. Played around quite a
>bit with the adjustments (mostly adjusting for max manifold vacuum at
idle)-
>but there
>was a point beyond which improvement just didn't seem to be happening.
> Because the truck had always run so well at full throttle, I had assumed
>that the ignition was doing fine. But while hooking the vacuum advance up,
>it occured to me that in addition to the secondary barrels of the carb
>opening up at full throttle, the timing was also changing because of the
>vacuum advance. So, I loosened it (the distributor) up, and starting
>experimenting with it- I haven't found any timing marks, and so I'm only
>guessing at this point. Sure enough, I could set the distributor so the
>hesitation was almost gone, response came on much sooner... but it idled
>like crap.
> The very best I could get, and where the truck is set now, is with the
>vacuum advance just disconnected.
> The motor has an Edlebrock Torquer intake manifold, tube headers, an
>Edlebrock air filter, and has almost certainly been cam'd. I'm thinking
>that because of these changes, the intake manifold vacuum profile may be
>very different than whatever stock motor my distributor came off of. So
the
>vacuum advance curve could be all wrong?? Does this make sense? I assume
>my distributor also has a (perhaps non-functioning) mechanical advance as
>well, but it probably is a weak advance, given that it counts on help from
>the vacuum advance too.
> I'm really tempted to just swap it out for a mechanical advance only unit
>(probably a Mallory Unilite: $150 from Summitt). But before spending the
>cash I'd thought I'd bounce this stuff off of you experts out there and see
>if there's maybe something simpler I'm missing and need to check first. I
>don't have much (well OK, any) experience with small-block chevy's.
> My theory right now is that the timing is getting advanced too far, too
>quickly. At partial throttle, especially at low RPM's, the vacuum is
>advancing the ignition too far, causing detonation and rough running. Open
>it all the way up, and the vacuum disappears from the manifold, the
ignition
>retards, and all is well again. Make sense? Oh.. yes, I do have the
vacuum
>advance hooked up to a port on the carb, upstream of the throttle plates.
>
>Ryan
I ran a mechanical-advance-only Mallory Unilite for some time and do NOT
recommend this for street use. I replaced it with a vacuum advance HEI and
the motor runs MUCH better. Mechanical advance alone cannot provide optimal
engine timing for light loads (i.e, idle and "cruise") where you need
another 20 degrees or so advance
The vacuum advance can which is stock with your distributor is probably not
perfect, but shouldn't cause driveability problems such as you describe.
You can get an adjustable vacuum advance from Mr Gasket, but It sounds like
your problems are elsewhere. It sounds as if your initial advance is set too
high; thus when you disconnect the vacuum advance the motor runs better.
Although, you didn't mention two other symptoms of too much advance:
detonation ("pinging") and slow cranking.
Assuming that all is well with your carburetion (home-made parts?!) I'd
recommend that you go through the drill of finding TDC, making some timing
marks on the front pulley, and getting your initial timing set correctly,
that way you can eliminate that as a possible cause of the poor running. You
can check the advance with a timing light as you increase engine speed.
Again, don't throw money away on a mechanical advance distributor - there is
often a perception that since many "high performance" i.e race car,
distributors do not use vacuum advance, thus it is not desirable. Not true
in most cases.
Jack / Winter Park FL
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