Single misfire

[Tod]

I had a similar issue several years ago with my Ruby. I traced it to a faulty lift pump. It may be worth checking.
Tod

[Peter Naulls]

Faulty in what way Tod? I did wonder if it was running short of fuel but haven't been able to catch it with anything less than a full carb.

[Tod]

Peter, I found that the diaphragm had swollen, (in my opinion by the ethanol used in petrol). This greatly reduced the effective working stroke of the diaphragm, hence the fuel delivery quantity . The car ran as normal when under a light load, however long hills especially, were a problem. It took a while to actually figure out what was happening.
Tod

[Reckless Rat]

The red coloured diaphragms sold by the Seven Workshop are E10 compliant.

["Slack Alice" Simon]

You don't mention it, I assume you have tried another set of plugs?

Is the distributor drive gear ok - some have a habit of wearing away to nothing. I can get mine to backfire by retarding the ignition, so timing can have a similar effect to an air leak.

Cracked cylinder head?

[Peter Naulls]

Many thanks for all your suggestions, on and offline. 

The pump does have a Seven Workshop diaphragm but I guess it could have developed a leak or valve problem. I'll check that next I think.

No I haven't changed the plugs. When it really fails it's not a distinct misfire, more a fluffing and hesitancy... and the stalling. However, I probably should change them just to rule them out.

Distributor drive gear is good, having destroyed a couple of chocolate replacements I now have a good original one back in there. Timing is fine too.

Cylinder head? I didn't think of that. Where do they crack typically? There's no water loss and the head gasket looked to be intact when I changed it.

["Slack Alice" Simon]

I don't think they do crack, typically.   But your post nudged some obscure memory that someone, on here or in a club mag, had a cracked cylinder head.

Can't believe it would provoke "fluffy" behavious, just straw clutching.

[Adrian Payne]

Hello Paul
Still sounds like a valve issue. Could it be a cracked cam follower where the cone nut has been done up too tight and now jams in the guide. These have been known to hold a valve open.
Adrian.

[Peter Naulls]

Hi Adrian, Simon etc,

The followers were all checked whilst the valves were out so I think it is unlikely now. I am beginning to think the original "single misfire at speed" was a separate issue (possibly the gummed up valve stem causing it to stick) and that the current problem is due to weak mixture. 

I need the car for work tomorrow but I'll check the pump valves and diaphragm at the weekend. If it's not that then it has been suggested that a worn butterfly spindle may be the cause. My spare carb is a bit of an unknown quantity but it may be a useful comparison.

I'll keep you all posted, your collective wisdom is much appreciated.