This is why a timing light...
This is why a timing light is a good idea, even if you're astute at tuning by ear. Timing marks allow us to see how timing events are unfolding. Watch them as you goose the throttle, which determines vacuum advance adjustment. Note how MCE Engines has marked this balancer: TDC, then idle timing at 12 degrees BTDC, then maximum advance at 34-36 degrees BTDC. This gives you a tuning window as a guide.
With the vacuum advance reconnected, check total timing, which can be done two ways. Total timing is checked at 3,500 rpm with the vacuum advance connected. Ideally, push for the maximum number of degrees before BTDC without detonation (pinging or spark knock). In most circumstances, total advance should be 34-36 degrees BTDC.
The rate of advance is checked by goosing the throttle to wide open and watching the timing mark with a timing light. If spark advance comes on too quickly, the engine will tend to break up (misfire) and ping (spark knock), which means you need to slow the rate of advance. If the engine noses over and doesn't rev quickly, speed up the rate of advance.
When the vacuum advance is dialed in just right, the engine revs smoothly and crisply. On the road, power comes on quickly without hesitation or spark knock. At sustained highway speeds, the engine should maintain a smooth demeanor without misfire. If you feel a slight misfire at cruising speeds or hear pinging under hard acceleration, you have too much timing.
Another area to check is manifold vacuum. Healthy manifold vacuum is normally 15-22 inches at idle. Checking manifold vacuum is just as important as checking compression and cylinder leakdown. It determines an engine's state of health. Note that manifold vacuum gets tricky when the engine is fitted with an aggressive camshaft or too much carburetor.
Good Carburetion
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