Triumph 3ta Dutch Army (tr35wd) Help Needed

Help!

I have a 1967 350 (TR35WD) that now only runs on one cylinder. It seems to be low miles (8000 kms). To explain there is an unusual starting technique on these bikes due to the Solex carb fitted for the Dutch Army, free clutch as normal - hold choke lever on carb all the way back - turn on ignition and kick not touching the throttle at all. When I took delivery of the bike in December it would not start at all - cleaning out the fuel lines solved that. It would start second kick. I did not ride the bike since the tyres seem to be 1989 vintage and as hard as iron - new tyres ordered. Being winter I left it in the back of the garage - just turning it over but not starting it every 3 weeks or so.

Last weekend I foolishly thought it may be time to run it up and down the road to check for any little issues before wheels were removed for new tyres to go on. Normal starting technique did not work, no choke and cracking the throttle half open got it to fire on the right hand cylinder. But increase the revs and it fires on both cylinders!

So normal checks - fuel fine but put in some new fuel anyway, compression by the thumb over plug hole method - seems the same on both pots, spark fit new plugs and check spark with plugs out resting on the head - good spark on both. Check plug lead connections ht and lt connections - fine. Run the motor up again just about ticking over on one pot - I took the plug lead off the dead pot put in a plug and held it to the head - no spark until the revs climb quite high. So try the old trick of swap the leads over so that the left coil feeds the right pot - won't fire at all but the bike is still on points so I'm not sure if it would work at all.

Is all this down to a coil that's failing? Or should I be looking at other things? Any help would be appreciated, thanks.

Trev