thought about doing something similar but from the end of the dynamo, and through the timing cover to a tacho drive, but this would show a reduced engine speed. Did you get it all working? Was it easy to install? Why did you stop using it, oil leaks? Do you have any pics? Sorry about all the questions! :-)

Re: Tachometer tribulation

Posted: Tue Feb 18, 2020 11:29 pm
by Jake slaminko
It came with some parts I bought years ago, I’ve never had it on a bike. I was going to put it on but decided I didn’t want to deal with the hassle. It would need an extension machined for the end of the cam and something to cover the back of the top half of the drive to keep oil in and it wasn’t worth it to me. Looks like it wouldn’t be too difficult too make work though.

Re: Tachometer tribulation

Posted: Wed Feb 19, 2020 8:12 am
by pappleton
282137944284 2013bestbuybest (ebay)
I bought this one last year. It's an inductive sealed unit, under a fiver, seems pretty acurate on my single, can be reconfigured for twins or fours.
BR,
Paul