Fellas - I'm in the process of timing a 350 SBC I rebuilt and have broken in. Would someone clue this dummy in on why the vacuum advance tube is pulled and plugged for fine tuning? If I'm using my light while the engine's idling, what difference should pulling the tube make (when, presumably no advance is needed)? See... told you I'm dumb!
You want static timing set with the vacuum advance disconnected . Then when you reconnect the vacuum ,you will see the timing advanced by the vacuum advance mechanism. The factory static advance was usually about 4 to 6 degrees BTDC.
Even when ported vacuum is utilized the factory procedure has always been to disconnect and plug the vacuum advance port. It's kind of a belt and suspenders thing, by doing this there is no possibility of vacuum advance interfering with setting the initial crank timing. If the idle RPM is high, then it will tip in to some degree (heh). But in practice at factory idle RPM there will be no vacuum advance, you can try it and see.
So, with the advance disconnected & plugged at the carb, I should set the mark to between 4 to 6 BTDC? And does it matter if the engine is warmed up or cold?
Depends on the engine. 4 to 6 for bone stock small horsepower, 8 to 12 for bigger power. Stop advancing and dual it back if you star getting kick back when starting.
This question covers a lot of territory, tho generally the initial timing can only be slightly advanced past stock spec before heavy pinging occurs (without first modifying the total advance of the distributor itself.) This is one reason why folks send their distributor off to guys like Bubba to have them "recurved", the factory curve is very conservative. Guys will often spend big bucks on carbs and heads and intakes and exhaust but leave a lot of performance on the table because of the factory curve. If you're hauling loads of gravel to Kansas City the stock curve is OK but most people like a little more zip. I run around 17° initial in a low compression Y block and it runs great.
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