Diagnostic query (clutch?)

[tomti]

Hi all,

On the way home recently, our Minor developed a new and slightly alarming quirk. It's hard to describe but is kind of like a juddering or spluttering, although both of those imply a noise of some kind and I actually mean in the delivery of power. As I pull away in first, all is well (usually) but when I change up to second, even at quite low speeds, the car starts to struggle a bit and fail to pull off with quite its usual gusto. This struggling/juddering/whatever is worse when climbing a hill and is present when I'm trundling along normally but not to anything like the same degree. I live in a fairly residential bit of London so I'm not getting up to any long stretches of stable speed.

I've replaced the points (and gapped them to 15 thou), condenser and spark plugs since it started doing this, all of which messing seems to have improved the quality of the engine's idle but hasn't affected the problem, so I'm thinking it's not an ignition thing. I've also recently adjusted the clutch linkage to bring the free play back from 2" to 1.5" as it was starting to crunch a tiny bit on synchro gear changes. I don't suspect this of being related but thought it worth mentioning as it's some fiddling I've done since the problem arrived.

I've never experienced a slipping clutch before, but does anyone with more experience than me (i.e. almost any amount!) think this could be what it is? The car has the same clutch it did when I got it a couple of years ago and I've done quite a few city miles in it since then...

Any advice would be much appreciated - I'd like to get to the bottom of this and have run out of ideas save for pulling the engine out to test my half-baked clutch idea and I'd like to at least feel a little confidence in that diagnosis before trying that for the first time!

Cheers,

-Tom

[MarkyB]

It doesn't sound like clutch slip, that shows up first when you are in top gear on an inclne, engine revs rise, speed doesn't.
Make sure there is some oil in the carb dashpot and that the carb piston is free to rise and fall with a clunk.

[tomti]

I just popped out and checked both of those - the piston moves freely and makes a nice click/clunk when dropped. The dashpot has oil in it up to the narrowing where the brass thing goes in - I think this is right?

The air filter's looking quite grey - could this be it if it's sounding like a mixture thing?

-Tom

[MarkyB]

Yes, I was thinking it could be weak but it could also be too rich, try another air filter, it is a service item.

[mike.perry]

Do a check of all the electrical connections and plugs to make sure that everything is tight. Check the carbon brush in the top of the distributor cap. Run the engine in the dark and look for stray sparks

[bmcecosse]

Sounds like head gasket - or perhaps just a burned valve if you are lucky. Compression check required (even just with the starting handle for a quick check) - don't run the engine until this has been checked. Also check the valve gaps.....

[tomti]

Ouch - sounds nasty. I've got a compression tester but have never got 'round to using it. The instructions mention using the starter motor to turn the engine over. Can I just pop it in each plug hole and wind the handle to get a reading, then - everything else the same as per the manual/leaflet?

I've swapped the air filter for a new one but won't try running the car again to see if this has helped or Mike's spark check before I've run down BMC's compression/valve gap tests.

If it is the head, that'll teach me to sit on my 12G940 head swap project for months without any progress!

-Tom

[kennatt]

have a look at the commpression 948 thread testing explained perfectly there (except for one poster)

[bmcecosse]

You do need to use the starter - in your case - just do it cold to get an idea of what's going on.

[tomti]

OK - I hand cranked the engine with the ignition off and the +ve lead removed from the coil, and put the tester in each plug hole. I got roughly the same behaviour on all four cylinders so tried using the starter motor (not warm, though, due to worries about running the engine with a leaky gasket/valve).

Although I didn't see the reference to the 948 compression thread (which was very interesting and informative, bar a slight detour as mentioned) until I'd done this, and therefore left the remaining plugs in, I got around 8 bar on each cylinder with the throttle jammed open.

Does this mean I can rule out the head as a place to investigate, do you think, or is there more a wise person would check? I checked the oil both through the filler cap and on the dipstick and I can't see any milky business going on. I haven't checked my valve clearances yet - would that be the next thing?

Thanks,

-Tom

[bmcecosse]

8 bar is fine (considering you left the other plugs in etc) - provided you zeroed the gauge before taking each reading...? If so - then you can rule out gasket/valve problem!! So - start looking at the Ign circuit - and if that all seems ok - look at fuel flow.

[tomti]

I definitely zeroed the gauge between cylinders - the benefit, I suppose, of not having used the tester before was that I was somewhat methodical about the process. Not enough knowledge to be dangerous/cocky on this particular gadget - yet.

Glad to hear it's not looking likely to be anything in the head - I could do without having to get involved in that area for a bit.

While running the compression test I noticed the leads to the coil seem a bit looser than I'd like so my first port of call will be to sort that out and see if it makes a difference. I'll have a look and see if the 'stray sparks in the dark' thing shows anything up, too. I also have a spare (new) distributor cap and rotor arm so I'll take a look at the current ones and see if they warrant a swap.

BMC - when you say 'look at fuel flow' (assuming I don't have any joy with the ignition side of things) do you mean check over the pump, carb etc. or something more adventurous?

-Tom

[MarkyB]

Have you taken it for a spin with the new air filter fitted yet?
On the ignition side, did you check the plug gaps when you fitted them?

[bmcecosse]

Well - if it's not the basic engine, and not ignition....must be a fuel or carb feed problem.

[tomti]

Have you taken it for a spin with the new air filter fitted yet?
On the ignition side, did you check the plug gaps when you fitted them?

I gapped the plugs to 25 thou - by which I mean I closed them down 'til a feeler gauge would slide between them but there wasn't any gap to be seen if I held gauge and plug up to the light. I hope that's a decent technique? By eye, the ones that were in there already were a bit variable in terms of gap, which I guess might have something to do with the fact that the idle seemed to improve when I fitted the new ones.

I haven't tried a spin with the new filter yet but will as soon as I get a chance and report back. We have a two-week-old baby in the house so I'm a bit all over the place at the moment!

-Tom

[tomti]

Hello again,

I've finally had another chance to look at this and I can report that replacing the air filter and tightening up the LT connections to the coil has made not a jot of difference.

However, the more times I do the lap of shame 'round the block, the more I think I'm becoming attuned to the nature of the symptoms. It does seem to my untutored ear as if the engine's burble is a little wobbly at idle and then positively patchy when on the move. It sounds a bit like a spark is either missing every so often or at least being a bit weedy every once in a while.

I've therefore ordered a new set of HT leads and treated myself to a new Accuspark (points) distributor with the more sensible non-riveted (red) rotor arm, as one of the posts that the advance weights attach to on the current one was looking a bit unhappy about a year ago when I had occasion to take a look at it and I've been looking for an excuse to swap it for a new one ever since. I think the current situation counts.

I'll fit all of the above and report back - does anyone think I'm on completely the wrong track or does this all sound like reasonable fault-finding to you?

-Tom

[bmcecosse]

Sounds good to me !

[MarkyB]

Sounds fine to me.
Intermittent problems and ones like this are always harder to diagnose than component failures.

[tomti]

Hi all,

Just a final report on this - I fitted all the bits mentioned above and the car drove a fair bit better but still had a sneeze at idle which hadn't been as clearly discernable before all the new bits went in. So I checked/reset my valve gaps - #4 cylinder's exhaust seemed a bit tight which I guess is where the misfire was coming from. Set the timing using a vacuum gauge (I don't have any hills nearby for the pinking-then-back-a-bit technique) and all seems much better now.

I think my cylinder head is in sore need of an overhaul as these gaps seem to close up very quickly - I do a lot of motorway miles and it's an unmodified head - do people here recommend the 'unleaded' heads available from the usual suppliers as an all-in-one fix or would I be better off going to an engine shop and asking them to do the individual jobs? And if so, should I go the whole hog and get new valves, guides etc. or just have the seats done? I've never done this before, as I'm sure you can guess!

Many thanks for all the help on this - I'm very pleased the car's behaving again! Although I just sheared off a wheel stud while going 'round checking the brakes before the MOT, but that's a topic for another thread...

-Tom

[bmcecosse]

If this is a 948 engine - then an unleaded head from a 998 Mini (cam 4810 marking I think) is perfect for you - and very cheap on ebay. If it's a 1098, then an upgrade to the 940 head should solve the problem - many of them are 'unleaded' anyway - but they do have better quality valves either way. With your existing head - be SURE to set exhausts to 15 thou gap (actually I now use 18 thou) to make sure the gaps don't close up when running hot. To convert your own head will cost plenty - and yes - you need better quality new valves (which likely need different caps and cotters...) to go with the hard seats - it's an expensive business.