Erratic Vaccum Reading

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69Chrgr

Charger/Cuda
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O.k., I'm learning just enough to be dangerous now with a vacuum gauge. My Eddy headed 408 is coming around now, especially now that I also learned how to use the 4 corner idle with the vacuum gauge. But my concern is that the vacuum gauge flutters, and the reading I get from my gauge hooked up to either of the lower vacuum barbs from the lower front of my QF 750 DP flutters between 7 and 9 inches. The Hughes roller cam has a duration of 242 and 246 @ .050 and .555 and .565 lift respectively. Someone told me it was due to the large cam and M1 intake. I have no misses or backfires, and everytime I jet it up it seems to gain a little on the vacuum gauge, however it just flutters. I have 82's in the primary and 86 in the secondary. Do I need to be concerned?

Thanks as Always,
Darren
 
If you have a lot of cam, as you do, you will have lower vacuum readings and they will flutter around. Totally normal. Just make sure that your power valve (holley or demon carbs) is 2" lower than vacuum readings.
Jet changes should not have any affect on vacuum readings. when you are idling, you're using the idle circuit and the jets aren't a part of that.
 
The vacuum reading sounds about right for the cam and cubic inches but it should not flutter. It may drift around slightly but it should never flutter. My cam is almost as big as yours although not a roller and it holds 9 inches of vacuum fairly steady. A fluttering vacuum gauge usually indicates valves not seating properly. Your power valve is about the right size. You want it to be about half of the idle vacuum or slightly above. The old rule of 2" lower than idle vacuum is incorrect. With a pv that's 2" lower than idle vacuum it would open every time you bump the throttle causing a rich condition. Some guys run em that way but they run better with more headroom so they don't pop open every little throttle opening.

Don's right about the jets have no affect on vacuum.

BTW: Those are huge jets for a 750 even with your stout combo. How do your plugs look?
 
A lean idle will make the needle fluctuate. A slow drift back and forth, not anything "quick", that's more a valve thing, as has been mentioned. Sometimes it's (leaness) remedied by a simple idle screw adjustment. Other times the idle feed circuit has to be opened a tiny bit. (I believe the idle air bleed can be soldered and opened smaller than original orifice, which causes a richer mix, too.) Big cams cause that. There's not enough vacuum at idle to pull fuel out of the idle circuit, thus requiring "Mr. Drill". The mixture screw only adjusts the amount of fuel/air already metered. If there's not enough fuel in the mix, the screw won't do the trick. Hard to explain. When the idle mix is right, throttle response is "right now".
 
SpeedThrills said:
A lean idle will make the needle fluctuate. A slow drift back and forth, not anything "quick", that's more a valve thing, as has been mentioned. Sometimes it's (leaness) remedied by a simple idle screw adjustment. Other times the idle feed circuit has to be opened a tiny bit. (I believe the idle air bleed can be soldered and opened smaller than original orifice, which causes a richer mix, too.) Big cams cause that. There's not enough vacuum at idle to pull fuel out of the idle circuit, thus requiring "Mr. Drill". The mixture screw only adjusts the amount of fuel/air already metered. If there's not enough fuel in the mix, the screw won't do the trick. Hard to explain. When the idle mix is right, throttle response is "right now".
Click to expand...

Well said. Only thing I'll add is I've notice that when I adjust them in drive I can get the best idle. If I adjust it in neutral it usually sounds fine until I put it in gear. Then it seems to be a little lean and surges a little so I have to richen it up a little bit.
 
Thanks, fishy68. I forgot: The idle feed pulls vacuum from a small slot in the throttle bore right below where the throttle blades close. (Sometimes there's two, for off-idle transition?) Thus, there's an adequate source of vacuum for the idle circuit when the throttle is closed. When the unmodified carb's idle speed is set on an engine with a big cam, it requires opening the throttle a bit, which uncovers the slot, weakening the idle circuit. Modifying the circuit as in my previous post, allows the throttle to be closed, allowing the idle circuit to function again. Some tuners will drill a hole in the throttle plate to allow the engine more air. This will still only alllow the "original" idle air/fuel ratio to reach the engine, when sometimes a richer one is required. In addition, when turning up the idle with the idle speed screw, the ignition advance can begin, further raising the idle speed!
I was raised on carbs and distributors. There ain't nothing like getting a combo set up right. EFI is neat, my 5.0 Stang (sorry!) will run low 12's and go anywhere w/ the a/c on. But for an ol' hot rod, gimme a carb anyday!
 
I can't add anythign except "New dont mean good". Every set of heads from any aftermarket source get sent to my machinist to be checked and corrected. The equipment he uses is state of teh art, and it makes the heads better than assembly line slammed together... Which "out of the box" they are. I've found wrong springs, valves with marginal angles, locks not in properly, seats out of round, and at least 5 out of 16 guides are too tight for my taste. Plus, you can elongate the pushrod holes while they are apart. If you spend $2K on heads, another $400 is cheap insurance, more power, and longer life.
 
Interesting. I notice when I adjust my 4 corner idle with gauge, and the vacuum goes up, it will get to a point where it "whistles" through the carb, like it is sucking a lot of air. Then, if I idle it down with the idle adjustment screw, it will stop. Is this strange, or am I missing something here. I appreciate all of this valuable feedback, as you guys are always on the money!
 
FWIW, I have a 408 stroker, with a .544 236 hydraulic roller cam, 9.9:1 comp, Edel Heads, and run a Carter 600 on the street, and a Holley 750 DP (72/84), only 2 corner idle, on the strip into a M1 single plane airgap, with Hooker comp 1 5/8" headers. I bought my Edel heads used, and had them gone through, reground, new seals, new springs, shimmed, etc, before I had them installed. I have a FBO distributor set up with 12 degrees timing at idle, and 32 degrees all in by 2900 rpm. (I want to try advancing it a bit at the strip this weekend). I have a vacuum distributor, putting in another 12 degrees at high vacuum. Everyone told me to run off of the manifold vacuum, which I did for a while, and experienced the exact problems you are experiencing. Because vacuum was coming from the manifold (throttle body) vacuum, I was getting around 20-24 degrees timing at idle, and it idled LOUSY! Vacuum fluctuated from 5 to 10 inches, engine was surging as much as 300 rpm, timing would jump around 2-3 degrees using a timing light on the damper. The engine would even spit and die occasionally, not to mention the occasional backfire.

On a whim, I switched to the idle circuit vacuum (on the Holley metering plate) for the distributor vacuum, and the aforementioned symptoms disappeared. I still have the vacuum kick in, but it is over 1200 rpm with partial throttle. Driveability went up, idle speed smoothed out big time. It still has some erratic vaccuum reading, and the idle speed does fluctuate some 50 rpm or so, but it will idle at 700 rpm, and runs MUCH better! Big cams are fun, but they will idle rough, which is kinda the point, eh?

So, my timing is now at 12 degrees, distributor vacuum unplugged. If I replug the distributor vaccuum into the Idle Circuit, the timing does not change at idle. Before, when I had it plugged into the manifold vacuum, it was adding another 10 degrees, which killed the idle performance!

Jim
 
69Chrgr: I haven't worked w/ 4 corner carbs. (Sounds interesting, though.) My experience has been that increasing vacuum by adjusting idle screws is good.
jimmyray: Somebody told me years ago that manifold vacuum was the way to go. I had the same experience as you. Maybe the manifold vacuum thing was a bandaid for smog motors? I think you'll find that 35* is a good max total. Get it in as soon as you can w/o "rattle". (2200-2500?)
I'd be interested to know how your experiment w/ vacuum advance goes. Mine is either w/ a strip, no vacuum car, or a street car, w/ a stock canister. Initial is a matter of not having too much, so that cylinder pressure won't allow it to turn over.
Anyone else have something?
 
Speedthrills I think your right on the assessment about hooking the vacuum advance up to full vacuum is a band-aid for low comp. smog motors. I used to do that to cars I had when I was a kid (they were all smoggers) and it always worked good but I tried it on a high comp. big cammed motor and it ran like crap like described above.
 
Well, I'm running an MSD "Ready to Run" distributor with Vaccuum advance removed, only running mechanical, one silver spring, one blue, total timing @ 35*. Funny thing though, although it is a stroker, it seems to like more timing. I just put it at 35 because everyone says that's the limit on strokers. I'm thinking of getting a QF 950 as that is what most seem to like on smallblock strokers for dragracing.
 
69Chrgr said:
Well, I'm running an MSD "Ready to Run" distributor with Vaccuum advance removed, only running mechanical, one silver spring, one blue, total timing @ 35*. Funny thing though, although it is a stroker, it seems to like more timing. I just put it at 35 because everyone says that's the limit on strokers. I'm thinking of getting a QF 950 as that is what most seem to like on smallblock strokers for dragracing.
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There is no set in stone timing max for any engine. Just guidelines. It is true that most small block Mopars don't need more than 34-35 degrees of timing max but every combo is different so it's always possible that it may be best at 36, 37 or even 38 degrees. Then others only require 32 degrees total. The only way to know for sure is to dyno tune it or track tune it for fastest mph. I have never read anything written on what timing to run on a stroker but I theorize that since a strokers piston speed is much faster they would need more timing advance to achieve complete combustion at 12-15 degrees after TDC which produces best power. Again this is just my theory. If someone knows different please speak up.
 
It all depends on the design. If your builder likes tight quench and the heads are good, and the static compression ratio is high enough, you need much less. Typically mine like less than 32, some as low as 28 total. If the quench is loser than .035", and dished pistons are used, you will have more tendency to detonate. But really, every engine and location is diferent. It's like a carb. If it's tuned very well, you will need to adjust for seasonal changes. Your timing will be diffeerent from a guy in Denver, or Utah, or me in CT. Best thing I can tell you is take it to the track and see what yours likes.
 
SpeedThrills said:
jimmyray: Somebody told me years ago that manifold vacuum was the way to go. I had the same experience as you. Maybe the manifold vacuum thing was a bandaid for smog motors? I think you'll find that 35* is a good max total. Get it in as soon as you can w/o "rattle". (2200-2500?)
I'd be interested to know how your experiment w/ vacuum advance goes. Mine is either w/ a strip, no vacuum car, or a street car, w/ a stock canister. Initial is a matter of not having too much, so that cylinder pressure won't allow it to turn over.
Anyone else have something?
Click to expand...

Depends on the type of vacuum can on the distributor. IIRC, some Fords are set up to run manifold vacuum to the distributor. I've never hooked a mopar up to straight manifold vacuum, always ported vacuum sources.

As a side note, drilling the throttle plates, IMHO, should be the last resort to obtaining a decent idle. Sometimes cracking the secondaries just slightly can make a world of difference in gaining primary side adjustments.

A little tip if you can't get to the adjusting screw on your secondaries. Grab a set of feeler gauges, maybe an .003-.010, start small, and pop them between the arm and screw on the passenger side. Once you get the thickness/number you want, figure out how far to turn it in based on the set screw thread pitch. This way you aren't taking the carb on and off making small adjustments.
 
Believe it or not, adjusting the secondary butterflies helped as far as vacuum, and I have a lot less "flutter" now. It seems I can get close to 9 inches using the 4 corner idle and adjusting the secondary butterlies, however it doesn't want to idle well in gear. So I fattened it up just a little, and it got better. I sure appreciate ya'lls help. Ya'll are lifesavers.
 
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