Steve,
There is no substitute for having your engine dyno-tuned. However, as stated
in one of the recent replies to one of your posts, you can get pretty close
to optimum ignition timing with just a good timing light and your
(approximately correct) tachometer. You will want all of the mechanical
advance in by 2500 rpm. My distributor lets me set total mechanical advance
in five degree increments. I currently have 25 degrees mechanical plus 10
initial for 35 degrees total from 2500 rpm up. I am using one very soft
spring and one of average (stock?) stiffness. (BTW, this is an Accel
all-mechanical distributor.) It sounds from your description that you have
the optimum situation bracketed. Try using one soft spring in place of the
missing one you replaced, or two of medium stiffness. Then use a timing
light to determine at what rpm you get full advance. If you also have vacuum
advance in the distributor, disconnect it for this test. If it looks good;
i.e., full advance at about 2500 rpm and total absolute advance of about 35
degrees BTDC, then try driving it. If the distributor has the typical 20
degrees of total mechanical advance, then at idle you will be at 15 degrees
BTDC. My engine doesn't like this much advance when starting, but most
engines like this setting. It's very possible that you are running too much
total advance, which causes your engine to ping and that adding the second
spring is just masking the problem. All of this will become clear if you do
some work with the timing light.
I hope this helps,
Bob
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