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| engine vacuum https://www.slantsix.org/forum/viewtopic.php?t=50191 |
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| Author: | kielbasa [ Fri Oct 26, 2012 6:40 pm ] |
| Post subject: | engine vacuum |
How much vacuum should a stock motor pull with a single barrel carter. At 800rpm I'm only getting 12lbs, steady needle. Seems low to me? (I do have a small reverse split cam) That's with the dist vacuum pod disconnected. Notsure if i should be tee-ing it off or not. Idle does drop when dist is disconnected and vacuum gauge connected. Was at 5degree timing, bumped it up to 10 to see if i could loosen idle screw a little. Did notice quicker accel. I'm just trying to fiddle with this thing. Been trying to set choke every morning but its not really progressing. With the choke flap bracket bent all the way to passenger side. I can still close flap by hand and idle raises (older piston type pull off). At a certian point of tweaking bracket it stops making progress as to pushing flap closed to raise the idle. And while motor is either cold or warm, cupping the top of the carb to a certian extent will raise Idle. I'd love to get a lower idle but even at 800 its a tad rough |
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| Author: | Aggressive Ted [ Fri Oct 26, 2012 7:18 pm ] |
| Post subject: | |
For reference, a stock 79 with over 280,000 miles and a very leaky stock steel shim intake gasket I get 18" of vacuum in neutral @750 rpm. In gear at 600 rpm (waiting at a stop light) I get 16" of vacuum. My initial timing is a 6 degrees, 50 degrees total at 2500 rpm cruise. |
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| Author: | kielbasa [ Fri Oct 26, 2012 7:28 pm ] |
| Post subject: | |
So where the hell is the rest of my vacuum haha |
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| Author: | hantayo13 [ Fri Oct 26, 2012 7:45 pm ] |
| Post subject: | |
old tired slant of unknown milage/use ...pulls 20 / 22 at 800 rpm ....check for leaks |
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| Author: | ceej [ Fri Oct 26, 2012 8:11 pm ] |
| Post subject: | |
12 PSI of vacuum is pretty good. Comes out to 24 inches of mercury. I'd be surprised if the vacuum was that high at idle. Higher engine speeds, sure. That would put you at 2.7 PSIA. The normal unit of measure when talking vaccum is Inches of Mercury, or IN HG. When using an OBDII system, the measurement is given in inches of Mercury Absolute. For 20 IN HGV, the reader would be somewhere just under 10 IN HGA. What are the units of measure you used for measuring the vacuum? It should be marked on the vacuum gauge. CJ |
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| Author: | kielbasa [ Fri Oct 26, 2012 8:17 pm ] |
| Post subject: | |
Sorry gauge doubles as a fuel pressure gauge didn't realize i was reading wrong unit of measurement. Its reading 12 in hg. Or 30 cm hg. |
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| Author: | ceej [ Fri Oct 26, 2012 8:21 pm ] |
| Post subject: | |
That's pretty darn low. Higher than my rumpity strip cam, but it's going to result in lousy fuel economy for you. The enrichment circuits in the carburetor will be adding fuel far too early. As mentioned above, look for vacuum leaks. Barring that, what cam are you using? Erson posts valve lashes that are too darn tight. Loosen the lash up a couple thousandths. Try again. My RV15M RDP ran well at .026" Lash Intake and Exhaust, producing numbers like 21 IN HG. CJ |
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| Author: | kielbasa [ Fri Oct 26, 2012 8:28 pm ] |
| Post subject: | |
Its an oregon cam #125. cam sheets say .012in .014ex. |
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| Author: | kielbasa [ Fri Oct 26, 2012 8:29 pm ] |
| Post subject: | |
As far as vacuum leaks i believe there are none. Maybe a very small one from throttle shaft but nothing seems out of whack |
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| Author: | ceej [ Fri Oct 26, 2012 8:38 pm ] |
| Post subject: | |
I run an OCG 34. It's supposed to lash at 20 and 20, I find it runs better at 24 and 24. Loosen up a couple thousandths. Try it, and let us know if that improves things. Did you request hardening? If not, think about adding something to protect your investment. My #34 went flat with the new oil change this year. Lost a few lobes. The cam is junk now. We're back to the EP additive discussion. You'll have to decide what it means for you. ZDDP content was decreased in DOT legal oil, again. Use caution. I now have a couple cases of oil that says, "For off-road use only." We'll see if the #183 holds together with that. CJ |
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| Author: | kielbasa [ Fri Oct 26, 2012 8:47 pm ] |
| Post subject: | |
I did not ask for hardening. But i broke it in with comp cams additive, and changed oil after brake in, then again at 500mi. I actually checked valves today if anything they are a little tight after about 2000mi if a cam goes flat its gonna happen durring the begining of its life. I think I'm gonna be ok. Tomorrow ill set them .016/.020 and see how well that works, now should i be checking vacuum tee'd with the distributor? And should i be adjusting valves to vacuum? Will there be a certian lash that reads a highest point and stops, or will actually drop after a high point? |
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| Author: | ceej [ Fri Oct 26, 2012 8:52 pm ] |
| Post subject: | |
After looking at the cam profile, just a quick question, what was the criteria you used when selecting that cam? With the lift available, cylinder fill may not be very efficient. This is a 225, yes? The best discription I would give of that cam is "Too tame." With lash figured in, you have extremely low valve lift. With the lunger of an engine we have here, your going to be choking it down pretty severely. For a good street cam, mild manners and such, the 346 looks like a better match, if economy is your goal. 2¢ CJ |
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| Author: | ceej [ Fri Oct 26, 2012 8:57 pm ] |
| Post subject: | |
Doc showed me an excellent method for lashing valves to get the most out of the lash ramps. Loosen the valve until it makes a loud "tack" noise, measure, then tighten a couple thousandths. If your close to one of the gurus down there, get some help. You will hear the difference between the typwriter stachato of a correctly lashed engine, and the hard noise of too loose lash. Vacuum should come up, but loosening further will not cause it to go the other way. It's not quite the same as adjusting mixture based on vacuum. CJ |
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| Author: | kielbasa [ Fri Oct 26, 2012 9:00 pm ] |
| Post subject: | |
So i really wouldnt want to find the highest vacuum # based on lash? I don't think any good slant guys are near me, if there is speak up! Im in oc / la area |
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| Author: | ceej [ Fri Oct 26, 2012 9:05 pm ] |
| Post subject: | |
It's a long drive..  Try the lash settings you suggested tomorrow, then take it another .001" and see what it does. You should be close. If vacuum doesn't improve, there is some other problem. I still would like to see closer to 19-20 in HG. Even with the low lift and short duration, you should have better vacuum than you are seeing. Is the vacuum gauge good? They don't last forever. (By professional background I'm a metrologist.) CJ |
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