Timing Question
- Thread starter dimagg
- Start date
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BillGrissom
Well-Known Member
Idle speed is kind of a preference and if you have a wild cam (lots of intake-exhaust overlap with rough sound), it may not idle below 1000 rpm. The lowest speed you can set and still have it idle smooth and not bog down in "D" is best. Usually 700 rpm, but I would love to idle smooth at 500 rpm.
Timing should be much more advanced than what the manual calls for. Best is the more advanced you can get without having it fire so early at low speed that it fights the starter. Many here run 20 deg advance at idle and 35 deg "all in".
One potential problem is that as the engine speeds up, the centrifugal advance comes in, making it run better, and speeds up more, i.e. a run-away "positive feedback" condition. In modern spark controllers, you design a "dead spot" near idle where it even retards spark a bit as rpm increases. That helps make a stable idle. I don't know how that is designed into mechanical distributors, and maybe why many of our engines like to "run away" in idle and bog way down in "R" or "D". I think if you set a fairly high advance at idle, you will have less run-away tendancy. Also, in a modern controller, it is smart to spark ATDC at very low rpm (while cranking), which assists the starter. I think those are some reasons modern cars start so easy and idle so smooth (plus IAC w/ rpm feedback).
Another thing that makes rough idle is a vacuum leak. It also leads to the "run away" condition. As the engine speeds up, the leak is less signficant, which makes O/F better, so it runs faster, ....
Timing should be much more advanced than what the manual calls for. Best is the more advanced you can get without having it fire so early at low speed that it fights the starter. Many here run 20 deg advance at idle and 35 deg "all in".
One potential problem is that as the engine speeds up, the centrifugal advance comes in, making it run better, and speeds up more, i.e. a run-away "positive feedback" condition. In modern spark controllers, you design a "dead spot" near idle where it even retards spark a bit as rpm increases. That helps make a stable idle. I don't know how that is designed into mechanical distributors, and maybe why many of our engines like to "run away" in idle and bog way down in "R" or "D". I think if you set a fairly high advance at idle, you will have less run-away tendancy. Also, in a modern controller, it is smart to spark ATDC at very low rpm (while cranking), which assists the starter. I think those are some reasons modern cars start so easy and idle so smooth (plus IAC w/ rpm feedback).
Another thing that makes rough idle is a vacuum leak. It also leads to the "run away" condition. As the engine speeds up, the leak is less signficant, which makes O/F better, so it runs faster, ....
I was going to answer, but it looks like I have bad "timing"....
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