400 and purple cam

well, this topic could open a large can of worms but worms can be a healthy nutritional snack so I will take a stab at it. If you twist the distributor to increase the timing and the rpms immediately jump up then you need the extra timing [more initial advance] if you twist it some more and it starts to gallop [not a technical term ] then you have gone too far. back down the timing til it sounds nice and smooth. adjust the rpms wuth the throttle adjustment. you really dont want too much timing. it causes hard starting and unpredictability of the idle [bad for emmisions too]use of a vacuum gage is also helpful. the needle on the gauge needs to be as steady as possible with a good signal without going to far. I would guess between 16 to 20. more leaning to 20 would be my guess. Of course you will need to check for toral timing. you should not need over 36-37 degrees of mechanical advance. this is important and may require a narrowing of the slot in the dizzy advance mechanism. my 2 cents