Hello Everyone-
My 55 2nd has a 350 small-block, installed by the previous owner. It's
always run very strong wide open, but has hesitated, shook, and run really
terrible at partial throttle. I assumed it to be problems with the
quadrajet (44 year old fuel tank, and no fuel filter when I got it) and so
tore it off for a rebuild. Confirming some of my suspicions, I found some
humorously fabricated (home-made) parts in the carb, a few things (like
float level) WAY out of adjustment, potential sources for a couple of vacuum
leaks, and a couple of stripped threads which I repaired with small
heli-coils. So, I was hopeful that the carb rebuild would solve my
problems.
Bolted it back on, cranked it over and the truck fired to life almost
immediately. A test drive showed some improvements, but there was still a
lot of rough running and partial throttle hesitation. 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.
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