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MG Midget and Sprite Technical - Strobescope fault

I have a Snap-On timing light, model MT 1261.
I haven't used it for several years now but its been safely in its case and worked fine on its last outing. Not so now!

All connected up the digital display works for the tachometer and the timing advance / retard can be dialed in as required. But the stobe light is erratic, not flashing uniformly. It gives out a couple of flashes, pauses a while, then maybe a single flash followed by a brief burst of flashes. Non sufficiently stable to give time to get a reading.

I've double checked the connections and gone through all the steps in the instruction booklet troubleshooting advice. Waggle tested the cables. No improvement!

Suggestions?
GuyW

Guy I saw this and thought 'capacitor' immediately

Follow up on this site https://www.electronicspoint.com/forums/threads/snap-on-mt2261-timing-light.71984/ indicates a similar fault on a later version of Snap-On timing light
He suggests that he replaced a 0.033uF HV ceramic cap connected to the low
voltage winding of the trigger transformer.

It might be worth your while getting hold of you can of the wiring diagram for yours the way he did.

I am having similar but different problems with my Lumix Camera, its flash has packed up and it seems likely I am having capacitor problems too..

I might take the camera apart to look and see if the capacitors have sprung leaks
Bill sdgpM

Make sure you discharge the caps before doing any work on them or check for voltage, the flash ones can give quite a jolt DAMHIK.
David Billington

Ah yes, awfully good advice (as ever David)

I have some similar experience to recall and a memory of being thrown across a reasonably sized room as a boy.
Bill sdgpM

Copper leads?
f pollock

I had a problem like this and it turned out there was a break in the wiring from the sender just before it entered the gun itself.
Bob Beaumont

Thanks, yes Bill, I wondered about it being a capacitor problem. That used to be the universal answer!
Fergus - copper leads. Good call. That is mentioned in the manual so I had already tried swapping out #1 lead for a silicone one.

Bob, that fault is mentioned as a weak point a couple of times on the web. I tried wagggling the cables but made no changes either way. Not conclusive so that need more inspection. I may just shorten them anyway to be sure.

I am using this on the positive earth frog and wondered if that might be an issue? I don't see why it should as I am still observing polarity for the gun. But I do have another battery so should I try powering it independently?
GuyW

The other thing I need to do is try it on the other car. But it needs to stop raining for that test!

The inductive coupler is very loose on my 7mm leads. Does that matter? The ferrite bar that closes around the wire is making good contact but the hole looks like it was designed for 15mm tractor cables
GuyW

I've no experience with the Xenon strobes, I still have a neon timing light (!) which can only be used in the dark. If I had to guess, I bet Bill is on the right lines but one needs a circuit really or general familiarity with such circuits. But trying it on the other car first makes sense although, as you surmise, negatve earth shouldn't make any difference.
Bill Bretherton

I saw a discussion about these strobes guns earlier and it mentioned the thin leads failing being such a common problem the Snap-on vans kept replacement lead set to swap into the guns.

Bill,

I borrowed one of those neon strobes once and found it useless even in a dark garage.
David Billington

I had a very similar problem with my strobe light . When I tried it on lead 2,3 etc it had a consistent flash . I changed the spark plug in cylinder 1 and it then worked fine . Don’t ask me how that would affect it
Mike Fairclough

Well, for now it seems to be behaving itself. Sort of.
I opened it up and poked at various connections and ribbon cables, checking they were seated properly. I discharged the capacitor and then checked its impedance which was 6.140k. Not sure what this means other than I guess it hasn't broken down internally.

I then redid the leads, sacrificing about 4" off each. After that I tried it out on the 1275 car and it worked, flashing more or less continuously, though probably in good burst followed by a momentary hiccup every second or three. Back to the frog and I am getting pretty much the same. Not 100% but certainly regular enough and for long enough to be useable.

Different question now - The white mark on the pully appears as a short white streak under the strobe, rather than a dot. Is that caused by wear in the dizzy or is that related to the length of the wiper arm on the dizzy rotor am as it whizzes around?
GuyW

Guy

The positive earth on my Frog makes no difference to the strobe. It has the odd hiccup but works well enough to set the timing.

I have to say mine appears as a white dot rather than a streak but it is a recently rebuilt dizzy by Aldon.
Bob Beaumont

Its a fairly short streak, maybe 4mm or so. I'm not greatly concerned
GuyW

Guy,the streak rather than dot must surely be due to distributor shaft wear and/or the advance jumping about a little causing points to open at slightly different times. As you say it isn't much.

David, it's a while since I used the neon strobe but I timed several cars with it. It has to be dark.
Bill Bretherton

Maybe, Bill.
But what would be the effect of the length of the brass contact arm on the rotor arm. They all seem much the same length these days but in the past I have come across some probably 50% longer than the norm. Would a longer arm sweeping past the dizzy cap terminals generate a longer duration spark?
GuyW

Guy,

That's just to deal with the advance variation so a part of the brass contact is next to the cap contacts, the point of initiation of the spark is what's sensed by the strobe induction sensor and what triggers the strobe. Longer arm sweep won't effect spark duration. The scatter you see is likely dizzy play, I expect you can find examples of the fault shown online.
David Billington

Actually I think David has explained it right. I do know that at any given compression the rate of flame spread through the combustion chamber is constant, irrespective of revs. At least that is what Vizard says. I suspect duration of the spark is irrelevant - from the moment the spark occurs, the flame spread is initiated and how long the spark continues for beyond that moment then doesn't matter. The flame is already on its way! Except I guess a long duration spark is going to eventually burn the plug electrode out faster or maybe make the plug run hotter.

Anyway, my white streak is fairly short and steady. I would be more concerned if the white dot was jumping around more, which it isn't.
GuyW

I think you are more likely to get a steady 'dot' reading with electronic ignition which is unaffected by spindle wear, and by using a chain tensioner which gets rid of snatch and grab present on the chain and timing gears. Be interesting th see what happens to the streak as you rev the engine.
f pollock

Having got the strobe to work, I will ser the timing today. Its a 1098 and is set at 8 degs advance at only 600 rpm, so a slow tickover. When thats done I will check how the streak behaves as I rev it. At the moment its not bothering me as it's pretty minimal and only on a standard road engine. I want to get it running right as it is before I consider any 'upgrades', if any.
GuyW

On my little portable light the inductive pickup has a direction arrow pointing at a sparkplug
On a positive earth car I have to have it right up near the sparkplug end of the lead with the arrow pointing away from the plug towards the dist for it to work but with my Bosch light which also has a direction arrow, it works either way round

Your scatter is probably a worn dist. drivegear

willy
William Revit

A photo from what Willy writes. You can easaly check if the drive is worn.

Flip

Flip Brühl

Thanks Flip, I will check, but the white line shows as a steady mark, not jumping around as I would expect if there was excessive lash in the drive shaft. The line is also pretty short so at this stage and unless it gets worse, I don't think it is a thing that I am going to worry over.
GuyW

Back to this as although I thought my strobe was fixed, it isn't!
It seemed to have settled and was flashing steadily as it should so today I had another go at setting the timing. What I now discover is that it flashes consistently at revs above around 1800rpm. Below that it becomes intermittent and at the speed that I need to set the timing to do it "by the book" (600rpm) the flashing stops entirely.
Does this give any clues?
GuyW

Guy, I'd like to help but don't know enough about the device. It sounds like a triggering problem but seems odd this is happening at lower revs as I'd expect more problems at higher revs. Have you tried moving the pickup around? (I presume it is inductive).
Bill Bretherton

If you have really good silicone insulated leads the signal mightn't be getting through to the inductive pickup
maybe try with a less performance rated lead in no.1 for timing purpouses
William Revit

Hi Willy. I could try that, though I am not sure that I have any other poor quality leads. A quick rummage will tell😁

Earlier, I changed the #1 lead from the solid copper that I have used, to a silicone cable, but maybe its too good. I could try it again on #4 which is still the copper sort.

Is a coil generated spark less at lower revs? It seems odd that this works at higher revs but is erratic or not at all at lower revs. I would have expected a fault to show up the other way around!

There's a large capacitor probably worth changing. They are very cheap. But I also wonder if the coil around the ferrite core in the inductive pickup might be breaking down? Its just that would surely give an all or nothing fault?

Ahh! Ping!!! I have just had another thought. I am powering this off the car battery. If voltage is low on the battery but picks up as the regulator switches on to a full charging supply from the dynamo that could explain a revs related fault. The strobe is rated to operate on a min 10V supply so its about possible. The display read out still works at low revs but that maybe more tolerant of a low voltage to operate.
GuyW

The HT voltage will be a bit lower at tickover on a dynamo car, maybe 8 to 10% but the strobe shouldn't be that sensitive i.e. it should easily trigger for a normal range of HT voltages. Either the inductive coupling is insufficient or the trigger circuit is underperforming imv. Capacitors often deteriorate over time so maybe worth changing it (for one with at least the same voltage rating).
Bill Bretherton

Bill, That last thought was that the power feed to the device might be the problem if it drops down towards 10V at low revs before the dynamo kicks in. Rather than the impulse from the inductive sensor being too low to fire the strobe.

But it could still be that capacitor if its charging, but leaking. At low revs could it have time to leak its charge before the next flash, but not have time at higher revs?
GuyW

Hard to say without seeing the circuit but changing it wouldn't do any harm. There'll be more than one capacitor, each having a different purpose. 10v would be low for a charged battery surely? Should be about 12.6v.

I've also read on a forum that the Snap on light was known for pickup leads going faulty and, although you said you've shortened them, I wonder if that's still an issue i.e. if it's partially arcing within the leads. A slightly higher HT voltage might be enough to make it work.
Bill Bretherton

Bill, it's not a new battery, just an old one that I kept as a spare for testing circuits etc. It spins the car OK to start so won't be much below 12v at first, but wouldn't hold its charge reliably enough for everyday use. After repeated starts in the garage it will likely have dropped further from a full 12.6v and I then rather assume there is another net drop once the car is running, but before the dynamo switches in via its regulator. Whether all this would take it below the specified 10v is what I need to investigate.

I am prepared to consider anything though and I am trying to then eliminate at least some of the possibilities.
GuyW

Why not adjust the timing at full advance and let the idle be what it will be? Better for power and torque and better if your distributor has any wear. Tippex or chalk a white mark at 30 degrees and have your assistant run the engine to full advance - probably around 4000 rpm.
Or will the mark become too blurry? At 600 rpm I doubt the bob weights would be moving anyway.
f pollock

I could do that Fergus, but I only have the official manual settings for static and 600rpm. That is done with the vacuum tubes blocked off and, as you say, with zero dynamic advance. Would the 30 degs include total with the vacuum and weights advance? I've never done it that way before.

Actually, at the moment it's not running right at all. There's no way it would run up to 4k anyway! It's popping and banging and not picking up at all so I do need to get the basics right first. I have set tappets. Checked compression, which is giving 155 across all 4. It's a standard 1098 with what I believe is an unmodified head so although this is low compared to my other car, I think it is probably ok.
New plugs, points, condenser, rotor, HT cables, and dizzy cap. Carbs are new, jets set 12 flats down but not yet balanced as I need to get it to run a bit more smoothly to do that.
GuyW

Since it's banging and popping have you checked to see if you've got the wires from the dizzy going to the correct cylinders?
Martin

Yes, Martin. Good call and that was an early thought, it was so bad! But they are correct, 1342, going anti-clockwise.

Only thing not new on the ignition side is the coil. Unknown vintage. Only test I can think of is the impedance across the secondary which is showing 3.95 on my digital meter, set on the scale marked 200, so how do I read that?

As car is positive earth I connected the dizzy LV wire to the coil +ve. That's correct isn't it?
GuyW

Yes, the LV wire from the dizzy goes to +ve or as my wiring diagrams call it, CB. (I checked on mine which is negative ground and it's attached to the -ve side and the later wiring diagrams agree with my car.)

As far as checking the coil reading, I have no clue since electrickery scares me.
Martin

Guy
If you measured the coil on the 200 ohm scale (probably the lowest on a cheaper DMM) then your reading sounds like 3.95 ohms which is about right for the primary winding (two outer terminals). The secondary should be several thousand ohms.
Bill Bretherton

Ah good.
Apart from muddling my primary and secondary windings!
'0' level physics was an awfully long time ago 😂
GuyW

Sounds like stale petrol. In scorchio summer temperatures petrol can go off in as little as 3 weeks. Might be worth cleaning the plugs,emptying the bowls and hanging a fresh supply straight to the carbs.
f pollock

The plot thickens!
Today I set the strobe up on the other car, and of course it works. Flash is still a bit erratic with gaps in its routine, but certainly enough to set the timing by. But equivalent timing on that engine spec is given for 1200 rpm, rather than the 600rpm given for the 1098 engine. The higher speed does help stabilise the flashing of the strobe. Other differences are the 1275 is neg earth and has electronic (MG Metro) ignition so its not a directly comparable test.

My current guess is that the strobe works, but has a dying capacitor, making operation marginal. The higher revs of the 1275 and possibly the electronic dizzy just helps enough to get it to work, whilst it doesn't achieve that on the the 1098 engine. I could probably succeed by following Fergus' advice and set it up for full advance but first I need to get it to run well enough to rev to 3500, which is well beyond it at present!
GuyW

Guy,
For initial dissy setting I usually just rotate the dizzy until the engine revs as high as it wants to! and then back off a tad - this has always given a starting point prior to using the guns.

In true BBS traditionI must say try a different rotor arm ! and check points gap again.

I know the cap SHOULD be OK !!! I hope so !

I assume you set the carb openings X screws each before locking the shaft.

Sorry if these are all obvious but when its all going downhill its easy to over look DAHIKT.
richard b

All good advice Richard. I think I did all of that, but there is no harm in treble checking!

I have always set static timing using a radio, tuned off channel. Rotate engine to the specified advance mark. Turn on ignition and then twist the dizzy back and forth listening for the interference click on the radio.

For the moment, it's all ground to a halt as the battery is on the charger again. It's an old and undersized Honda battery I have been using to check the various loom electrics and isn't going to be any good long term. I think it's about time I bought myself yet another birthday present.
GuyW

This thread was discussed between 25/08/2020 and 09/09/2020

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