Technical Fitting a tachometer

[lazylobster]

One or both of your parameters will be wrong. The standard 126 final drive does have a better ratio than the 499 engine and gearbox but I'm not sure about the 594. In any case you would not be getting such a low RPM reading.

Your speedo will be reading wrong; mine was displaying an optimistic speed with a standard engine and box, an exact reading when I fitted the 126 setup and it is now reading about 10% slow with a 9:39 crownwheel and pinion.

I did soem calculations a while ago; they are probably wrong but I think that the principle is correct.

https://www.fiatforum.com/500-classic/463651-top-speed-increase-126-bis-pinion.html?p=4364748

Thanks Peter. I've got a GPS speedo on my phone so will take some measurements this coming weekend. See what the score is.

Cheers
R

 

[lazylobster]

Apologies Rob, forgot to answer your other question---a good condition 594 engine should be able to rev to 6,000rpm. If you take it much higher there is a chance that the fan will shed blades---NOT recommended!

Hi and thanks Tom. OK, I'm now starting to wonder if my new electronic tach is not 100% as the engine does not seem to rev much past 4K. But I'm taking the tach feed from the neg of the coil so ??????

Next question, apart from a tach setup, how can I measure the RPM of the engine?

Cheers
Rob

 

[The Whitakers]

Hi and thanks Tom. OK, I'm now starting to wonder if my new electronic tach is not 100% as the engine does not seem to rev much past 4K. But I'm taking the tach feed from the neg of the coil so ??????

Next question, apart from a tach setup, how can I measure the RPM of the engine?

Cheers
Rob

My experience of fitting aftermarket Tach's is that you need to select the amount of cylinders your engine is......I bought a couple of cheap ones to use as set up tach's when working on my aircooled dubs but they had provisions to be used on 2 to 12 cylinder engines but no way of setting how many cylinders you were using them on?! like you state my engines only revved to around 4k regardless of how much you squeezed the throttle...




they are in the back of the garage gathering dust and used as shelf fillers....

 

[the hobbler]

According to my 'Haynes', the 594 (Mk1) 126 has a final drive ratio of 4.875 (8/39). In the 'Supplement' section for the 652cc (Mk2) version, it gives no details for the gearbox or diff, which can only lead one to believe that the gearbox/diff details for two models are the same. Just forthe recordthe 500s diff ratio is 8/41

 

[the hobbler]

The best way to ascertain the accuracy of your speedo vis-a-vie the tachometer reading is to put it onto a rolling road. A workshop with diagnostic machinery (like a 'SUN' or 'Crypton') might be able to check the accuracy of the tacho.

 

[rizzo360]

Hi Guys,
Anyone have a wiring diagram on how to connect a tacho to a 126 engine using a 123 ign dissy . The tacho I intend to use has 8/6/4 cylinder option.



Cheers

 

[Bigvtwin996]

.......Next question, apart from a tach setup, how can I measure the RPM of the engine?

Cheers
Rob

there are non contact tachometers... they are quite cheap on fleabay...
not too bad, obviously prices vary....
you can use them to measure rpm of all sorts....
you have to attach some reflective strip to the rotating part....

(Or you could buy a good automotive Multimeter with Dwell and RPM)


I'm sure someone will explain the use of a Dwell meter and RPM..
or if you have a good multi meter with Hz you can use that....
(Old Skool eh!!! )

 

[lazylobster]

Hi Guys,
Anyone have a wiring diagram on how to connect a tacho to a 126 engine using a 123 ign dissy . The tacho I intend to use has 8/6/4 cylinder option.

Cheers

Hi, you'll need a convertor to take your 2 cylinder engine and double the signal output for 4 cylinder option on your tach. Otherwise, your tach will under report.


My tach takes the feed from the negative of the coil.

R

 

[lazylobster]

there are non contact tachometers... they are quite cheap on fleabay...
not too bad, obviously prices vary....
you can use them to measure rpm of all sorts....
you have to attach some reflective strip to the rotating part....

(Or you could buy a good automotive Multimeter with Dwell and RPM)


I'm sure someone will explain the use of a Dwell meter and RPM..
or if you have a good multi meter with Hz you can use that....
(Old Skool eh!!! )

Great, thanks for the reponse and advice. Will look into.

 

[rizzo360]

Thanks for your input guys. I have a big face tacho which has oil pressure and oil temp gauges all in one, was looking at using it on my 500 hill climber.
So how do I wire it up to my ignition. What converter do I use.


Cheers

 

[lazylobster]

Hi for my digital tach that expects 4 cylinder input, i’ve installed as per the link attached. No affiliation.

https://www.ebay.co.uk/itm/UNIVERSA...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649

Regards
Rob