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MG Midget and Sprite Technical - Rev counter with electronic ignition

Hi

My 1098 1965 Sprite Mk III has been fitted with electronic ignition. The rev counter reads about 2X the actual revs. E.g it reads 2000 revs at idle.

I'm tempted to go back to traditional points ignition.

Will that sort out the rev counter?

Thanks!
B M Le Page

Was the rev counter working properly before converting to electronic ignition?
Bill Bretherton

The problem may be linked to the electronic ignition or it could be failing components in the rev counter, if it’s the latter I can test, repair and calibrate them. I don’t do this as business, but I have repaired a reasonable number for friends.
R.A Davis

Early electric tachos exhibit more frequent and sometimes considerable calibration drift. I understand it was due to the use of germanium rather than silicon as semiconductors.
I have heard of electronic ignition confusing tachos but never directly encountered one. I have found quite a number where this was alleged and found not so.
But most electronic conversions I have come across have coexisted perfectly with the tacho. This could vary with electronic unit type, the ones we usually encounter are Lumenition/magetronic and Pertronics, and we have never had actachbisdue with them.
The RVI tacho units have no direct electrical connection to the signal circuit, as it works off induction. The RVC tachos of later cars have their signal is taken from the voltage at the distributor side of the coil, so there would be more potential for interference.

Paul Walbran

I used a Pertronics positive earth system with an RVI tacho and found that reducing the induction loops from 2 to 1 fixed my overreading. That worked fine for about 10 years till it packed up, and then I had John Ostick install all new internals.
f pollock

Assuming that the electronic unit is taking its ignition switched feed from the coil, you could try temporarily connecting it to the white circuit at the fuse box instead, just as a test. See if that makes a difference.

Dave O'Neill 2

I run an RVC with an Aldon Ignitor: no problems in sixteen years on my Sebring Coupe. You could get a s/h unit from Andy Jennings breakers.
Alan
Alan Anstead

If the tach is orginal I presume it is an RVI, not RVC. The RVI will not work properly with some electronic ignition systems. Also it will have ageing components which "drift" in value, especially the capacitors. But we still do not know if it worked properly before fitting electronic ignition. If it didn't then it seems R.A. Davis has made the OP an offer he can't refuse!
Bill Bretherton

Hi All these expert comments have given me a lot to think about. First the electronic ignition was fitted by the PO who included the original mechanical one in the sale.

Now I know what to look for its an RV1. And it has the white wire loop on the back.

So with this info and all the advice above I'm going to try this approach unless someone stops me for being daft.

Step 1 try original mechanical ignition
Step 2 if that doesn't work try reducing the wire loop by 1 (although from the photo will I be left with 0 loops?)
Step 3 try Dave's diagnostic wiring change (but I'm a bit nervous about doing that because I'm not that familiar with the terminology and would need more of a walkthrough)
Step 4 if that doesn't work I should talk to RA about diagnosing the instrument itself.

Does that sound like a plan?

As always hugely grateful
Brendon






B M Le Page

On the 1971 1275, it was necessary to lower the needle when the electronic ignition was installed using the set screw on the rear of the tachometer. Use an external tach and adjust accordingly. Mine read about 150 RPM higher with the points changed out for an electronic ignition.

If the transistorized tach is exposed to severe exterior heat, like when the car is parked in the sun on a 40°C day, then it may read very high until things cool down.
Glenn Mallory

Brendan As a fan of mechanical ignition its certainly worth doing. Just a couple of comments:
The aftermarket items like condensers and points are of variable quality. If the ones you have are old or tired then get replacements from the Distributor Doctor or NOS. Try and avoid the green boxed Lucas stuff.
Bob Beaumont

Brendon, yes, that's a plan although I doubt you'll get beyond stage 1 as that will solve the problem probably. Alternatively, you could get an RVC tach which should work with electronic ignition.
Bill Bretherton

Great thanks. MGs are for tinkering so I like the mechanical points too. Something more to play with. :-) I've got some new points and what is apparently a better quality rotor arm. So now I will have to adjust the timing as well I guess! I will hope Bill is right and I wont have to go past step 1! I'll make the changes and report back.
B M Le Page

Brendon,
the cars are for driving not fiddling around with - but if you're into that sort of thing then many modern made CB points, capacitors (condensers), rotor arms and some dissy caps will give you hours of fiddling around. Be very careful what parts you buy and where from.

Oh, and very cheap spark plugs and HT lead sets - apart from that the ignition side is fine, well perhaps some ignition switches - you could be in fiddlers heaven.

Each to their own thing but for me, service, maintenance and repairs yes (reluctantly), fiddling definitely to be avoided, you've no need to go out of your way looking for it it will find you.

Of course, the more you drive it the more you'll need to fiddle with it (well at first, drive it enough the fiddling will diminish).

What's the electronic ignition, there's a few different sorts, all should give better overall starting, idle, through the rev range, better economy and performance by being more constant - but a lot less fiddling.

As long as you're enjoying yourself then fill your boots (not quite sure what that means).
Nigel Atkins

This is what I wrote for the Masc magazine a couple of times because of ignition woes. I also send it to participants on my tours. If only some would take note.
Dry joints and a lack of spark can affect many of us, of a certain age, over the winter months but can also affect our Sprites and midgets at any time of the year. I have broadcast this information before but the problems still arise and may not be known by newer members.
Firms such as Aldon Automotive and Distributor Doctor are able to provide better quality alternatives to the commonly available reproduction rotor arms which routinely fail and leave people stranded.
Electronic ignition can be fitted concealed in the distributor and replacing the contact breaker and unreliable modern condensers but the rotor arm, depending upon which sort of electronic ignition is used will still remain. My own choice has been the Aldon / pertronix ‘Hall effect’ system that has proven reliable over fifteen years and many miles at home and abroad.
Some rotor arms manufactured in recent years have been failing because the typical ‘mix’ used in the injection moulding process now contains more carbon blacking than of old and is more conductive to electricity. It has been found that the rivet which holds the brass contact inlay into the moulding, on reproduction rotors, is slightly longer than the original bringing it too close to the spring clip on the underside. The High Tension (HT) current, averaging 30,000 volts, is always looking for the easiest route to earth and shorts out, from the tip of the over length rivet, through the reduced thickness of the more conductive plastic and the spring clip on the underside of the rotor arm to earth out down the distributor shaft. This results in no spark at the sparking plug and a ‘failure to proceed’ from your Sprite or Midget. The system sometimes rectifies itself on cooling but then repeats itself with increasing frequency until the rotor permanently short circuits.
Distributor Doctor offers ‘Lucas red rotor arms’ which are modelled on the original 1960s design but made from modern materials on the latest machinery to very close tolerances and have achieved a good reputation for reliability. The integrally moulded brass inlay dispenses with the troublesome rivet. The increased insulation properties of the body makes them suitable for use with high voltage ignition coils and electronic ignition kits. The spring clip on the underside, in conjunction with a revised internal tolerance assures a really snug fit upon the distributor spindle even on old and slightly reduced diameter mountings.
Even with the uprated rotor arms it is worth being vigilant and occasionally checking the security of the brass contact.
When fitting a rotor arm consider applying some petroleum jelly to the distributor drive shaft rotor arm seating to eliminate moisture as I have been called upon to work on a car where the distributor drive seat rusted such that it grew so much in size that it cracked the rotor arm that had to be chiselled off. I also tend to use petroleum jelly on most electrical joints to stop them being effected by damp and corrosion.
Part number for Sprite / Midget 25D distributors is DRB101
Part number for alternative 45D type distributors is DRB104
Distributor Doctor can be found at www.distributordoctor.com and Aldon Automotive at www.aldonauto.co.uk.
Electronic ignition systems are easily fitted to Sprite and Midgets and are available for both Positive and Negative earth configurations fitting inside a vehicle’s distributor with no outward sign of conversion. It is easy to carry a spare distributor base plate set up with points and condenser in case of the rare, but not unknown, instance of the electronic system failing at the roadside. Why not also consider carrying a spare coil? I have used a Lucas DLB105 “gold’ coil on both of my Sprites for years, for they are relatively cheaply purchased at the car shows and can be affixed to a suitable bolt in the engine compartment, there being no need to clutter the boot, and be easily accessible if they are ever needed.
Alan Anstead

Hi Nigel I definitely agree with you about driving. My son who owns half the car and I spent an hour or so driving around Kent today for no particular reason other than to be out in the nice weather and having the best experience of the countryside you can get from a car. We would have stopped at a pub in normal times. However as a pensioner with four day weekends its tempting to try to neaten something or try to make idling a bit less lumpy etc. when the weather is bad. (And after every tinker you need a test drive 😄)

Alan that's brilliant. I'd like to cut and paste it in my collection of really useful stuff if that's ok. On the back of that I might wait to buy a full set of points, condenser, rotor, coil, leads and plugs which will play nicely together from a specialist supplier.

The car has only done a legitimate 30 000 miles so I think there won't much wear on moving parts like distributors.

B M Le Page

Brendon
We have a run on 29th June if you want to exercise your car.
alan dot anstead @ btopenworld dot com.
Alan
www.masckent.org





Alan Anstead

Brendon,
you certainly have the right idea, if you must touch the car then you must reward yourself with a drive after. A stop at a decent country pub keeping its ale well is a convenient comfort stop.

I'm afraid you're wrong about the dissy not wearing on so few miles, the dissy were out of wack within a few years of the car leaving the factory and generally time and lack of maintenance on them makes sure they're not as accurate as they could be. The top of the dissy has been dealt with, with hopefully a good quality igniter head(?) so no need to carry any spares unless it wasn't fitted correctly. This still leaves the bottom of the dissy perhaps needing attention, perhaps just a clean and lube or parts replaced.

I didn't hesitate to as soon as I could replace the whole dissy with a fully electronic dissy, electronic bottom as well as electronic stop. They're reliable so no need to carry all these spares. If you've got an iffy component or part get it repaired or replaced don't carry spares for the 'fun' of roadside repairs. CSI or 123 ignition fully electronic dissys are fit and forget, have the comfort of the pub not rainwater down your back as you play what some think of as a roadside warrior.

30k-miles has benefits and detriments, while the lack of mileage and use can help it can also hinder, lot of wrinkles to be ironed out before consistent good running. I always suggest with a classic new to you to carry out a staggered 36k-mile check and service whilst regardly using the car on reasonable distance runs and not to make any unnecessary improvements or cosmetics until 12 months of year round regular use.

Follow (more or less) the service schedule in the relevant Driver's Handbook, which will also tell you how and maintenance and how to properly operate all the controls. Reading, referring to and following the Driver's Handbook will mean you'll know more about the car than many owners, some very long term.

Good luck, enjoy.
Nigel Atkins

Thanks Nigel.
Happily, I've been doing pretty much what you suggested as it happens - making sure its safe and runs well to start with. So new shoes and pads, and callipers refurbished with new pistons and seals and flexible hoses. Gone round all the grease points and normal service items, changed diff oil because it was dirty black, renewed brake fluid because it was gungy, petrol filter to stop the frogs in the petrol tank. Topped up gearbox which was half empty but clean. front shocks bouncy - need replacing on the exchange basis. The front wings are rusted beyond repair but the PO threw in some really clean front wings. The indicators are the wrong height for the year, but this car ain't going to be concourse.

You are right, there is an ever growing to do list even for a 31K car, but for example I refreshed the whole clutch hydraulics and now the gear box is quiet and gearchanges are smooth as silk. My intention is to drive as much as possible this summer, doing a few things each week.

I will take your advice about giving the distributor a more thorough inspection. But I'm going to go for mechanical points. (ducks head).

I definitely take your point about the 'fun' of roadside repairs. Cars only break down when its raining.
B M Le Page

When I put at roadside repairs water running down the back I was being polite. :)

Really good to see you have the priorities correct with brake first, many starting with the least important components like engine and carbs (for fiddling with). Bear in mind the tyres count into brakes (and steering, suspension, road olding, ride comfort and noise) so regardless of tread depth left if the tyres are old and or/little used changing them can transform the car.

Another word of caution, replacement dampers often range from piss-poor to abysmal, having been "reconditioned" badly perhaps, many times over, and modern made new ones don't seem to have a good reputation either.

If your dampers are original, and not previous more recent "reconditioned" ones then have a look in the Archives here about how you my be able to revive them yourself.

Otherwise best replacements are from Peter Caldwell, World Wide Auto Parts - http://www.nosimport.com/british-cars-nos-parts/British-bmc-austin-mg-morris/ah-sprite-mg-midget-parts/mg-midget-front-shock-rebuilt-better-than-new-by-world-wide-auto-parts-nosimport

Or you could try Stevson Motors over here(not sure if the jury is fully in on these) - http://www.stevsonmotors.co.uk/Dampers.html

It's a lot quicker to name good quality modern parts - Hardi petrol pump, er, 123-ignition, er CSI, er, there are others just not coming to mind at the moment, whereas if you want a list of piss-poor parts . . . :)

A photo of your car or Vehicle Profile to view would be nice - 3.5" steel road wheels and twin-rod spokes steering wheel, floor dip and other stuff I've forgot.

Nigel Atkins

Hiya,
Get your Rev counter to Bob Davis, who is basically a Magician!!
I have an RVI gauge that was crap , Bob worked his ( VERY reasonable priced !!) magic and it is absolutely fine now.
Can't recommend him enough .
Cheers colin
colin frowen

Hi Colin thanks for the tip
B M Le Page

Hi all

I've just got back to the rev counter issue after having to attend to other more urgent things on the car. I did step 1 today swapping out the electronic points with the mechanical ones. The car actually ran ok (!) but rev counter still read 2X actual speed. So I put the electronic ignition back. I'm going to work down the list of suggestions and if all else fails I'll take the car + rev counter to the suggested experts.i still have a nagging feeling it's got something to do with wiring and take off as the wiring has been messed about with the the PO's rather than the rev counter itself.

I'll report back on progress.
B M Le Page

Hi this is addressed to Nigel.

Re your shock absorber advice I did what you suggested and refurbished them myself. Lots of dirt and required several rinses. I rinsed with the 20W fork oil I was going to use. Cleaned all valve parts in an ultrasonic cleaner fitted a new gasket and spent a happy few hours chasing bubbles but they work really well and no double bounces. Car drives better too of course. 2 shocks done for around £15.

Invaluable advice thank you.
B M Le Page

Brendon,
follow my hyperlinks rather than my advice!

On another thread I put links (to others instructions or advice) on how to do the oil change, as you found it takes quite a bit to get the air out and arms fully working.

I do remember that it is important to have an air gap above the oil so check you have that air gap on both dampers.

What I do often put is that much of the servicing, maintenance and repairs on these cars can often boil down to clean and lube, as here it seems so far, and as here the clean and lube has to be done properly, which is often just thoroughly (but sometimes is also in a particular manner or order or technique).

I also think it's very worthwhile having all electric wires and connections from battery posts to end item and earths clean, secure and protected so it's worth checking and cleaning all and making good as required.

Doing this even if it doesn't uncover the issue at hand I bet it finds other issues or helps to prevent them or at the very least cleans or checks so you know they're good.
Nigel Atkins

So I've got to step 3 reducing the number of turns on the induction coil from 2 to 1. I found the instructions for a this on the web as well. On removing the rev counter I found the cable from the ignition switch looked like the attached. Melted, wires exposed and the insulation on the 2 turns fused. Ouch. so I replaced the cable and tried 1 turn but no difference. I guess the overheated cable fried the rev counter internals.So I have to turn to the experts and ask Mr R A Davis to take a look. I'm not sure how to contact him other than hoping he reads this thread.

(I'm updating the thread for interest and to show I'm following up the suggestions and the process I've followed and eventually the solution. If it's boring tell me to stop)




B M Le Page

Brendon
The white wire will not damage the tach. It is not electrically connected but forms an inductive loop which acts as one winding of a small transformer, the other winding being the input to the tach circuit. It is probably the case that one or more internal components need changing, then it needs calibrating and the meter movement (that drives the needle) needs checking. The fact it is basically working is good news.
Bill Bretherton

Brendon, if you want to drop me an email to eppingmg at gmail dot com we can discuss it offline.

Bob Davis
R.A Davis

Brendon,
with hindsight that wire looks rough in your original photo. How far can you chase the white wire back to check it and it's connections. I'd check all the white whites and their connections and switches just in case.
Nigel Atkins

Hi all

Bill - of course I was being dumb because the wire is just an induction loop. Smh. Pretty poor for a physics grad (1981!) I really hope you are right and the internals can be sorted.

Bob - email follows

Nigel the white ignition - rev counter- distributor has good continuity. I completely take your point about refreshing all electrical contacts. At the moment I'm still trying to get the few remaining things to work and improving things as I go. I solved the broken headlight problem. It wasn't too hard to diagnose - see pic. But the wing was so rusty I had to fit the sound wing first so the light wouldn't fall out.

Now that it works I've discovered the high beam cuts out when the right hand indicator is on. I know bad connections can lead to completely random faults. But this stuff is mostly due to botched wiring by DPO who reckoned that twisting two pieces of wire together and taping it up is a 'connection '. Each little victory leads to a new issue.



B M Le Page

Time for the pic. :)



Nigel Atkins

Brendon
If the tach is giving a reading it is working. In case you are interested in its operation this is as follows: Basically the white wire has a current pulse (from the coil switching current) which is electromagnetically induced in the secondary winding of the tach circuit. The induced voltage pulses are used to switch the circuit on and off rapidly (using one or two transistors) and, in effect, the pulses or "frequency" are converted to a variable voltage, the higher the frequency (proportional to engine speed) the higher the voltage. This variable voltage is connected to a voltmeter which is what indicates rpm. Yours is working but is reading high probably because a capacitor or two have "drifted" in value with age, maybe a resistor as well. They can be replaced and the device re-calibrated. I assume Bob has made you an offer you can't refuse and might elaborate on my brief operation description!

Check the earth connections from the front lights btw.
Bill Bretherton

Can anyone confirm Bob Davis' email address please, as I'm getting
The following recipient(s) cannot be reached:
'eppingmg@gmail.com'
David Smith

David, that is the correct email address, I typed it that way in an attempt to stop spambots picking it up. It’s the email address for the Epping area section of the MGOC and you can find it about halfway down the first page of our website. I use this email address as I don’t like advertising my personal address on an open BB. http://www.eppingmgers.co.uk/

Bob
R.A Davis

Bill thanks for the run down I am always interested to add new knowledge to what I know. I remembered electromagnetic induction only works with a continuously changing current and I was wondering how that worked so thanks. Its inspired me to look up how the speedo works - I bet there is a hall effect in there somewhere 😄

I've just posted the pic to show the quality of wiring I'm dealing with. 🙄


B M Le Page

Brendon,

Speedos of this period work like an eddy current brake as do early tachos like on the frogeye, no electronics. The odometer is mechanically geared directly to the speedo cable but the speedo needle is magnetically coupled to the cable. The speedo cable spins a magnetic bar inside an aluminium can and that induces eddy currents in the aluminium which drag the can around against the hair spring, the faster the magnet spins the more the drag and so the more the needle deflects. https://en.wikipedia.org/wiki/Eddy_current_brake
David Billington

Horrendous but I suppose at least it soldered..........!
Bob Beaumont

Brendon,
based on that photo I'd suggest you also, if you've not already, have a look a the column switch contact points and clean if required and check the wires and their contacts to switching.

Although it's earths and crudded bullet connections that are are favourites for many issues, go easy with existing original bullet connectors as the modern made ones are not as well made. Often just clean, secure and protect with resolve and/or improve matters.

If you've not already got it you can download at least a couple of different pdf copies of the Smiths book 'Care of the Instruments' for the general ideas.
Nigel Atkins

Bob,
thanks; yes I understand about making email addys less obvious to bots.
What I'm saying is - that email address doesn't work (for me).
I've tried it several times over a few days now and every time I get the following message back:
'The following recipient(s) cannot be reached:

'eppingmg etcetera' on 02/08/20 16:56
550 spam, or phishing or malware. Please check or edit your message and try sending it again. (6-1-3-2) ID (5ED9AA6E09D31EA1)'

Perhaps you could try emailing me -
midget1380mackerel atcarp btsharkinternet dotcodcom
remove the fish!
David Smith

David,

It may be that something somewhere thinks you are sending spam, perhaps because certain words in your email that are being misconstrued by a spam filter.

If you go to https://emailcopychecker.com/ (there are other sites available as well) you can copy and paste your text into a window and the system will tell you if there are any 'naughty' words. It has worked for me in the past, and you'd be surprised at what words cause problems sometimes.

Good luck!
Jon
Jonathan Severn

many thanks Jonathan, first time I've ever come across this issue, you live and learn.
It seems it didn't like a video hotlink I'd included as after deletion it seems to have sent OK.
David Smith

Glad it worked. Email spam filters can be a real pain in the wotsit sometimes!
Jonathan Severn

Hi again

David you exposed some ignorance ☺ on my part and when you mentioned the eddy currents I remembered a fun demo from teaching science decades ago when you drop a piece of aluminium (non magnetic) through a magnetic field and it stops in mid-air.

Rev counter posted to Bob today.

Nigel the column switches and steering column are from a later model 🙄 as the diameter of the splines is different(searching archives shows the expletive thing was changed four times over the life of the marque). And the column switches include bright dip as well. The indicator switch wouldn't stay on so I replaced the assembly with one I had lying around.I read this too late re the bullet connectors - I had already replaced them all with (properly soldered) shiny new ones.

This car is such a mongrel. But I went out for a long drive yesterday (to test the indicator switch 😉) and it runs so beautifully and quietly I'm happy with what I bought. It will need a lot of refurbishment/replacement on everything except the whole drive train which is super but at least I can drive it in between times.

I am changing Murphy's Law.

"If something can be bodged it will"

Brendon

B M Le Page

Oh and that reminds me - I always cut off the end of the cable tie when I use it to join wires. Smh.

And should I start a general thread called "a Sprite called Bodge" or something because this is starting to drift from its original question- some forums don't like that. I'm enjoying it thoroughly though.
B M Le Page

Brendon,
just about all but the very rarest few classics are mongrels whether some owners know it or want to admit it, not even the rust is original on mine.

I prefer crimped to soldered connectors but as long as they're good it doesn't matter, it's more the bullet connectors (slit tube joiners, covered over with loose sleeve) that are poor quality (well the ones I've got anyway) than the actual modern bullets.

Spend more time driving your car than getting it fully 'right'.

If items work and are safe put them on the to-do or rountoit lists as using the car, as they really need to be used, regularly on reasonable length journeys (at least 15-20 miles) will throw up things that need ironing out that will take priority over the lists.

After about 5-10 years you may think that you have the time, opportunity and willingness to tackle some of the jobs on the roundtoit list but then realise they've continued to work and not bothered you for so longer they don't really need doing.

As long as the car is safe, and reliable, I always recommend reading the relevant Driver's Handbook and carrying out a staged 36k-mile service as soon as you can but in between driving and enjoying the car and learning how to drive it and what it is/could/should be capable of.

Below the cover of my (reprinted) Driver's Handbook, the ones that were with the car for the owners when the cars were new.

I had to look up bright dip, I thought you meant headlight flasher. :)



Nigel Atkins

Cut the cable-tie, a man of high standards indeed.

Thread drift is rife on here, I'm one of the biggest culprits, as long as the issue of the thread has been or gets resolved does it matter.

I think it's best to start new threads for each issue or enquiry though as it'll probably attract the attention it requires and keep the threads more concise.

With a bit of practice you can consult the Archives for information but it can also be a way to loose a lot of time by being distracted, or finding some longer and meandering threads.

Always worth putting up a new fresh live thread though, if only to confirm what you've found in the Archives or elsewhere as information, parts, opinions and even facts can change with time.


Nigel Atkins

Brendon,

It's quite easily demonstrated by dropping a magnet, particularly a modern neodymium super magnet, down a bit of copper tubing like 15mm plumbing stuff as it takes much longer to drop through compared to a drop in free air. Even understanding what's going on it seems weird sometimes but an effect put to all sort of useful purposes.
David Billington

Aluminium cans are sorted using eddy currents at waste recycling centers.
L Langley

Hi

While I wait for Bob to breathe magic dust on the rev counter can anyone tell me the mph per 1000 revs in each gear?

Even Horler doesn't say and I'm not even certain of the diff ratio - I assume 3.9:1

1965 Sprite 1098.

I've been searching everywhere!

(The South African Car Magazine does proper technical road tests- a page of graphs with power curves in each gear, kph per 1000 revs, speedo accuracy using a fifth wheel, standardised fuel economy at different speeds etc.

Not just 'there are two cup holders in the front and one in the rear' stuff.
(although probably this sort of thing is what many people base their choice on.)

We also used the'bright/dip' nomenclature

Brendon
The moany ex-pat of 30 years

Interesting about the aluminium cans - I just assumed they used magnetism but that only separates steel from non-steel of course.
B M Le Page

From the Specail tuning booklet:

MPH/1000rpm = 60000 divided by axle ratio x wheel revs per mile This is for direct top but speeds in the indirect gears can be calculated by dividing this by the gearbox gear ratio.

the wheel revs per mile @ 30 mph for a 145x13x80 tyre is 934
so for a 3.9 diff and 145x 13 radial is 3.9 x 934 = 3642.6. divide this into 60000 gives 16.472 @ 1000 rpm

for a 3.7 ratio diff its 17.36 mph per 1000 rev and for a 4.22 diff its 15.22 per 1000 revs
Bob Beaumont

Brendon the '65 1098 Sprite would originally have had a 4.22 diff - any reason for thinking it now has a 3.9?
David Smith

Mine is a 65 1098 with 4.22 and does 15mph /1000 so almost spot on with what Bob quoted.

Trev
T Mason

Hi David the ratio was just a guess - my previous 1275 was 3.9:1 but I forgot the 1098 has less power so thanks for putting me right.

Bob thanks so much for the detailed explanation of the calculation. Thank goodness you gave some examples 😄

I completely forgot about the tyre size I'll check mine tomorrow I hope they are 14s. Then I can be sure it's close to 15.22. (Unless I over-inflate the tyres😉
I've heard of people doing this to improve fuel consumption)

Brendon
B M Le Page

ETA: I think you meant 13" tyres (145 width) and yes a higher inflation is used to lessen rolling resistance, even on moderns or a sports setting on 90s Jags but you do have to do it within limits so as to not over-effect handling and road holding or cause extra wear or damage to tyres, suspension and your fillings.

Round numbers for 4.2 diff with 145/80 r13 tyres -

1st - 5 mph

2nd - 8 mph

3rd - 11 mph

4th - 15 mph
Nigel Atkins

Brendon,
if you put up your 'Vehicle profile' for view then it saves you repeating some details of your car and could cut down confusion - but only for those that bother to click the link of course.

As you know from Terry Horler's book even within a year, or model year, there were lots of small changes but this doesn't allow for the changes made by owners in the decades since the car left the factory of course.

BTW I allowed 3% minus for rolling circumference but it disappears in the round figures as would higher sensible :) inflation of the tyre. :)
Nigel Atkins

Hi Nigel Sorry I misread - 13" tyres. Great to have figures in every gear thanks. And at 15mph/1000 revs in top it's easy to get all the useful figures. The engine seems quite flexible and seems happy at 25mph on the flat in top. Compared with my 900cc silly Citroen where I have to stir the gearbox all the time, because top in the Sprite is a bit low geared I seem to be able to drive comfortably between 30 and 65 (which is my chosen top speed) in top which is very satisfying - except for real hills. If I've got the wrong idea please tell me!

At your prompting I filled in the profile but didnt think to add the link!

B M Le Page

Hi guys

Just to close off the thread I got the rev counter back from Bob Davis today and fitted it. It works perfectly, looks new and is as accurate as they can be.

Great tip and a successful outcome. And I've learned so much from the contributions.

Brendon
B M Le Page

Brendon

If your wiring is really shot you can buy the correct wiring harness for the car from Autospiarks: https://www.autosparks.co.uk/finder

Nigel's comma net about getting the car safe and useable (I.e. Making safe dodgy bodges and living with the acceptable bodges) and using it it sound and having a long list of round to it jobs for sometime later. You will be fixing and replacing other stuff first (like rubbish rubber parts as well as new things that crop up as ten car gets used and settles down).

My car is previous owner bodge job too and finally nearly on the road after many years. I am getting to the stage of what can I live with - otherwise it will never be driven - before being tempted to redo jobs I did previously or 'fixing' acceptable bodges of stuff that is safe and works.

Cheers
Mike

M Wood

This thread was discussed between 18/06/2020 and 14/08/2020

MG Midget and Sprite Technical index

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