Hey Oli,
Throw a vac advance unit in the distrib. or advance the timming to about 16 * initial and plug or remove vac hose. The bad vac unit is causing a vac leak so running leaner than ideal and engine wants more timing anyway.
I remove vac advance units from all my distrib. to get more reliable timing adjustments ( vac unit allows variations from slop or slack in the plate). Then I add all the initial the engine will take and fill in with mechanical until total is about 38 to 40 max in by 2500 rpm.
Vac Advance unit helps milage and emissions but little more.