The lightest spring will advance your vacuum advance very quickly. I check the vacuum advance with the engine at an idle using a pump to record the amount of advance and the vacuum needed to achieve it. If you ran into spark knock and did't have knock with the vacuum disconnected then you probably have your timing over advanced or a vacuum leak. I set my vacuum advance to begin at 10" Hg and to be all in at 15" Hg. That way there is very little chance of getting vacuum advance when the charge into the cylinders is dense enough to cause knock. Set your timing to 34 degrees BTDC at 3500 rpm with the vacuum advance temporarily diconnected and plugged. After the total advance is set you can play with your vacuum advance to get decent mileage - thats all its for.