Morris Minor Owners Club

Hello all,

My poor Moggie doesn't sound very happy. It appears to be only firing on three cylinders. With the car running, I can remove each HT lead in turn. This makes the engine run worse for each plug except number two. This same plug is also the muckiest where the others look in good condition.

As I understand it, the low tension circuit acts on all four spark plugs evenly, as does the coil. This leads me to suspect that either the distributor cap, HT lead, spark plug or cylinder compression is at fault. At a push, I suppose that it could also be the rotor arm, although this seems unlikely. Could the timing be out? Would that cause just one plug to miss?

I have tried replacing the distributor cap and HT leads to no avail. However, the replacements were bought when I had a similar problem a couple of years ago. We never got to the bottom of that one as we eventually gave it to our Minor specialist who ended up replacing everything again and adjusting the tuning, etc. The probability of having two distributor caps with the same problem (on the same plug) isn't tiny but it does seem unlikely. When I had the problem before, the compressions were all checked and were fine.

So what to try next? Will it do any harm if I remove the plug, connect it to the HT lead, earth it and run the engine to see if the spark is good? The spark plugs are fairly new but I could swap the plugs around and see if anything changes? Anybody else had similar issues related to parts - cap, leads, etc?

As always, all suggestions and diagnostic tips gratefully received.

When you pull the plug lead on number 2 is there a spark?

If there is I suggest you check the compression on that cylinder, and the tappets.

It's probably just oiled that plug - try another one !

take the plug out and refit the pluglead not the ht lead and clamp it (The metal head) to the engine block dosen't need to be mega tightly clamped turn the engine over and see if there is a good spark at the plug.Alternatively simply fit a new plug and see. From there the next stage is a compression test.and report back

Thanks for the suggestions.

Roy, I didn't mention it, but I had cleaned up all the spark plugs.

MarkyB, I pulled each lead in turn from the spark plug and I could see a good clean spark on all plugs. I also tried Kennatt's trick of running the car with the number two spark plug out and, yes, I could see a good spark.

With all the plugs in and the engine running, I'm still sure that the running gets worse when I pull off each of the leads, except for number two.

I guess that the next step is a compression test. Something for which I'm not adequately equipped. Maybe a trip to my local, tame garage is in order.

MarkyB, you mentioned the tappets. Is there something I should check there? Alternatively could it be something related to either timing or irregular firing of the low tension circuit?

Any more ideas or suggestions?

You should indeed set the valve gaps. Cleaning the plugs doesn't always work - try some others. A compression tester is not expensive - buy one !

You can get a good idea of the compression just by putting your thumb over the plug hole while turning the engine over on the starter.
Take all the plugs out first and disconnect the small wire to the distributor to avoid getting any HT shocks.

It's only a comparative test but should reveal if there is a discrepancy between the cylinders.

With the plugs in turn the engine over on the starting handle, you will feel if you have got a "soft" cylinder

Thanks for the compression tips, people. Hopefully I find some time tomorrow to try them out (and to try using new spark plugs, just in case they are knackered).

Also ordered some bits, such as a new condenser and the LT lead from the dizzy case to the points - mine is looking a bit "thin". Could cause the problem, I suppose. Also got a compression tester on order.

Money and time, is all it takes

i have complete sets for £7.50 with postage
check the for sales

So after a manic week, running around after various family members, I finally got some time to have another look and, just in time, my compression gauge turned up yesterday.

First of all, I checked all the valve clearances. All were good except number three, which of course if for cylinder two - the one with the problem. A glimmer of hope I thought. The hex nut was very tight and so it clearly had not slipped. What concerned me was that Haynes says that if the clearance is too small, there is a real danger of overheating and damaging the valve. The clearance on number three was indeed too small.

Anyway, got it adjusted, put it all back together and started her up. Luck wasn't with me - it still sounded bad and now there was a lot of smoke out the back. So I let her gently warm up and tested the compressions - 155, 30, 140, 145.

I've obviously found the problem. So, anybody want to tell me how bad this is? I'm guessing it's a big job.

I think that difference could have been detected with the thumb method!
The head will need to come off for sure, a manual would be handy and you'll need a head gasket set.

How tight was the tight tappet, was there any gap at all?

The chances are it's the exhaust valve has burned, it may be savable but it'll need to come out first to see.

To put it back together you'll need a torque wrench or the loan of one.

Thanks MarkyB. That's exactly what I thought when I saw the numbers - "that difference could have been detected with the thumb method"

There was certainly no chance at all of even getting the feeler gauge part way in, but I must admit I didn't look to see if there was any gap. I do have a torque wrench and two or three manuals including the workshop manual. Is it a big job? Are there any difficulties or gotchas?

Hopefully it is just a burned exhaust valve - you will need a new one - perhaps consider changing all 4 ? This is why i suggest setting 15 thou on exhausts - and check/reset every 3000 miles. And of course - grind in ALL the valves.

Is it possible the head gasket gone inbetween the cylinder and a nearby oilway? As far as I can tell that would explain the lack of compression and the smoke, presuming of course that its not a burnt valvep

Could be - but unfortunately I feel it's more likely to be piston/ring failure........ Let's hope not!

Such pessimism! There is every likelihood that it's a burnt out valve.

It's an easy enough job, undo the nuts that hold the manifold on then tie it out of the way of the studs, this saves disturbing the joint to the exhaust and saves undoing the cables, unless you have to in order to get clearance.

Don't yank the push rods out or the followers may come with them, a twist and a gentle lift should ensure that doesn't happen.

Take pictures as you go, and post what you find here.

Oh! don't forget the bypass hose that goes from the head to the water pump, that's a good gotcha! all the nuts are off but the head won't lift!

Is have thought head gasket is

more optimistic than burned valves! Less work and he's gotta take the head off anyway.

Unfortunately smoke with low compression and oiled plug suggest a more serious problem. But it's always hard to tell from a distance! Rings and piston problem would have heavy 'breathing' from the oil filler etc.. We haven't been told about that. So - fingers crossed!

well if it is piston rings it'l be damn odvious when the head comes off and the bore looks like a peice of modern art! i suspect your probably right, but its such an unpleasent thaught im inclined to go with a more ositive prognosis till the heads taken off ;)

ifd it is coming off, then i suggest doing as was advised ot me and leaving the manifolds on the head and undoing the exhuast at the manifol clamp... made life much easier.