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Hi Richard. Interesting comments. I did have gauzes top and bottom on both sides and this worked well but restricted power so I took the bottom ones off and it has started again but I also moved the needle one notch down too.  I have a 276 carb, which might be too small and all standard size carb components that are new ish and this was clip 4 from the top, so set rich. When the bike is running down the road it does not spit at near main jet  speeds and it only seems to occur on the overrun or at lower revs. This suggests to me that it is not a timing thing, nor is it a porting thing but, as you suggest, a fuel air thing. I messed about with the timing lever to see if this made any difference but it doesn’t and just as I was getting home it seemed to like being choked at lower revs.

The snail shell on mine has a big bolt going through the middle and is not restrained from the frame as I have seen on others, so the gases have to negotiate the curved tube and this bolt too which is hardly a smooth path but there again we are only talking a few horsepower. Thank God mine doesn’t go as fast as others seem to because I have yet to master the art of braking in any situation resembling an emergency stop in 100 yards. My seat covering is already the right colour brown as it is.

As far as my brain goes with the spitting, this is either a case of it being too lean or too rich at either the air screw or the needle so I will mess about until I end up back where I started and with no sensible conclusion realised.

Before that I will try the left hand gauze thing too because that makes sense. There is a lot of swirl going on which Yamaha, my second favourite bike company, researched enormously. I wonder if reed valves would do anything?

Fitted a Weldonia fork brace incidentally and this has made the front end a much more confident thing. A work of art to be honest because that was designed by hand and if you fit a big brace along the back, it also works as a steering stop too. It would have definitely needed to be designed by CAD such is the way is passes around everything and it still works really well.