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MG Midget and Sprite Technical - unsteady at the timing light

We are working on my father's 65,000 mile early run 1968 Sprite with 1275 engine. The smog stuff has all been removed. When we put a timing light on it there is at least a 5-7 degree random but consistent wander and it occurs at all RPM ranges. The engine has never been rebuilt to our knowledge. The distributor is an original points type and really does not feel to have excessive or abnormal shaft play. The advance is working. You can detect that the engine could run smoother at steady RPM. I am afraid it could be wear anywhere in the drive line for the distributor but I was wondering if there was a common problem that this story would flush out. Have at it guys!
BEW Brett Wright

I think, might be wrong, but a bit of wander is common including for all the reasons you've put plus quality and functioning of timing light

dissys were often out within a few years of leaving the factory so decades on are probably worse

to set up timing you first want to check/adjust tappets, CB points and plugs and possibly lubricate the CB points and dissy as required

65,000miles (if correct) suggests not enough use and too much laying up for the car to be anywhere near set up or reliable

I'd look at a full 36k-mile service/check up of the whole car then some regular reasonable distance use and you'll probably find things smooth out a lot and the parts that are truly worn beyond reasonable use
Nigel Atkins

Depending on the type of timing light you have, the symptoms you describe can sometimes be caused by an intermittently weak signal from the No.1 HT lead to the timing light's lead-clamp. Assuming the timing light is fully functional and the contact breaker is new and the clamp is working correctly, you might have hair-line cracks in the distributor cap, the rotor-arm or less than perfect HT leads.
Nick Nakorn

Im guessing the engine rpm is rising and falling over and over agian...if so, hmmm

I agree with nigel, a good complete servicing and recalabration is needed.

This could be a number of anything...my guess would be out of tune carbs and an ineffiacant fuel pump, as in the fuel pump points might need replacing and a fresh diaphragm (SU), or the high speed idle is set to high... but thats only a guess

If only you knew somebody that has a manual


Prop
Prop and the Blackhole Midget

This sort of timing scatter is very common in B series. 99% of the time it occurs due to wear in the gear on the camshaft which drives the distributor spindle, or wear on the spindle gear itself. The wear allows the spindle a bit of lash, and that affects the timing. I have seen this wear in A series engines as well, but not as frequently because unlike the B series the worm on the camshaft is not also driving the oil pump.

The scatter is also caused if a gasket is used between the boss which supports the distributor and the block. This boss also prevents axial travel of the spindle, so if a gasket is used then the excessive endfloat allows the spindle to move in and out along its axis. Because of the angle on the gear teeth this also causes the spindle to rotate a bit as it does so, affecting the ignition timing.

When either of these conditions is present, I have found that addressing (new or mint second hand camshaft or spindle, or removal of the gasket) instantly removes the scatter - and the associated loss of power.
Paul Walbran

Wow paul,

Thats deep !!!...any chance that gasket is made from weed....haha

Seriously, thats some good insight, thanks for sharing...it makes good scence, and something I doulbt id ever thought of.


Prop
Prop and the Blackhole Midget

Well, If those are your symptoms - then check out this site.

http://advanceddistributors.com/services07_1.htm

I sent them my distributor from my 1972 midget - and the rebuilt dizzy changed the car's behavior.

The timing light scatter issue plus any "drift" went away and the car still drives like a dream 5+ years later.

I was thinking of taking out the drive line, replacing the UJ-joints and other drastic measures.
You will NOT believe how a correctly set up distributor can make the Midget a much more enjoyable experience.

Bottom line? You can wiggle the distributor shaft all you want and convince yourself that the 45+ year old distributor is still in acceptable condition, but better to give this job to somebody who really knows their stuff!

Best of Luck !

Mike
Buffalo, NY
USA
Mike Pelone

To clarify, I wasn't saying that wear in the distributor doesn't cause scatter - it does - but that there are other causes that can simply be checked first and which rebuilding the distributor doesn't address (which many have found to their cost). The other spaect of distributor wear which affect performance greatly is in the springs which control advance rate. When these get out of spec performance suffers hugely, as the timing is right only at the speed you set it out and wrong, by as much as 10 deg or more, most other places.
Paul Walbran

I recognize the scatter problem. It all disappeared when I started using the 123 programable ignition. I did not rebuild the drive train (Is has 100.000 KM) Uneven running stationary is gone. It gave me even more torque and HP. I don't us vacuum advance anymore and the motor pics up al lower refs and cold. I think the 123 solves the scatter electronically.

Flip
Flip Brühl

Thanks to everyone for the comments and suggestions. Here is some more information:

We have removed the distributor, lubricated all of the vacuum and mechanical advance joints, replaced the o-ring seal to the block and there was not a gasket at the boss.

The RPM's are actually quite steady when we get the fluctuations on the strobe light so I don't think what we are seeing has anything to do with change in advance.

From my earlier description you might think the car has just come out of a long hibernation. In fact it has been used quite regularly over the past 10 years.

The carburetors are fully correctly adjusted. The spark plugs all read like the mixture is correct or close to correct. Certainly no fouling from running rich or burning oil.

The wandering happens whether or not the vacuum advance is connected or not.

The ignition wires are relatively new.

The valves are correctly adjusted

I am thinking the least cost approach would be to send the distributor for a rebuild. If that is not it, I sense it is likely wear at the cam drive gear
BEW Brett Wright

if you're thinking of sending the dissy away for a rebuild I'd also suggest replacing it with a fully electronic version like the 123

like Flip I have a 123 but it's the standard model and I do use the vacuum advance, the 123 gave improvement throughout the rev range from idle to top end

fit, set up and forget, no annual or twice a year checking or adjusting, just drive the car

whether a rebuilt or new electronic dissy fully solves your problem is a different matter but they will improve matters especially when the problem is resolved so worth the investment
Nigel Atkins

My major reluctance with a fully electronic distributor is that the tach will not work without an expensive conversion
BEW Brett Wright

I'm not sure if that's correct but Paul Hunt and others would know better than me
Nigel Atkins

Check the distributor drive spindle, it is easy to access and any wear between cam gear and that will show in it too.
Paul Walbran

Yes you need an electronic tach. So no more replacements of the reduction box and you can sell your mechanical dynamo for more than a tach.

http://www.ebay.co.uk/itm/MG-Midget-Austin-Healey-Sprite-Smiths-Negative-Earth-Tachometer-/171487658947?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item27ed7603c3

current bid 26 pound
Flip Brühl

The car already has an electronic tach. Nisonger (US authorized Smith Gauge service) says that it would be need to be converted in some way to work with electronic ignition correctly.
BEW Brett Wright

Will my tachometer (RPM meter) work with a 123ignition distributor?
Yes, by far most of the tachometers work in combination with a 123ignition.
RVI type tachometers manufacturedby the Smiths Company, will not work properly though.

We have found two companies that offer solutions to this problem. They use the existingrev.counter to replace the electronics with another printed circuit board.
In the UK this is done by JDO-instruments ( www.jdo1.com ), and from the Netherlands
this is done by CASU-Utrecht ( www.casu.nl )
Flip Brühl

for info, my RVI Tacho works just fine with my K series, using both the original Rover ECU and an aftermarket Emerald ECU.
Rob Armstrong

As does ours.
The current loop one works well too.
Paul Walbran

Or just use a pertronix conversion and the RVI taco will work fine.
dominic clancy

This thread was discussed between 03/10/2014 and 15/10/2014

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