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MG Midget and Sprite Technical - Sudden death

After a 70 mile high way drive, partly on top speed (85 mph), the motor suddenly stopped while parking. (parking is dangerous with this car. Last time I was stopped down the road was also while parking; the axle shaft snapped after about 200.000 miles). During this drive there was something I never encountered; backfire if I took the right foot from the throttle.

It is a more or less standard 948cc, > 75.000 miles after rebuild, without a problem, 123 ignition, Maniflow LCB, yes I can hear the double chain better than when it was new.
There is a spark, the spark plug colour is ok (light brown) the fuel level in the float chambers is normal, the valve train is moving.
What will I control next?

Flip
Flip Brühl

Compression check. Could you have blown a hole in a piston?
Mike Howlett

has the distributor moved, or if you have points has the heel of the points worn away or has the condenser failed
Dominic Clancy

Compression on number 1 and 3 is 10 bar. No points no condenser: an expensive programable 123 ignition.
Flip Brühl

Flip
Backfire = Lean burning? Does your engine have the fuel it needs? fuel pump? Lines?
Just some brain storming......
Jan Kruber

Backfire = air leak in manifold causing weak mixture on overrun. Check the manifold gasket is still intact, especially in the underside edge?
GuyW

Whilst checking your dissy hasn't moved I'd also have a look at your rotor arm and dissy cap just in case they're scarred.

If you have a more or less standard engine why do you need a programmable 123, are you running it at a setting too fine for everyday use.
Nigel Atkins

Thank you so far for your suggestions. Nigel, I have a 123 because the "modern E 95" fuel is another thing than the RON 100 fuel in 1959. Peter Burgess programmed it on his rolling road for E 95. I also drove in France and Eastern Europe where E 98 is hard to get as I have only 7 gallons in the fuel tank. One more thing is that I could not get the right dissy springs, the quality of "original" condensor, points and rotor arms was so that I did not trust them. (a black rotor arm once stopped me dawn. I did not know about the red ones those years)
I23 is Bosch, German and brown, not made in PRC. (People Republic of China)
I drive 4000 miles every year and after 150.0000 miles the original dissy needed a rebuild.
I am going to check the rotor arm now. Guy, tomorrow I 'll check the gasket.

Flip
Flip Brühl

Flip,
sorry I forgot you wouldn't be running on UK fuel!

I'm all for fully electronic dissy, I have had a standard 123 for nearly 10 years, it has also been set up by Peter, for 95 octane fuel but I normally run on 99 for the cleaning additive package in with it.

My 123 is set at bog standard setting even though my engine is modestly "tuned".

My cap particularly, gets scarred at the posts so I clean the rotor and cap at least once a year and have replaced them at intervals, hence my suggestion to check them also.

I use Beru or Bosch for rotor arm and have Intermotor cap. I have a couple of the less expensive "red rotors" (not the Dissy Doc ones) and they look fine but don't have the weight or sturdiness of the Bosch or Beru.

If you have a look in the Archives you'll see I put up lists of equivalent part numbers for the 123 Bosch 1 235 522 332 (right angle) dissy caps and Bosch 1 234 332 024 rotors.
Nigel Atkins

The dissy was spontanously timed 180 degrees different from original. I suppose that I did not press the 123 deep enough in the distributor spindle. Can the bearing of the spindle sink as the bearing wears? Anyway the firing order now is different and the motor goes faultless.

Flip
Flip Brühl

It was not the dissy deep enough, there is a lot of play between the distributor spindle and the camshaft about 15! degrees.

Flip


Flip Brühl

What do you want to show in the picture?
J.W. Vlaanderen

The order of the spark plugs I think
G Lazarus

Yes Garry is right. Is this a well known problem? What do i have to renew?

Flip
Flip Brühl

Rotating the distributor will not change the firing order, it just changes which lead needs to get connected to which plug in order for the timing to be correct and the engine to run.
David Billington

Correct David, what happened inside the motor, why changed the the order of connection of the leads spontaneously? I had a lot of scatter in the ignition timing at low RPM for years. There was a rattle, I thougt that was the timing chain.
Flip Brühl

Flip,

Difficult to see how the dizzy could be inserted 180 degrees out as I thought all the A series dizzy had offset drive dogs to prevent incorrect insertion into the drive spindle. Is all present and correct on the dizzy drive end and is the drive spindle in good order, IIRC a 5/16" UNF long thread can be screwed into the drive spindle to aid removal.
David Billington

Hi Flip,
please excuse my nosing in but just a thought, is the 15 degree play actually what we might call "backlash" caused by the centrifugal ignition advance system in the distributor shaft IF the 123 dizzy still uses one?
Your earlier comment hints that it may not.

Cheers, John
j c macleod

David, I ordered a long 5/16 bolt to see and feel if the spindle and/or the bearing is gone. I suppose so, because the valves are timed right, it is unlikely that the chain causes the stop.

John, there is no centrifugal advance, it is all electronic. You can program the 123 advance in Windows with an USB cable.
Flip Brühl

Flip, one can remove one of the front shock absorber mounting bolts and use that to remove the lower half of the dizzy drive. Don't disturb the other mounting bolts and I wouldn't drive anywhere like that, but temporarily borrowing it won't do any harm. Then just replace it when done.

I haven't really followed why you think the drive needs removing. You will just upset the timing!
GuyW

Front shock mounting bolts are 3/8" UNF IIRC.
David Billington

Pretty certain those bolts are what I used, and more than once!
GuyW

I think you may be wrong Guy. I use a rocker shaft stud ( the UNF not the UNC end) and that is smaller than the shock absorber bolt.
Bob Beaumont

Memory is off then. But I distinctly remember removing the shock absorber bolt to do that job as a long one is needed. At the same time I used a helicoil in the mounting plate as one of the tapped holes was worn.
It was the drive from an A+ engine but I doubt that would make a difference.
Maybe I removed the bolt, found it didn't fit, and used something different.
GuyW

Guy, item 71 here - https://www.ukmgparts.com/catalogue/mid-midcat-7-submid19-front-suspension

I can't remember what it's like to have a bad memory.


Nigel Atkins

Not sure what you thought you were reminding me of, Nigel. I have no doubts about which bolt I was referring to.
GuyW

They list a different shocker bolt for the 1500 and 1275 and the 1500 one is more than double the price. Is there actually any difference?
David Billington

The size Guy - if you click on item 71 you get - BOLT 3/8 INCH UNF X 4.0 INCH.
Nigel Atkins

That's the one. I guess I 'll have to buy another one and a new camshaft. Of cause I 'll have to make an insert for the distributor spindle bearing in the block.


Flip Brühl

After consulting the archives I decided to see if I could replace the distributor drive. With my flexible scope (Lidl €45) I could see that the camshaft was hardly worn. I had another distributor spindle somewhere in another garage where an old dirty and incomplete 948 waited 21 years for dismantling. That spindle was not worn al all. (Why the difference?) . The bearing end was 0,0I0 mm thicker. I have replaced the spindle: No more scatter, no backfire, even idle, no rattle! I am going to change oil, filter and later the camshaft.



Flip Brühl

I’ve never seen wear like that on a dissy drive gear.
Dave O'Neill 2

Having looked at the photo again, on a proper screen, the replacement also seems quite worn.
Dave O'Neill 2

This thread was discussed between 05/04/2019 and 16/04/2019

MG Midget and Sprite Technical index

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