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Discussion Starter #1
I have a 383 with a radical (what I think to be radical) cam, although I do not know the specifics on lift, duration, etc--just that it is a roller with a very "lumpy" idle...

I measured the manifold vacuum (I just rebuilt the carb--Holley 650 DP, so I am now in "tuning mode"), and it is only showing at around 5 inches of vacuum at idle (this is with the initial 1.5 turns on the idle screws as a starting point)...

How do I (do I need to) improve the vacuum at idle? I do not run anything that requires vacuum except for the PCV system, and the vacuum advance on the dizzy.

I should also probably note that I have set the initial timing at 14* BTDC--I do not know "total" timing as my damper is not degreed, but the HEI appears to be stock (but for the ACCEL Super coil)...
 

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Since your idle vacuum is that low, and your hei distributor appears stock, you should probably buy the correct can for it, having the right can will make a world of a difference.

As for the idle vacuum... you can try advancing the timing to 16*-20* initial and it should pick up a few (probably a band aid fix). When I had a single plane and a 236 @.050 110lsa cam in a smaller ci motor, I couldn't get the vacuum above 6" no matter what I did. Didn't seem right, and it really messed with my vacuum advance.

Get the correct power valve.

From what I have seen, the stock type hei unit's advance curve comes in really quickly and you can get well over 60* (total) if you are not careful, especially with the wrong can.

Timing tape is cheap but will probably have to be ordered, unless you are skilled enough to mark your balancer in degrees by hand.
 

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Discussion Starter #3
aside from an occasional backfire pop while warming up, the car actually runs very well (rough idle excepted, but that is more cam than it is carb/dizzy)...

Of course, I've only had it around the neighborhood while I was "dialing in" the timing/carb/idle, so I cannot yet speak to highway performance, but she seems oK around the block...
 

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aside from an occasional backfire pop while warming up, the car actually runs very well (rough idle excepted, but that is more cam than it is carb/dizzy)...

Of course, I've only had it around the neighborhood while I was "dialing in" the timing/carb/idle, so I cannot yet speak to highway performance, but she seems oK around the block...
A must read is John Hinckley's "Timing 101" I have it on file. It's invaluable. Do a search here, it will come up a million times.
 

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Good off-idle performance is a combination of timing and carburetor adjustments.

Earl Parker of Earl Parker Carburetion sent me the following setup memo. We used it on a 4779 Holley used on a 331" road racing motor and it worked very well.

One method of setting up your idle system is a follows:
To start with, invert the carburetor and check the position of the throttle
butterflies. Turn the idle speed setting screw to set the bottom edge of the
primary throttle butterflies about .020" from the bottom edge of the transfer
idle slot. Don't worry about measuring anything - your eyeball is good
enough. Positioning the throttle butterflies near the bottom of the transfer
idle slot at curb idle is absolutely critical for maximum acceleration.
Next, turn the idle mixture needles in until they are lightly seated. Excessive
force here will damage both the needles and metering block and make the
idle fuel mixture difficult to set with any accuracy. After seating them turn
them out 1 1/4 turns, which is a good baseline setting. Now you're ready to
reinstall the carburetor and setup your idle system.
Before you start the engine, examine the fuel bowl side of the throttle body.
Hopefully you'll see a little tube, covered by a rubber plug. This vacuum port
connects with a passage in the throttle body that 'sees' manifold vacuum.
Remove the plug, attach a good vacuum gauge to the port and position the
gauge where you can see it clearly. Don't forget to zero out the gauge.
Without touching the carburetor, turn the engine over until you have
pumped fuel into the bowls. Work the throttle a few times then start the
engine. If it dies, which is likely, you'll have the turn the idle speed setting
screw to increase the RPM to get it to idle while it's cold. Since throttle
butterfly position is critical, count the turns and fractions of turns so you'll
know exactly where you're at. The whole idea is to be able to return the
throttle butterflies to the position you originally set them at. As the engine
warms up it should gain rpm, so you should be able to reduce the throttle
opening at least somewhat without the engine dying. Now the fine tuning
begins.
With the engine idling, pick one of the idle mixture needles and turn it in 1/4
turn while you're watching the vacuum gauge. Give the idle a few seconds
to stabilize. If manifold vacuum increases repeat the process, letting the idle
stabilize each time, until it starts to decrease. If turning it in hurts manifold
vacuum then try turning it out. When you've found the 'sweet spot' (i.e. the
manifold vacuum is as high as you can get it) repeat the process with the
other idle mixture needle.
Presumably you'll be able to pick up enough idle speed by optimizing the idle
fuel mixture that you can close the primary throttle butterflies down to their
original position near the bottom of the transfer idle slots.
As a final check give each idle mixture needle a slight turn in then a slight
turn out. If any motion hurts manifold vacuum, you know that needle is set
properly. At this point if the idle is stable and the engine responds quickly
when you just crack the throttle, you should be good to go.
One final note: Make sure your timing is set correctly before starting this
process.
If your distributor has a mechanical advance system there is a much better,
though more involved, way to setup the idle system.
Position the throttle butterflies and idle mixture needles as described above,
attach the manifold vacuum gauge to the vacuum port and start the engine.
Turn the idle speed screw to increase the RPM, again taking note of exactly
how much you have to turn the screw to open the throttle butterflies
enough for the engine to idle while it's cold. Allow the engine to warm up,
the close the throttle butterflies as much as reasonably possible without the
engine dying. Attach a timing light, check to see how much initial ignition
advance you have and make a note of the figure.
Next, loosen the distributor hold down clamp and turn the distributor so as
to increase the initial ignition advance. When the initial ignition advance is
increased the RPM should rise as well, allowing you to reduce the throttle
butterfly opening. Simply turn the distributor to increase the initial ignition
advance and continue to reduce the throttle butterfly opening until they're
in the original, correct position and the engine is idling at the desired RPM.
Lightly snug the hold down clamp to make sure the distributor can't move,
then adjust the idle mixture needles for best manifold vacuum. Once they're
properly set if the idle RPM is higher than desired, loosen the hold down
clamp and turn the distributor slightly to achieve the desired idle RPM.
Recheck the idle mixture needle position then tighten the hold down clamp.
Once the idle system is setup you'll need to correct the distributor's
advance curve. The first step is to attach a timing light and recheck the
initial ignition advance. Let's say, for example, that it was originally 15° and
now it's 22°, a 7° increase. If your total ignition advance was originally 35°,
in order to keep that figure the advance curve will have to be shortened by
7°. Assuming you have a centrifugal advance system you'll have to limit how
far the advance weights can move outward, which will limit the total
advance. The method required will vary from distributor to distributor, so I
won't get into that here, but any competent technician with a good
distributor machine should be able to do it for you.
If you don't have access to said technician/distributor machine and you can
come up with a way to limit the outward motion of the advance weights,
you can do the same thing using your engine as form of distributor machine.
Limit the motion of the weights somewhat, make sure you have the correct
initial ignition advance then check to see how much total ignition advance
you have. If the total ignition advance is still too high, just continue to limit
the motion of the advance weights until you achieve the desired total figure.


AED has some good tips on carb setup, also.

http://www.aedperformance.com/Tuning Tips.htm
 

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I was able to get mine from 8" up to 12" just with idle circuit tuning. I'm running 16 degrees initial with vacuum can hooked to ported vacuum and PCV. Getting your idle up to 800-1000 or so if you're not already there will pull the vacuum up some as well.

The PCV valve you'll probably want is a 736V (variable) and is what I'm running. It's what is specified for most of GM's big cam motors....302ci, 375hp 327ci, etc.

Are you at 14 degrees initial with the vacuum advance disconnected and plugged? If so, that's probably a pretty good place. A stock HEI should have around 20 degrees of mechanical advance in it. And I'm with Steptoe on one thing: keep those GM weights if you have them. I haven't found any others that are worth a crap other than the ones that came in my MSD HEI.
 

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onovakind67
Pst above..that 1st paragraph GET THAT RIGHT on the se butterflys....
the 20/100 is the starting piont....
Find out how many cent degres u have in the hei...set the dizzy at the correct all in degrees....that will be your intial cent idle...(No VA)
Set the mixture and idle stop to middle of factory....and adjust for best idle with the sec butterfly stops.....connect the VA and fine tune the mixture /sec idle .
"And I'm with Steptoe on one thing: keep those GM weights if you have them."
Thats a definate....and use stock bushes or even turn a couple up out of brass.
 

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Discussion Starter #8
I was able to get mine from 8" up to 12" just with idle circuit tuning. I'm running 16 degrees initial with vacuum can hooked to ported vacuum and PCV. Getting your idle up to 800-1000 or so if you're not already there will pull the vacuum up some as well.

The PCV valve you'll probably want is a 736V (variable) and is what I'm running. It's what is specified for most of GM's big cam motors....302ci, 375hp 327ci, etc.

Are you at 14 degrees initial with the vacuum advance disconnected and plugged? If so, that's probably a pretty good place. A stock HEI should have around 20 degrees of mechanical advance in it. And I'm with Steptoe on one thing: keep those GM weights if you have them. I haven't found any others that are worth a crap other than the ones that came in my MSD HEI.
It is 14 initial with advance disconnected and plugged...which way do I adjust the idle mix screws to increase vaccum--in or out?
 

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It is 14 initial with advance disconnected and plugged...which way do I adjust the idle mix screws to increase vaccum--in or out?

On mine, I went out but that's because they were too far in out of the box on a Street Avenger.

Use the vacuum gauge on the manifold vacuum port like you are now and just turn one out at a time, but only an eighth turn at a time. If vacuum goes up, turn the other one out the same amount. If you didn't already, it might be good to bottom both of them out and then make sure they're turned out the same amount to start.

What's your idle speed?

I think mine were out about 3/4 turn to start and I didn't have to go out much further than that. Maybe a half turn or so.
 

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Discussion Starter #10
My idle speed is about 800 RPM, and started turne dout 1.5 turns on both sides
 

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It is 14 initial with advance disconnected and plugged...which way do I adjust the idle mix screws to increase vaccum--in or out?
No one will be able to tell you that because it is entirely based off of the reading from the vacuum gauge.
 
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