Triumph Repair: TR 250 tune up, vacuum leaks, dieseling

Question
Scott, We are talking about a 68 TR 250 (CD220). I have had the ZS carbs rebuilt, installed a fuel filter, new fule lines, all new distrubutor parts, lubed distributer, new plugs and wires, new coil, dwell=32degrees, timing set at 4 degrees ATDC (retard disconnected) but I am still getting backfire through the exaust and run-on when the ignition is turned off. Carbs are ballanced using float guages(they are older ZS with no needle valve adjustment) and every thing seems to be set up to spec. She runs well under load but at idle or when low RPM there is backfire (pop)from the exaust. To me it seems that it is running rich but I have no way to lean it out.

Answer
I can't say for sure, but I would look at a couple things, if you haven't done so already:

1) check for vacuum leaks near hoses and the intake manifold
2) check the bypass valve for proper setting
3) use an air gun, pipe cleaner and needle to make sure all passages in the carb are clear.
4) check the choke valve on both carbs, synced, to make sure it isn't leaking & on all the time.

The run-on indicates either the ignition isn't grounding out when you shut it off (condensor, wiring or similar), or the mixture is so lean & ignition retarded you are getting dieseling from carbon buildup heat...

When I've had dieseling, the timing is too far retarded, or the mixture too lean.
When I've had backfiring and am unable to set the idle, the carb isn't clean, or there is a leak somewhere.

Hope that helps...