2.25 petrol engine of a 1982 series 3. Before Christmas we (my son and I) completed new pistons/rings, little ends, big ends, new valves/with new springs. Tightened head again after couple of hundred miles - going beautifully - best it has ever sounded. Engine spluttered and jerked then stopped and would not start AA man came after a while and it started, ran fine, AA followed to home. All fine, next day started and ran but running eratically again and stopped. Sounded like fuel to us but loads of fuel getting there, cleaned blew out all jets no difference, spark is not as big and bright as I would like but it was running well - Maybe ignition - renewed coil - No go. Fitted new electronic distributer still spluttering and flameback through carb. Fitted new carb (Webber 34 ICH from memory) No go. Still splutter and back fire through carb. The new items were planned but brought forward. Someone said timing but why would it be running beautifully then do all this spluttering. Oh, plugs are sooty of course but were also new just before new pistons. Really stumped. Help !

I’m also going to say timing.
6’ BTDC
Mark the dizzy before you start as it runs already.
Hold the revs up(1500/2000) and slowly rotate the dizzy. You wil find a sweet spot. Mark the dizzy again (incase it don’t start again)

It wont actually run properly but just keeps spluttering then gives up and when it is doing that, there is no response to accelerator.

When you say plenty of fuel - do you mean at the delivery pipe to the carb?

There's a fine mesh filter in the carb inlet - it's underneath the housing where the delivery pipe connects to the carb upper body, it can get clogged.

Sounds familiar so here's my experience.
The first time i had the "run fine for a while followed by no start" it was the timing caused by a worn sprocket and stretched chain which became a bit of a lottery and actually an easy weekend of spanners fix.

Then I had very similar problems after my first head gasket change and in desperation after jet and seal replacement bought a new carb (Zenith) , yes the copy and it was a night mare.
I found I had over fueling issues and not matter what I did to the timing She would run fine from cold but as soon as the engine got up to temperature the problems started.
There was an air leak between the carb and the manifold for a start and I also replaced the dizzy with an electronic one resulting in sleepless nights try to work out which one of the 12 available curves matched the engine.
Regarding the Zenith there's an issue with the top half becoming deformed if I remember rightly which causes flooding (i think).
My fix was ditching the Zenith for a new manifold and SU plus getting a proper dizzy from Distributor Doctor and now She runs just fine and now i can test the spark at the dizzy.

I would go back to the start, set the static timing, check the spark is happening with a timing light (12v bulb), you can check the coil is throwing a spark by holding the HT lead near the engine (use insulated pliers or it's going to hurt!), i am not familiar with weber carbs so can't say anything there.
Battery check, alternator check, are you running on battery.

Alpine greetings

Simon

Are you using the lead replacement additive in the petrol? I know its unlikey given the newness of the valves but unless the head has been modified to use unleaded fuel, burning of the valve seats can result in carb blowback and bad running.

Col