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Originally Posted by dlampe
At this point I think matt is more concerned with why he is not getting 28 deg. in the advance curve than his total timing. The problem is he has the dizzy set up for 28 deg. mechanical advance and he is only seeing maybe 14. The timing light is good and his car idles great. I don't understand why his dizzy is not advancing. It did not feel like it was binding.
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OK, let's try this again. By using the light springs, it's likely that the rpm for zero mechanical advance is down somewhere around 700 rpm (based on the MSD installation drawings on their website:
http://www.msdignition.com/pdf/8594_frm23872.pdf). Assume for the sake of the initial argument the following items: (1) the curve of the mechanical advance is linear between zero advance and full (28 degrees in this case based on his selection of bushing); (2) Mr 951mat's engine is idling at 12 degrees advance and is achieving full advance at 2300 rpm; and (3) he has his idle set at 1200 rpm (this is a guess on my part). Based on a linear advance curve, the distributor will feed in about 2 degrees of advance per 100 rpm increase above 700 rpm. In that case, the distributor is already feeding in close to 10 degrees of mechanical advance at his idle rpm. This means he can only get an additional advance of 18 degrees out of the distributor using the bushing he selected, because he is already using 10 degrees at idle of his 28 available. Assuming (again) that everything is functioning perfectly inside the distributor and he has his idle timing at 12 degrees at 1200 rpm, he can only get another 18 out of the distributor for a total of 30 at full advance.
In the real world, the advance curve of the distributor is not linear because the springs do not exhibit a linear force-per-displacement curve. The advance will be quicker at the lower rpm range, probably closer to 3 degrees per 100 rpm increase, and will slow down at the higher engine speed since the spring constant (force per displacement) increases as the spring is stretched. This may be observed by again referring to the MSD advance curve drawings. Using these numbers, Mr 951mat is probably using up closer to 15 degrees of his 28 available, and so has only 13 left. He observes a delta between idle and full of 13 degrees, pretty close to what the numbers predict. Also, as Mr Parker says, the repeatability of the spring setting at idle is not 100%, and you have enough error to account for the differences which Mr 951mat is experiencing.
Having said all of that, it is always possible that there is something wrong with the centrifugal advance in the distributor. I had problems with my MSD which turned out to be corrosion between the advance plate and weights. I purchased the MSD unit to replace a Mallory Unilite and found that MSD had tried a new anti-corrosion coating that didn't anti-corrode very well. I returned the unit to the fine folks at MSD and they rebuilt it at no cost with new parts; problem resolved.