To amplify a bit, the 17.5 curve increases the total advance beyond ideal
performance as part of coping with the 'smog pump,' a U.S.-imposed
contraption that injected a portion of exhaust fumes into the fresh air
intake manifold.
Removing the smog pump was supposed to be combined with recurving the
distributor to the superior, non-U.S. specs for mechanical and vacuum
timing advance. (Vacuum is not changed, but make sure it's functional,
since the diaphragm and line can fail).
There are still examples out there where the pump is long gone but the
distributor was never recurved. Worth checking. It's a very easy retrofit.
Use a dwell meter to check shaft wobble while you're at it.
On Monday, May 28, 2012, Joe Russo <jrusso07@rochester.rr.com> wrote:
> Weights are the same, springs are different. 7.5 is the non-smog advance
cam.
> For that cam, one spring is real heavy wire and the other is a lighter
weight
> spring. The heavy spring goes on the cam side with the longer slot. The
> shorter loop of the heavy spring gets connected to the weight not to the
cam
> slot.
>
> Happy Memorial Day
>
> Joe
>
> Sent from my iPad
>
> On May 28, 2012, at 1:35 PM, Dana Scribner <dscrib2@hotmail.com> wrote:
>
>> This being Memorial Day I hope everyone has taken the time to thank those
> who
>> gave their lives so we are able to live ours as we do. I have been
having
> a
>> problem with the idle on my 1969 2000 SU carb no emission car. I can set
it
>> then go for a ride and when I stop at a light the idle keeps dropping off
> it
>> 100 or less or it stalls. At high idles it's fine. I gave up and took it
> "The
>> Guy" wit 40 plus years on SU knowledge. Long story short he thought is
is
> a
>> distributor problem. I opened it up and there it was a broken spring
> sitting
>> at the bottom. So this leads me to my questions. On the timing lever
plate
> it
>> is marked 7.5; I looked at my supply of parts and some are marked 17.5.
>> Why?Are the weights different on the 2 distributors? How do you tell them
>> apart?Are the springs different between the one marked 7.5 and the 17.5
>> distributor? How do you tell them apart?Where the vacuum advance is
> attacked
>> to the housing I see the a 17.5 stamped in ink on the 17.5 marked timing
>> lever. Are the 7.5 ones not marked?is the vacuum advance different
between
> the
>> 2? Dana S1969 2000
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