Hello all.

I recently had some running problems which I put down to crap in the tank making its way to the pump and blocking the filter. So having cleaned the filter(s), stripped and cleaned the pump and got it going again I thought all was well.
This only lasted a few days and it was running fine until Tuesday morning when I left it at work. I then went away for a few days and come back to it friday night. It started (just) and then a mile up the road the engine bogged down and let out an almighty back fire.
It wouldn't start again so I had it towed home (I was thoroughly pissed at this point having taken > 8hrs to get back from chesterfield and actually driven passed my house in a fully functional hire car 2.5hrs prior to finally getting home)

So yesterday morning I had a look over it and decided it must a continuation of my 'crap in fuel' problem so thought id look at the carb. I followed mac's guide and the jets were a bit blocked. So the carb was cleaned and although I didnt have a rebuilt kit the diaphragms and gaskets were tidy and reusable imo.

But its still not running.
Theres a good spark on all 4 plugs.
Fuel is making it to the carb (he tank pickup doesnt have a filter, but the pump is working on the starter as per haynes test)
Theres no immediate signs of HG failure (oil and water condition/levels are ok)
The engine has decent compression

Any ideas?

Hummmm, you have spark and you have fuel and it was running. The backfire could point to a sudden air leak leading to a very weak mixture. (or excess fuel igniting in the exhaust)

With a vario there are a lot of vacuum pipes that can "fall off". Start with the connections to the pre heat plate and manifold, check the pipe work to the 4way valve, and to the clutch servo valve and servo. Make sure the pipe to the brake servo is intact.

The spark tend to rule out an ignition problem and the dizzy is obviously turning (all 4 plugs spark) so we can rule out something wield like a broken timing chain (also the pump " pumps" so the cam is being driven) - which leads us back to fuel - you have some but is it the right amount? If the float chamber fills you obviously have enough - but too much is possible (not really likely though).
Also concerning is "the jets were a bit blocked" - crud is obviously still making it's way through - you've cleaned the jets but what about the fine passages from the jets to the Venturi, (really need an airline blow gun).

Best guess at long range is too much air - you'll be looking for a significant leak to prevent it starting though.

It's back to basics,
Is the basic engine sound, compression etc.?
Do you have a spark, is it at the right time,?
Do you have fuel, is it at the correct mixture,?

Oddball long shot - an internally collapsing silencer causing wadding to cause partial blockage of the exhaust - any signs of debris in the tailpipe?

Good luck - Mac.