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Unread 04-05-2005, 10:08 PM   #1
Dimension4
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Join Date: Sep 2004
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Manifold Vacuum Spec?

In diagnosing my ride, I am trying to see if I have a manifold leak. I am running 15 inches of mercury (Hg) on a 304 with an edelbrock torker manifold and 650 4 barrel (if it matters). Am I in spec?

Also, when you set the timing to 8 degrees BTDC, does it idle like crap until the vacuum advance is connected? It would help me a lot to know.

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Unread 04-06-2005, 09:51 AM   #2
John C
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Don't know the spec, however if you spray starting fluid around the base of the carb and anywhere else you suspect a leak you will hear a noticable difference in idle. Lets you locate the source area quickly also. The Edelbrock carb is very finicky. I have the same one (performer 650/electric choke) I am about done with it. It has incredible tuning capabilities but mine doesn't seem to hold a setting. I am constantly fiddling with the thing. When it's on it's great, but the next time I drive it it may not be the same. And I don't have vacuum leaks. You should time you engine without the vacuum advance. In fact the vacuum advance only works durring part thottle cruise.
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Unread 04-06-2005, 10:24 AM   #3
MR.CJ-7
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I run between 15 and 17" Hg at 800 rpm depending upon carb setting. I'm running a 304 with a MC2100 2bbl carb. My timing is much higher, around 14 or so. I don't remember off hand, but I know it's pretty high.
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Unread 04-06-2005, 02:00 PM   #4
Dimension4
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Ok, maybe I'm not too off. Do you run 14 degrees with or without vacuum? Will my torker manifold reduce my manifold pressure?

Also, I tried the old "spray starter fluid" trick, but the thing idles pretty erratically as it is, so I can't really tell. I was hoping the manifold vacuum pressure would be a better indicator. If you're running 15-17"Hg on a 2 barrell, then maybe my 15" should be ok?
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Unread 04-07-2005, 10:43 AM   #5
MR.CJ-7
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I time mine with vac. advance disconnected. I don't have any experience with the torker and what it will do to vacuum. It sounds to me like you may have a leak or maybe a really "hot" cam if you have an erratic idle. Mine idles very smooth.
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Unread 04-07-2005, 11:09 AM   #6
CJ5_Fan
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I am no expert, but I have a 304 w/ mc2150 and I run at 15 HG and 12 BTDC.

It runs like crap with the vacuum advance disconnected (as it should) mainly because of the way a distributor is designed to work via ported vacuum instead of raw, and the physics behind ingnition/combustion and the dizzy advance requirments at speed.

I have found that using a vacuum guage to time my motor instead of a light makes me a bit to far advanced (adjusting dizzy for highest HG) and I Ping a bit so I run it at 12 not 14. At 8, I have no umphf......

I have been told that 15 is a bit low on a stock 304 (performance cams also tend to make HG lower) and I have acutally added a vacuum storage canister to assist with my power brakes (which seems to have helped) but my motor seems okay and overall, I run fine with no signs of vacuum leak anywhere. I have not done a compression check to see if I have worn rings, but via vacuum testing, I am pretty confident my valves are okay (with a vacuum guage, you can really test alot of things on a motor related to wear and adjustments)

So to net it out, 15 works for me, and as long as you are running okay and have no vacuum leaks evident, then I wouldnt worry about.

Just ensure the dizzy is around 10-14 unless you are a CA vehicle and trying to pass smog (then it should be 5-8 BTDC) and ensure your dizzy is on ported vacuum. The 304 has a 3 port CTO switch on the manifold. Put it on the center one marked with the D. Some people attach to the EGR port on the front of the carb as well since that is also ported. Set base timing with the dist. vacuum disconnected and Idle at about 800 RPM. Once set, re adjust your carb using your vacuum guage for the higest HG being careful to only adjust outward until the HG level is reached, then go back in until it falls, then back out again just until the higest level is first achieved again. That sets you to the best lean condition and should work fine for you. If it wont idle and you choke is correct, then check your carb internals for wear

Good luck.
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