I recently posted to know how to optimize ignition timings on
my 350 CI - L82 ( 1973 Vette ), Manual transmission.

So here's what I did, following the ( good ) advices I was given :
Set total timing advance (centrifugal + initial ) to 36° BTDC @ 3000 RPM
As my maximal centrifugal advance is 28°, that gives my a 12° BTDC advance at idle, which is 4° more then what i had before but as you all say : "I don't mind"
Hooked the vacuum advance to manifold instead of ported vaccum on the carb. My diaphragm vacuum unit is adjustable and I set it at :
0° @ 9"Hg
15° @ 11" Hg
My carb is a brand new Holley #0-4776 ( Model 4150 ) double-pump, square flange, mechanical secondaries ( 600 CFM ).
It is mounted on a Holley Street Dominator intake manifold. It's a 360° ( single plane ) manifold and I know it's not the best choice for a "street use" as it gives its
best performance only after 2000 RPM, but it was already installed on the car when I bought it, so...
I only get about 8 to 9"Hg of vacuum at idle ( 950 to 1100 RPM ) but I think it is normal for this kind of manifold.
That also means that the vacuum advance unit is not active at idle...
After a few tests on the road, the results are that the engine performs better than before, except for one thing ( and I had this problem before tuning ) :
When the engine spins at 1600 - 1800 RPM, it has a short hesitation at WOT. It doesn't happen if I do the same thing at 2500 or more RPM.
There is no detonation when the problem occurs ( which could be a lean condition ) and I don't get either a " black smoke" behind the cars ( which could be a too rich condition ).
I know that lars said that these kinds of manifolds can be a real challenge to tune for street RPM use, but maybe someone has an idea to fix it ?

 

It sounds like you need to work on the accelerator pump squirt a little. Double - pumpers need a big shot.

 

Low vacuum at idle speeds is more a function of camshaft timing than the type of intake manifold you have. This sounds like the power valve is not opening soon enough. That may be why it happens at the lower RPM and not the higher. Your cruise metering is a little lean. The progression goes like this, idle gives way to transision which gives way to cruise metering which gives way to the power system. The power piston is designed to cover up the holes between these systems but it can only do so much. Power piston tuning can get rid of this problem but I think the real answer is to fattin up the transision/cruise metering circuits. This is power valve tuning.

BigBlockk

Later.....

 

It seems that I solved the problem with your help,
many thanks to all of you guys
Here's what I did :
I slightly modified the pump cam on the primary only, so that the pump shot starts earlier.
As my carb was equiped with standard .028 & .032 pump shooters on primary & secondary respectively, I tried larger & larger shooter sizes.
I stopped at 0.039 for both, as I didn't have any more bog or hesitation with this size.
I red that Holley recommends to use the "hollow" screw (PN-26-12) when shooter sizes over .039" to allow more fuel to flow to the shooters.
I will try this as soon as I can...

 

I think you're still conservative on your timing curve. On my '73 L82, I was running 18 degrees advance at idle, with an additional 20 degrees of distributor advance all in by 2600 RPM for a total of 38 degrees from 2600 rpm up. Doing this really woke the lower part of the torque curve and upped the vacumn at idle (900 rpm) from 10 inches to 13 inches. More advance at idle equals more vacumn and a steadier idle as well as more power off idle.
To be able to do this you need to change out the bushing on the top of the distributor shaft (on the underside of the top plate that the rotor is attached to). This will change (limit) the amount of distributor advance from the 28 degrees you have now (which is too much) to about 20 degrees or less. My Crane distributor curve kit came with the bushing.
The reason that your L82 has low idle vacumn is because it has the combination of a fairly healthly duration cam (approx 222 (?) degrees at .050 lift intake) and low compression (9:1 or less). Extra spark advance is a good crutch.

 

Sounds pretty good Red73Vert
And at a so much low cost I need to try and see.
You're right when you say that the torque is very low under 2000 RPM on my engine, so a little extra would be welcome at low end RPM.