Advance is pulling the timing ahead to far . Disconnect the vacuum and you'll see it goes away. 2 bbl an 4bbl have different ratios. 2 bbl are 11.0 4 bbl are 8.5 marked on the vacuum rod. They are also adjustable with a allen inside the vacuum port on the dist. The more open your exhaust is the less timing you need. One reason many go with a mechanical advance.
