Which port do I use for vacuum advance? Holley 1850 - Team Camaro Tech
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post #1 of 12 (permalink) Old Jul 22nd, 03, 09:10 PM Thread Starter
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Red or Yellow?

I have this 600cfm on my 4.3 liter as a temp solution to some idle problems. The red one pulls vacuum, and the yellow doesn't do anything...

I have this set up with a spread bore to square bore adapter, will that cause the yellow to not pull vacuum?

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post #2 of 12 (permalink) Old Jul 23rd, 03, 02:25 AM
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Yellow. The port is above the throttle plates, thus, as throttle goes open, it'll have a vacuum signal on it at rpm's above idle. Check it with a gauge when installed. Going from a square-to-spread bore pattern does require an adapter plate to be used to prevent vacuum leak. Summit has this plate, 1/16 inch thick.

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post #3 of 12 (permalink) Old Jul 23rd, 03, 06:09 AM Thread Starter
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the Yellow is not pulling vacuum off idle, could it be clogged?

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post #4 of 12 (permalink) Old Jul 23rd, 03, 06:29 AM
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Either one of two, the yellow can be clogged, or not enough rpm's to produce a vacuum at the port. Usually starts around 1200-1500 rpm with stock camshaft. OR leak in baseplate to manifold interface.

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post #5 of 12 (permalink) Old Jul 23rd, 03, 08:52 AM
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The red is probably manifold vacuum and the yellow one should be ported. If the yellow one has vacuum at cruise but not at idle, it is ported. Vacuum at idle is manifold vacuum. You can use either tap but I prefer the manifold vacuum tap when you have a lumpy cam as it improves idle quality and reduces temps at idle. Otherwise, the performance would be the same but the tuning is slightly different. [img]smile.gif[/img]

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post #6 of 12 (permalink) Old Jul 23rd, 03, 11:48 AM Thread Starter
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ok, the Yellow does not pull vacuum at any rpm
the Red pulls 9" of Hg at idle.
Also the vacuum secondaries are not functioning. I can move them by hand, but reving the engine does not open them. I need a vac source for the vacuum advance and also a tip in switch for the TH-400 I believe.
Just to recap, the set up is Holley 1850, gasket, squarebore to spreadbore adapter, gasket, then intake. I just got an email about how to set up the vacuum advance to work off of manifold vacuum. I'm gonna try it and I'll let you know how it works.


I could try puting my 670 holley street avenger on it, but it's only a 262ci motor....
I still need to take apart my old carb, to see if I can find the problem.

[ 07-23-2003, 02:12 PM: Message edited by: JIM68 ]

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post #7 of 12 (permalink) Old Jul 23rd, 03, 12:01 PM
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its the yellow, there is no vac. at idle. if there was, you would start advancing. you want the advance to start afterwords. i rode in dude's car that was set up on the other port it was sucky.

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post #8 of 12 (permalink) Old Jul 23rd, 03, 01:35 PM Thread Starter
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Ok, I got the top(yellow) one to pull vacuum off idle... [img]smile.gif[/img]
Now I just need to figure out how to get the vacuum secondaries to function.

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post #9 of 12 (permalink) Old Jul 23rd, 03, 06:56 PM Thread Starter
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this is the carb plate i'm using:

http://store.summitracing.com/partde...58#largerimage

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post #10 of 12 (permalink) Old Jul 23rd, 03, 11:29 PM
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The secondaries will not open when you blip the throttle. You must be actualy driving with a load on the engine for them to open.

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post #11 of 12 (permalink) Old Jul 24th, 03, 02:22 AM
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You might put a small tyrap, 4", on the secondary diaphargm shaft and butt it against the body.

Then when you go test the secondary opening, return to the garage, check for distance between the body and the tyrap to see if secondaries actually open(ed). Look for movement of 1/4-5/16 inch.

You will need alot of engine load, or a condo cruiser (heavy car, i.e., Impala, Caprice, Buick Park Avenue, Olds 98, etc.).

With 9" of vacuum at idle, you either have a crummy running engine or not a stock cammed engine. It will take some fine tunng to make this 1850 work. Use a feeler gauge to open the secondaries, at the setscrew by the diaphargm shaft. This will allow the primaries to close further, thus allowing the idle mixture screws to do their intended function. After adjusting for best tidle, measure the height of the secondary shaft bracket and turn setscrew to this new height. Sometimes if you remove the setscrew and install from the top, this helps to do adjustment

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post #12 of 12 (permalink) Old Jul 24th, 03, 05:54 AM Thread Starter
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I have 9" at the the carb, but 20" directly at the manifold. I'll keep checking stuff. When I had the other carb on, if I gunned the throttle all the way I would see the secondaries open.

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