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Original Message
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| RE: 6,8, or 10 degrees timing on a 390? |
| By Gerry Proctor - 02/23/2004 7:15:53 AM; IP 207.133.188.254 |
Sounds like you're using manfold vacuum to achieve that 10 degrees of advance at idle. You can use manifold vacuum advance but it requires a slightly different tuning method. Or you can more easily just go back to ported vacuum and call it good. You first need to sort out the vacuum advance taps on the carb. Get yourself a vacuum gauge to hook up to the vacuum ports on the carb. A port that has vacuum at idle is a manifold vacuum port. A port that has no vacuum at idle but shows vacuum as soon as you start to open the throttle is a ported vacuum port.
I'll address tuning with manifold vacuum first since it appears that's where you are right now. Disconnect the vacuum line and set your base timing at around six degrees. reconnect the vacuum advance line and read your advance now. It will be much higher than the base six degrees. If you don't have a fully-degreed balancer, it may be off the marks and the only way you'll know just how much advance you have is with a dial-back timing light. It's not all that important, though, since you are only interested in base timing (remember, your base timing is timing without any advance). Distributor vacuum advance cans vary as to how much advance they contribute. Some as little as ten degrees or so and some over 20 degrees. You'll probably have to adjust the carb idle speed since your total idle timing will be close to 20 degrees now. Tuning with manifold vacuum can be helpful for engines with big cams that have trouble generating a stable idle or enough vacuum to work the power brakes. Go ahead and test drive the car to see if you're pretty close to where you want to be. Just to be clear on this; You set up your base timing without the advance connected and once you do that, reconnect the vacuum advance. Do not pay any attention to the jump in the timing mark. Do not readjust the distributor to bring it down.
Tuning with ported vacuum (where there is no vacuum in the tube at idle) is pretty easy. You set your base timing to six or whatever degrees that works for you and that's about it. The vacuum advance canister won't contribute any advance until you crack the throttle open.
There is a lot of myth and lore (Net truth) regarding ported and manifold vacuum. Many folks believe there are differences that lie beyond what occurs once the throttle is cracked open. There isn't any difference between the two once the throttle is cracked open and it's not hard to prove by anyone who has a vacuum gauge. Setting them up with the engine at idle is different, but that's about it. And in many cases, it's not a one size fits all. The savy tuner will use whichever method, manifold or ported, that suits their engine. |
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