My 305 seems to run best when set between 5 & 7 btdc on the timing light, idled down low w/vac advance disconnected and port plugged while setting timing. Have had some trouble with the stromberg carbs wanting to flood when idled down-needle and seat leaking. If you can't get consistent timing, you might pull the distributor and check the pin through the distributor gear and shaft bushings for slop, and the vacuum and mech advance components before tearing down to the timing gears. Other problems I've had with these engines include gummed up pvc valves and breathers and previous owners over-tightening the valve lash to quiet valve noise (makes solid lifter engines run like crap after they warm up).
Joe
