I was fairly sure it wouldn't be that valve which is why i didn't mention it beforehand.

Something that did cross my mind is it could be a partially blocked main or idle jet in the carb, often they can be cleared by getting up to ~40mph then changing down to 2nd and maintaining the speed, close the throttle for a few seconds then literally floor it as fast as possible then repeat.
The sudden shots of vacuum hitting the jets as the throttle is opened quickly can often clear a jet blockage but you need fairly high revs and hence vacuum to do it.

For now i'd pursue the condensation idea as that seems the most likely.

Although the valve seems to be operating OK now, I think I had problems with it many years ago.

I also thought of a blocked jet, already tried your suggestion.

Never taken a jet out of a Webber before. Hope I don't have to.

As you say, I'll pursue the condensation thing. Not had my travel out in the car today.

Are you sure yours is a Weber? Fairly sure they were all Solex and/or Pierburg-Solex on the 340, especially with the anti-diesel valve.

If it is a Weber, it's fairly simple to remove the idle jets (they screw into the side of the carb at the top of the venturi usually) but the main and air correction jets will need the top half of the carb body lifted off and a new gasket (preferably) fitted.

The main jets are normally accessed from within the float chamber and are at ~45deg angle from vertical at the bottom of the float chamber adjacent to the venturi they serve. The air correction jets sit in a drilling that connects via the emulsion tubes with the main jets which also draws the fuel up and into the "spray bar" (can't remember the correct name for these, Auxiliary Venturi i think but could be wrong) that sits in the actual venturi.

All fairly simple and easy as long as you're methodical on a Weber. Solex are a French manufacturer so expect some Gallic influence but usually a similar construction, might find the jets aren't removable and are cast in the housing though.

Hopefully it's my first suspect of condensation, especially as you've already tried the jet clearing trick.

https://www.carb.parts/pdf/Exploded_...4_DMTL_USD.pdf

That link is useful as it shows the exploded view giving a good idea where everything is.

Thanks for the above. Well, the carb is a Weber, says so on it! Otherwise I would not know!

Started engine again, same rough idle.

Thoughts I'd take out the anti-dieselling valve, tested it again, works. It has an O ring seal on the threaded bit, it looked a bit compressed, so put another on as well, took the advice and did not tighten valve up much. Started engine, went up to about 2000 RPM (I had days ago adjusted it up to stop car stalling). Unadjusted idle screw, probably to where it had been prior to this problem, engine idled nicely at about 900 RPM. Continued running engine for about 20 min, all seems OK. I can't really understand this!

I will probably take the car for a good run tomorrow and report back.

Some boring pics of 340 carb below.

https://live.staticflickr.com/65535/...8bf7bc91_b.jpgDSCF0220 by A60man, on Flickr

https://live.staticflickr.com/65535/...a76a3e04_b.jpgDSCF0221 by A60man, on Flickr

https://live.staticflickr.com/65535/...ff07ef63_b.jpgDSCF0222 by A60man, on Flickr

https://live.staticflickr.com/65535/...ea809546_b.jpgDSCF0218 by A60man, on Flickr

https://live.staticflickr.com/65535/...5655d730_b.jpgDSCF0219 by A60man, on Flickr

That definitely looks like a Weber conversion, i'm wondering if what had happened was the "O" ring on the anti-run on valve had perished and was admitting air - essentially a small air leak, just enough to make it run excessively lean.

When you removed the solenoid for the anti-dieseling/run-on valve, does the plunger in it have a conical tip?

If so, is it gummed up around the edge where it seals the airways inside the carb off?

No, the original O ring on the valve was not perished, just a bit compressed, put another on top. Yes, tip of plunger was conical, no gumming up.

I will probably drive the car a few miles tomorrow.

For now, thanks to all.

The carb is a Weber DIR as fitted to the later B14's

The most common cause of rough idle is air leaks with that most often being carb base warpage. Testing for air leaks is the very first step.

Your problem could relate to the fuel and could be worth running off and filling with some branded fuel to prove one way or the other

I did a sort of air leak test early on, just sprayed WD40 around carb / manifold joint. Did not find anything that way.

Did you pull the idle jet off the end of the solenoid and check it for dirt as I explained yesterday ? It sounds as if you might have disturbed the dirt when you removed the valve ..

Now to clear up some misunderstandings ..

ALL B14's from 1976 to 1991 had the DIR Weber carburettor which is a very good quality carburettor which does not suffer from wear , there were several variations over the years ...

The B14 DIR never had problems with a warped base , that could happen on the low quality solex used on the 1.7 engine , mainly by being overtightened my ham fisted people ..... I must have flatted off the base of several hundred of these carburettors when the cars were current !

The 1.7 engine had the rubber pip on the end of the idle valve that could stick when old and the car not used for a period of time hence the tale of the tip being cut off in an emergency to get you home . The idle jet itself was screwed into the side of the carburettor and if blocked was replaced as you could not get inside behind the jet to clear it ...

The idle valve is also an economy device , it shuts off the fuel when your foot is off the throttle over 1200 rpm .... this function was fitted around 1986 ...

There was a sort of brass bush on the solenoid valve, I did remove it, it was clean, but washed it in WD40 anyway.

My car is a 1.4 GL 1989, and I have a vague recollection that the previous owner may have fitted a replacement carb. I have had the car over 10 years and generally, it has run very well.

I did not know about the economy feature of the valve.