I seem to be having a similar problem with my WC51. On an ANZAC day excursion, after running well for a year she decided to suddenly run rough as and backfire alot.
She now does not want to idle without full choke or alternately throttle being out and lacks power on the upgrades or when being worked.
I have checked the carby and other than it being a little wet at the lower housing (Has the carby for a Chev Cargo on her) that is all good. Replaced/gapped plugs, checked air/fuel mix, checked gap and cleaned points, replaced manifold gasket (As noticed it had blown), new exhaust pipe to manifold, cleaned air filter to ensure good air flow through there..
I dont have a timing light but have used my eye-crometer, what else could i check.