My 1st Big Cam/ Trying to adjust Carb/Low Vacuum - Team Camaro Tech
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post #1 of 21 (permalink) Old Oct 2nd, 02, 06:56 AM Thread Starter
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I just got my rebuilt motor back and I just fired it up and got it running. Tried to adjust the air fuel mixture screws and couldn't get any more than 6 in vacuume. I have 396 .30 over Rect port heads (840's). Motor is 9.3-1 with comp cam xe284 (240,246 @50) (574,578 lift) and 110* lobe sep. This is the first big cam I've ever had and didn't know if I just have my carb set wrong or if this 6 in vacuume is correct. What could cause this vacuume to be so low?
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post #2 of 21 (permalink) Old Oct 2nd, 02, 07:15 AM
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hey perks you should be able to get more than that out of it, i have a 454 rect port heads with comp cams 294s cam, .595 lift and 248 [email protected] and i pull 10" at 950 idle. where does yours idle?? if you have idle screw adjusted all the way out you might be into the primary circuit, check that as you should have more vacuum, goodluck
Jake
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post #3 of 21 (permalink) Old Oct 2nd, 02, 11:53 AM
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Your engine is going to like a lot of advance real quick.Try shortening the amount of advance in the dist and setting the inital at about 20degs that will help. Make sure the carb is controlling the idle with the idle circuit. If it isn't you may have to drill the throttle blades. You have a lot of cylinder head and cam for a 396 with only 9.3 to 1 compression it may not get much better no matter what you do.

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post #4 of 21 (permalink) Old Oct 2nd, 02, 02:23 PM
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That vacuum might be about it. I agree with Oger, I would think that you would want at least 10:1 with that cam. You've got a lot of duration there.

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post #5 of 21 (permalink) Old Oct 2nd, 02, 02:43 PM
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i agree also, i have about 10.25-1 comp in my 454 and rect port heads, and my cam likes about 18 degrees of intial timing, goodluck and let us know
jake
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post #6 of 21 (permalink) Old Oct 2nd, 02, 03:06 PM
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Your total timing should be independent of the idle speed, and is set at a point at which the distributor is at max advance. The initial timing is based on what the engine likes at idle. Earl Parker wrote a nice paper on setting the idle mixture/initial timing that I copied. Do you have vacuum advance?

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.




------------------
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406, 10:1, 224 cam, Q-jet, 700R4, 3465# w/driver
11.75 @ 117 thru the mufflers
18 mpg on the road
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post #7 of 21 (permalink) Old Oct 2nd, 02, 03:40 PM
 
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I not sure if this will help your problem, but I had the same problem last week. On my camaro I was reading a "6" on the vacuum gauge. My 350/300 has a BIG cam, causing a low vacuum, the fix was replacing the power valve in my holley carb. The holley part number is "125-85". I now have a 13 reading on the vacuum gauge.

Hope this helps.
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post #8 of 21 (permalink) Old Oct 2nd, 02, 07:26 PM
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I agree in principal with onovakind67's method but have run into some problems doing it that way. First warm up the engine before you start. Second with a really big cam you end up with so much advance it won't start. I usually set the initual advance to a reasonable level. Then I set the throttle blades to .020 and try the idle. If it is too low start by drilling 1/16 in holes in the primary throttle blades. If it is still too low go up 1/32 in at a time until it is where you want it. If the cam is really big (over 280 at .050) you may find it better to drill the sec. blades also.

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post #9 of 21 (permalink) Old Oct 3rd, 02, 04:06 AM
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I originaly set my timing the way the article describes and I started breaking starters. I now use a vac adv cannister once again. If you disabled your vac adv. I suggest you look into getting one on the engine. It did wonders for my idle quality!!!

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post #10 of 21 (permalink) Old Oct 3rd, 02, 04:07 AM
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Oh by the way you didnt mention if you broke the cam in or not?

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post #11 of 21 (permalink) Old Oct 3rd, 02, 06:14 AM
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I didn't hear anyone suggest runnng manifold vacuum advance. Manifold vacuum advance will add timing at idle which increases idle speed. Then you can turn the idle speed screw down to where you want it which hopefully be low enough to get the carb idling on the didle circuit once again. I'd try this before drilling the throttle blades.
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post #12 of 21 (permalink) Old Oct 3rd, 02, 07:08 AM
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He said Didle

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71 BB SS Chevelle (clone)
2002 35th anniv SS Z28 conv.
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post #13 of 21 (permalink) Old Oct 3rd, 02, 07:10 AM
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Yes using manifold vacuum advance will clean up the idle with a big cam but I have found it to end up being a real pain. If there is less vacuum (cold motor or you let the clutch out too fast or such) you loose the vacuum advance and the engine will refuse to run at idle. You end up playing with the throttle to keep it running. I will usually run ported vacuum and set the idle as described before. That seems to work best for me.

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post #14 of 21 (permalink) Old Oct 3rd, 02, 10:49 AM
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Good point Oger . . .
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post #15 of 21 (permalink) Old Oct 3rd, 02, 11:46 AM
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Oger what do you feel would be an appropriate starting point as far as advance goes. I set mine up at a low (5*) starting. point and hoped to use manifold vac adv to maintain an adequate base idle. So far it has but I dont have any real time under my belt. I dont have a ported source but if i did It wouldnt have vac at idle would it ? and even at acceleration, ported would never provide more than available (manifold) vac would it?

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