OK, background is of Nellie idling a bit unevenly for a while but basically running ok (usually goes very fast in fact!) but at MOT was told it was running very weak and MOT man tweaked it a 1/4 of a turn. Didn't seem to make much difference at the time but...
On Weds drove to Glossop and back. Ok on way there, started to hiccup (misfire) slightly on the way back. Not done much apart from put petrol in on the way there (Buxton).
On taking it out on Thursday to Manchester airport and back misfire got a lot worse, especially when going up hills/ pressing throttle down more (and particularly after foot being off to go downhill then on again to go uphill). Much hotter day but don't know if that is relevant.
Ok, work done/things checked so far (some just general servicing too):
Oil/filter change
Changed air filter sponge for new one
Spark plugs change
New points but not condenser as I thought it was working ok (don't they just die suddenly and completely?)
Changed HT leads but not for new ones just less old ones, then back again as didn't seem to make a difference
Tried 4 different coils as the 2 I have on bother overheat (hence 2 in there) but known good ones didn't make any difference
As had a wiring melt down issue a while ago due to short, causing me to replace some of wiring loom and especially the main live feed to coil, ran her with wire straight from coil to battery to make sure no breaks there but made no difference
After Jono did a check for air leaks in the carb block by using WD40 (not tried that before!) it seemed to make a difference to idle/erratic running for a short time and could hear hissing as if air being sucked in, so took top off carb and checked block - looked ok so cleaned it and put back with hylomar. Seemed to run better for a bit after this (more even and no misfiring) but not for long, started to cough again after going slowly through Matlock Bath traffic in very hot heat.
Done lots of playing with idle and mixture screws on carb to no avail. Every time it seems to be ok (idles well) it coughs like hell as soon as you put your foot down!
Oh also checked oil breather was working with some syphon tube with water in to check there was some pressure there. Didn't move much but did a bit. Feels like it has some pressure (but would this make a difference anyway?)
Oh and took whole of carb apart and checked float height (fine), checked all jets clear (fine) and for any crap in the petrol (there was but cleaned out now and made no difference)
So... what do I check/change next?! Other suggestions are that fuel pump dodgy (would that cause this though?) or small crack/hole in fuel line somewhere near fuel tank allowing air in. Thoughts on this and other suggestions please...
Or do I just decide to go to Spain in Poubelle who seems to be running well at the moment?!!

Thanks, Nikki.

Change the condenser to start with to eliminate that, clean all your jets, check float level and action of needle valve. I would also check your fuel pump is working correctly and that the short length of tube connecting the fuel take up on top of the tank is not perished,this can cause problems like you have.OK you'll have to drop the tank but it's easy to do.

Condensers can and do fail gradually, with the resultant misfire becoming more and more pronounced until the stage is reached where the engine struggles to start and then runs very badly.
I've just finished servicing another car for its WM trip and because it was on points, decided to fit a pair of condensers mounted on the fan cowling, remote from the points box.
During test drives, one of those condensers proved to be faulty straight out of the box, but it was a 5 minute job to replace it...

ken

Somewhere in your description you said you could hear hissing and took the top off the carb. Did you check the four nuts holding the carb onto the manifold. They have a habit of working lose. Bastard to tighten unless you have the tool.

One of your fav tools Paul

Yes, the nuts were all tight, but air was clearly still being sucked in. Personally, I still suspect a problem with the carb mounting block, even though no crack was visible I believe.

If the carburettor mounting nuts have ever been over-tightened, the base of the carburettor may be warped.
This can be corrected with careful use of a large file.
However, an air leak of any significance would cause problems with starting and as soon as the engine was running, whereas a faulty condenser tends to make things worse the longer the engine has been running.

Neat petrol is better than WD40 when checking for air leaks, applying it with one of the small syringes supplied with those kits for refilling ink cartridges.

ken
( p.s. My cranked 12mm carburettor spanner seems to have gone walkabout at Registers.
Last seen in the vicinity of Louise's 2CV, the one which had a dodgy 'super-duper' plug lead... )

I'm sure you did, but there's no mention of whether you checked the accelerator pump. I helped Dan Creed with a similar problem on his van a couple of years ago. The pump diaphragm was fine so it looked like it was working perfectly when the lever was operated by hand, but it turned out the cam on the end of the spindle had worn right the way through the lever, so it barely did anything when it was actually in use!

I guess you tried the WD-40 trick at the other ends of the manifold as well, where it's fitted to the heads?

If your idle mixture is too weak it will cause a bit of hesitation if you bury the accelerator when the engine's been ticking over, but it would only be very brief as the fuel drawn through the main jet catches up with the increased airflow through the carb.

Having said that, the condenser would definitely be my prime suspect, too, and hot weather will always show up a dodgy one. I've had the same bloody problem all weekend on the bike - made even worse by the fact it uses a magneto so the ~60 year old condenser is buried deep within the coil windings..! It was misfiring at the same specific throttle opening each time, so would have been very easy to mistake for a carburation issue. Ken's idea of resiting the new one to the cooler area above the fan shroud, with a spare next to it for easy swapover strikes me as a very good one, particularly for hot climes. Unfortunately your new points are likely to have been burnt a bit now, as well... They sell some nice cheap sharpening stones in the pound shop! FWIW, if I was hoping to save a couple of pennies on a service, I'd always fit a new condenser and just clean the old points up a bit. Best to do both though!

Bad move, condensers are not the best thing these days, for reliability if in doubt keep old condenser & clean existing points with a diamond file, if I thought points were required I would fit the new points alone.

Well I've changed the condenser, and things appear to be running a lot better. An Ecas super coil is on order, to replace the collection of duff coils Nikki has been acquiring.

Thanks for the input folks. I'd sooner get the thing running OK here than on the side of a Spanish highway.