If the thing runs good throw the gauge away and drive the wheels off of it.
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| vacuum https://slantsix.org/forum/viewtopic.php?t=53937 |
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| Author: | '67 Dart 270 [ Sun Nov 24, 2013 12:10 pm ] |
| Post subject: | vacuum |
I have a 225 with dual 2bbl Webers. Newly rebuilt, runs great. Just checking my vacuum at the intake and at idle I'm getting 30 cm Hg, about 12 psi vacuum. The vacuum is very steady, no needle jumps whatsoever. My vacuum gauge has the "green" area that says "normal" vacuum, starting at about 43 cm Hg (17-22 psi), so I'm way short of that. I've checked for vacuum leaks, the intake shouldn't be leaking as I have a remflex and it's at about half crush, the carb gaskets are snug, extra ports blocked off with new snug rubber caps. I don't think there's a leak. The cam is an "RV grind" I'm told, a "chevy" style as the intake and exhaust lash are both 0.012". I'm set at 10 deg BTDC on the timing, it idles very smoothly. Is my vacuum OK or should I be changing something? |
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| Author: | Sam Powell [ Sun Nov 24, 2013 12:16 pm ] |
| Post subject: | |
I don't speak psi or cm's of mercury vacuum. Mine idles at 18 inches with a similar cam. How does that convert? Sam |
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| Author: | '67 Dart 270 [ Sun Nov 24, 2013 12:32 pm ] |
| Post subject: | oops... |
Sorry, mine is at 12 inches of Hg. |
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| Author: | DusterIdiot [ Sun Nov 24, 2013 1:32 pm ] |
| Post subject: | Ouch... |
Quote: Sorry, mine is at 12 inches of Hg.
That's awful for a comp cam, something is really off, either the cam degree process was off or the timing is off (but you're driving it so it's not off).Where is the reading taken from? (port at the carb base or at runner #6 vacuum tree?) Does the reading improve at higher RPM? Does it improve with a looser valve lash? -D.Idiot |
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| Author: | '67 Dart 270 [ Sun Nov 24, 2013 3:47 pm ] |
| Post subject: | reading |
I'm taking the reading from a port between the two carbs on the intake (Offenhauser), should I be taking it from the #6 port? It runs wonderfully, I was surprised as well. |
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| Author: | '67 Dart 270 [ Sun Nov 24, 2013 3:50 pm ] |
| Post subject: | Offy intake |
That port I'm using is not in any of the runners, it's between the carbs, maybe that's the issue? Maybe it's a relative dead spot, i.e. downstream from where the real vacuum is - the runners? At higher rpm it improves slightly then goes down from there. bg |
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| Author: | '67 Dart 270 [ Sun Nov 24, 2013 4:04 pm ] |
| Post subject: | photo of port I used... |
See in the center, the brass fitting. That runs to my PCV via an oil catch can. I took off the hose and put my vacuum gauge on there. http://s785.photobucket.com/user/67dart ... sort=1&o=0 |
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| Author: | WagonsRcool [ Sun Nov 24, 2013 7:35 pm ] |
| Post subject: | |
What's your local altitude & barometric pressure? The generic reccomendation for 18-20" of vacuum at idle is dependant on near sea level baro of 30". As altitude increases- baro decreases & engine vac goes down proportionally (as does engine compression pressure). Otherwise late cam or ignition timing will reduce vacuum(as will a "big" cam) . |
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| Author: | '67 Dart 270 [ Sun Nov 24, 2013 7:51 pm ] |
| Post subject: | altitude |
We're at about 900 feet above sea level, so I don't think that's it. Before I get all panicky...is there any chance that vacuum from the port I'm using is not the proper place to measure vacuum? The two other things are late valve timing, but I'm at 12 BTDC so I don't that's it, and low compression. I can certainly do a compression test, but it appears to have lots of power when I'm driving it, it only has 1500 miles on it, but the rings surely have seated by now, break in went smoothly, oil change after break in and then after 500 miles, oil is clean as a whistle. It runs great, idles smoothly, I'm getting close to 20 mpg, no smoke, no oil leaks. AFR is set within specs. If I had a vacuum leak of that magnitude, to drop vac from 17-20 range down to 12, wouldn't it idle rough or something? It's as smooth as glass at idle. I can put a nipple in the #6 runner and check vac there...thoughts? Any chance my gauge is just wrong...maybe I should check that first. bg |
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| Author: | slantsik [ Mon Nov 25, 2013 12:51 am ] |
| Post subject: | |
I am having similar results to yours. I have the standard 1bbl set up. Gauge says 12hg at idle and steady as a rock. drops to about 5hg on acceleration then op to about 25 hg on de- celeration. compression test shows 165-170 psi on all 6. I will do some vacuum leak tests soon ,but I suspect the gauge. Try putting the gauge on another vehicle ,or borrow another gauge. Brendan. |
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| Author: | Aggressive Ted [ Mon Nov 25, 2013 7:35 am ] |
| Post subject: | |
Maybe the way the gauge is calibrated....very odd! At 12 degrees initial with vacuum leaks (original steel intake gasket) mine shows 19" at 750 rpm idle. 1920 Holley one barrel. |
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| Author: | slantzilla [ Mon Nov 25, 2013 8:53 am ] |
| Post subject: | |
If the thing runs good throw the gauge away and drive the wheels off of it.
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| Author: | '67 Dart 270 [ Mon Nov 25, 2013 11:44 am ] |
| Post subject: | the gauge... |
Take a look at the gauge....is it just me or should the needle be at zero to start with...? http://s785.photobucket.com/user/67dart ... 0.jpg.html Do vacuum gauges get calibrated like ohm meters? That extra distance between zero and 0.2 would put me dead in the green zone... Sorry if I was just an idiot on this one... BG |
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| Author: | '67 Dart 270 [ Mon Nov 25, 2013 12:25 pm ] |
| Post subject: | conversion... |
So, my pressure gauge is reading about 3 psi when it's not hooked up, and 3 psi converts to 6 in Hg, so 12 + 6 puts me at 18 in Hg...does that sound better? Who knows how far off this gauge really is.... |
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| Author: | Joshie225 [ Mon Nov 25, 2013 12:32 pm ] |
| Post subject: | Re: conversion... |
Quote: So, my pressure gauge is reading about 3 psi when it's not hooked up, and 3 psi converts to 6 in Hg, so 12 + 6 puts me at 18 in Hg...does that sound better?
Yeah, that's much better. Nice gauge. Who knows how far off this gauge really is.... 
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