Morris Minor Owners Club

So the symptoms are:

1. when the engine is warm and idling it (seemingly) randomly misses a beat. This has been the case ever since I've had the car and means that the idle speed has to be set at around 800-900rpm to avoid it occasionally stalling; and
2. when not under load the engine will also miss a beat at higher revs.

I now want to try to sort it.

It's a standard 1098 with an HS2 carb ('AN' jet) and a fairly new 45D dizzy with points (installed because the 25D's vacuum failed).

I have:

1. set the carb (13 flats worked out best);
2. set the dizzy points (about 0.012" gave ~60 degrees dwell);
3. carried out static timing (can't take it out for a run as there's a huge scaffold outside my garage door!!);
4. swapped the standard old DLB101 Lucas coil for a spare DLB101 (both show 3.2 ohms when tested with a meter);
5. replaced the NGK plugs with a set of Champion ones set with 0.025" gaps;
6. replaced the plug leads with new ones;

also:

1. the pump ticks happily and appears to be delivering sufficient fuel (engine revs freely);
2. the carb float bowl is pretty clean;
3. the fuel feed pipe is sound and not leaking;
4. the compression is 155psi all cylinders;
5. oil pressure at idle is around 40psi (according to my capillary gauge);
6. battery voltage is fine and dynamo is charging;

However, I still can't get rid of the random miss-fire and now I've run out of ideas. Help....please!

Can only suggest a loose/poor connection somewhere in the ignition system.

Try running the engine in the dark and look for stray sparks, you may have weak insulation somewhere or arcing from HT leads.
Check that HT leads are tight in the dizzy, coil and spark plug caps.

Hi

We down here in Oz were recently following the story on one of our web forums of a Minor (albeit with a Datsun motor) that was exhibiting this type of misbehaviour. The owner had spent months and a considerable sum of money changing engine ancillaries and having an auto technician work on it. Eventually, he took it to a different, clever auto technician who quickly tracked down the problem to a lack of an earth strap to the motor. Obviously, a strap making a poor connection would have the same effect. The motor can function to some degree with earthing provided by accelerator, choke or speedo cables, but they won't be adequate for good and reliable running.

A search on this messageboard revealed a few similar cases over the years.

Just a thought.

Cheers
Don

Thanks to all for all the suggestions.

Responses as follows:

loose/poor connection somewhere in the ignition system.

The problem does seem to suggest an intermittent connection somewhere but if there is then one I haven't found it; In particular I have checked all the spade connectors that I can find for continuity and firmness of connection.

Try running the engine in the dark and look for stray sparks....Check that HT leads are tight in the dizzy, coil and spark plug caps.

No visible sparks and connections seem tight. I've tried two sets of leads now.

...lack of an earth strap...a strap making a poor connection...

Earth strap is in place and resistance between block and battery is <0.5ohms. Just to make sure I placed a jump lead between the block and battery negative but it made no difference.

By the way the (replacement) coil seems to get pretty hot after even a fairly short time. Almost too hot to touch. I know they get warm but is that normal? (...and I know "it's 'never' the coil" BMC )

Any more suggestions please?

silloyd wrote: By the way the (replacement) coil seems to get pretty hot after even a fairly short time. Almost too hot to touch. I know they get warm but is that normal? (...and I know "it's 'never' the coil" BMC )

When I was trying to diagnose a problem I left the ignition on without the engine running for 10 mins and after that the coil was almost too hot to touch. My problem was the rotor arm breaking down when hot, the coil was fine.

Although the dizzy is relatively new I've now tried changing the rotor arm and cap as well - sadly no improvement.

Still searching for a solution and I spotted the exhaust blowing slightly around the joint with the manifold.

(Underside of clamp looking up towards manifold)
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(Top side of clamp)[frame][/frame]

There are no signs of blowing at the back of the clamp, top or bottom.

Would this be enough to upset my idle?

I doubt it's the exhaust connection - but won't do any harm to remake it. Coil will always get hot if left with Ign on, and points closed. But try the original coil (because it's never etc) and see if it still 'misses'.

BMC,

I replaced the original coil and the hiccups continue. An interesting quirk is that when connected to my multimeter the original coil over-reads RPM (double!?!?), whereas the 'spare' doesn't. By the way, the coil was getting hot with the engine running rather than just left switched on, don't know if that makes any difference(?)

To continue the process of elimination I have also

1. replaced the condenser; and
2. put a temporary direct feed from battery +ve to the coil, i.e. by passing the ignition wiring;
   
   ...with no resultant improvement.

I'm sure there is a really simple (but immensely annoying) cause/solution somewhere ...I hope.

Hi

Have you checked the condition of the little cloth covered earth braid within the dizzy and also the LT wire within the dizzy from the external connector to the points. These can sometimes be fractured within the braiding and connecting intermittently as the advance operates.

Taupe

The dizzy is pretty new and the earth inside looks to be OK. I've tried shiggling it (technical term ) and changing its position slightly, no change. The LT lead is permanently attached to the condenser on a 45D and therefore got changed when I replaced the condenser. I've also tried bypassing the short (white with black stripe) wire between the coil and the dizzy.

Mine, huccups occasionally, especially when warm.... but I've never considered it an issue as the missing sounds like a random quiet tap of a base drum. It doesn't detract from the driving experience one bit.

One thing a trip to the Morris Minor centre taught me, was the points they have in stock don't fit the early dizzy's very well. There is a tiny gap between the points and the plate they are mean't to sit on. They are not aware of any problems caused by it, but it may be best to check yours is sat correctly (incase this is what happens).

Simply chamfering one of the holes would sit the points down flat.

The coil gets warm - it's consuming ~ 50 watts, running or not. Try another rotor arm perhaps?

Have you checked/reset the valve rocker clearance?

BMC - I've tried two rotor arms, the latest being brand new red one from Accu-Spark, and I don't have another spare I'm afraid.

Katy - I haven't rechecked the rockers, may get a chance to do that tomorrow, though I wouldn't have expected an imbalance to cause a seemingly random miss, but rather a consistent problem with the affected cylinder/

Thanks for the continued suggestions, keep 'em coming

Is the carb set up correctly? Sometimes if the mixture is set either slightly too rich or slightly too lean it can cause the engine to appear to miss a slight beat. I call it fluffiness in the exhaust tone, but is noticeable more from the rear of the car.

'katy's idea is possible - a valve may be sticking open slightly from time to time.

Dean - Mixture could be off but I don't think so. However, in the spirit of double checking everything I will check the mixture again but will have to be a static method as I can't take XOG out on the road at the moment.

BMC (and Katy) - Fair point. I wasn't thinking of that, just the gaps themselves. I will check the clearances tomorrow but would that help a sticking valve, or would I need to so something more radical like remove the head? Is there anything I can do to clean the valve guides(?) without removing the head?

Not really.... But set inlets to 12thou and exhausts to 15 thou - and inspect the springs for broken coils. Unless the whole lot is a nasty mess of crud - no need to pop the head - unless of course it's bone dry with no oil reaching the rockers ?