Very Low Vacuum

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SpeedThrills

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Recently got my 360 running in my Duster. Cant drive it, LOTS more work to get there. lol

Anyhow, it's 11.2:1, MP 292 508 cam, 2.02 intakes, 915 heads are ported, Torker II intake and 750 vac Holley.

It's idling at 900 rpm @ 3" of vacuum. I have this PCV, M/E Wagner Performance Products – High Performance Crankcase Ventilation Systems it's not set up yet, but I've plugged it, with no difference.

It hasn't run in 10 years, and I know I played with idle feed hole size, but don't remember what I did. I also drilled holes in the primary plates. (Maybe 1/8"?) Today, I plugged the holes and cracked the secondaries. Still @ 900 rpm w/ 3", but it doesn't "run away" so much.(It was hard to get it to return to idle before I did that.)

I cover the airhorn w/ a rag slowly and it doesn't pick up, like a vacuum leak. I sprayed around the carb base and manifold, no rpm increase.
 
Pull the initial timing up. That will fix it.
 
What all else is the camshaft matched with? This camshaft is MASSIVE for a hydraulic camshaft. It has to be MATCHED with other engine components in order to run well. Help us out here. What "else" is in the engine?
 
Anyhow, it's 11.2:1, MP 292 508 cam, 2.02 intakes, 915 heads are ported, Torker II intake and 750 vac Holley.
Other than this, it has a mech advance Unilite ignition. What else needed?
EDIT: Also, it has Hooker 5204 1 3/4 headers, Summit turbo mufflers and turn downs.

I'm going to check timing again. I'm pretty sure it is set at 35 total, which should be 15 initial. I haven't be able to idle it low enough to check that. Mallory's 20 in the distributor is what I have.
I have the damper marked, but I'll check again.

When it ran 10 years ago, It had idle 'issues". That's why I was messing with the idle feeds back then.

I've had it completely apart to look it over since then. Only changed bearings and rings, added Angelucci rocker arm hold downs.

I don't expect an MPG cruiser, I'm aiming for 11's, street legal. It'll be light, minimal interior, glass fenders, hood and bumpers.
 
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Don't worry about total to get it to idle, just make a mental note not to get into it when you are doing this........go ahead and advance it and see what it does for the idle. If that works then shorten up the curve some more.
 
I'm going out to try more initial soon. Good idea.

One other thing I have to look into: It has a 6.5 PV, and it doesn't seem to be open at only 3" vac.
 
Needs more initial timing, kick it up to 20.

This^^^^^^ It's a good cam, but it can suck cylinder pressure right on out. You can make it run though with a lot of initial.
 
Recently got my 360 running in my Duster. Cant drive it, LOTS more work to get there. lol

Anyhow, it's 11.2:1, MP 292 508 cam, 2.02 intakes, 915 heads are ported, Torker II intake and 750 vac Holley.

It's idling at 900 rpm @ 3" of vacuum. I have this PCV, M/E Wagner Performance Products – High Performance Crankcase Ventilation Systems it's not set up yet, but I've plugged it, with no difference.

It hasn't run in 10 years, and I know I played with idle feed hole size, but don't remember what I did. I also drilled holes in the primary plates. (Maybe 1/8"?) Today, I plugged the holes and cracked the secondaries. Still @ 900 rpm w/ 3", but it doesn't "run away" so much.(It was hard to get it to return to idle before I did that.)

I cover the airhorn w/ a rag slowly and it doesn't pick up, like a vacuum leak. I sprayed around the carb base and manifold, no rpm increase.
Big cam equals no/low vacuum
You can adjust the carb and timing all you want, you may get it to come up a couple of inches but you will never see vacuum like a stock engine creates.
How much vacuum do you need? And what are you going to do with it?
 
Big cam equals no/low vacuum
You can adjust the carb and timing all you want, you may get it to come up a couple of inches but you will never see vacuum like a stock engine creates.
How much vacuum do you need? And what are you going to do with it?
I don't expect much vacuum. I thought I'd see more than 3", though. No power brakes, nothing that needs vacuum.
 
I don't expect much vacuum. I thought I'd see more than 3", though. No power brakes, nothing that needs vacuum.
If that is the case you will need to work with what you have.
You will wear out that Holley trying to adjust it for street driveablity, they dont transtion well with low vacuum.
 
I'm going out to try more initial soon. Good idea.

One other thing I have to look into: It has a 6.5 PV, and it doesn't seem to be open at only 3" vac.


That's correct. You need to measure vacuum at a cruise and set your power valve opening by that, not idle vacuum.
 
I bumped the timing 7*. Should be 22 initial now. (I saw 42 total.) It has 6" vac now. The idle went up and I lowered it to about 1100 now. Much more stable. (Considering what it is. Wouldn't idle consistently before.)
I set up the Wagner PCV, didn't affect it, but now it's set. I'll be able to fine tune when I can drive it.
I'll limit the mech advance the 7* that I added to the initial.
It smells a lot cleaner, too.
I just tried to post an audio clip, no luck. Same when I tried to post video. I've only been able to post pics.
Thanks everyone!
That's correct. You need to measure vacuum at a cruise and set your power valve opening by that, not idle vacuum.
Okay, I thought it would be open and rich because it's idling below 6.5".
 
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Trying the sound clip again. Beats me.
 

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I bumped the timing 7*. Should be 22 initial now. (I saw 42 total.) It has 6" vac now. The idle went up and I lowered it to about 1100 now. Much more stable. (Considering what it is. Wouldn't idle consistently before.)
I set up the Wagner PCV, didn't affect it, but now it's set. I'll be able to fine tune when I can drive it.
I'll limit the mech advance the 7* that I added to the initial.
It smells a lot cleaner, too.
I just tried to post an audio clip, no luck. Same when I tried to post video. I've only been able to post pics.
Thanks everyone!
Okay, I thought it would be open and rich because it's idling below 6.5".


How many turns out on the mixture screws are you? You may need to use a bigger idle air bleed or maybe a smaller idle feed restrictor. Kinda depends on what you have in there now.
 
That's a big cam.
You HAVE to set the Transfer port exposure under the throttle blades just right.
To ballpark yours; Set the T-port exposure to a little taller than wide, but not 50% taller! maybe 20 to 30%, and leave it alone. Set the idlespeed with timing; ~14* with a manual trans, or ~18* with an auto. Defeat the vacuum timing. Put a known good 10.5PV in there, or a PV-plug, and a known to be working PCV.
That leaves just bypass air to fool with. I like thru-holes, others like secondary cracking. After you get it set nice, don't forget to re-install the PV.

A little explanation.Properly set;
At idle your engine is idling on the low-speed circuit which is the Transfer slots. But at idle there is not quite enough mixture coming thru there, so Holley gave us two small additional trimmer holes.
Getting this this carb to work with a big cam is a delicate balancing act between those two discharge ports and getting the timing right. All three have to be co-ordinated.
You start with setting the trimmers (Idle-mixture screws) to the center of their adjustment range. Then setting the Transfer slot exposure, then throwing a bunch of idle timing at it until it runs. At idle the engine will easily take 20 to 30 degrees.....or more, don't worry about it.
After it warms up, you take timing out of it to get the idle-rpm close to some arbitrary target you pull out of a hat; I like 800 with that cam and my manual trans.. With a big cam, very soon idle quality will begin to suffer, and you will be tempted to start turning on screws. This is the wrong thing to do.
Now,
>cracking the secondaries introduces dry air into plenum, which in my case, caused the rearmost cylinders to run lean, and the exhaust caused my eyes to water.
>1/8 holes in the primaries is too much; the transfer ports will be closed too far and run lean, So then in compensation the mixture screws have to be opened up, maybe as far as they go. This makes the idle speed too high, so you have to back off the timing. It's messed up.
> idle runaway occurs for a number of reasons that I can think of;
1) the mechanical timing is unstable; or
2) the V-can is being tickled at the sparkport because the throttles are too far opened; or
3) the carb is vacillating between the T-port supply and the trimmer supply; or
4) the PV circuit is dribbling thru a ruptured diaphragm; or
5) the fuel-level is unstable ;
basically it all comes down to a tuning issue.

The big-cammed engine wants extra air only; not air-fuel mixture. Here is one method I have used to determine how much extra air she wants;
Grab the PCV line, (which should be connected to the carb port at the front of the carb and dumping into the same airstream as is passing by the transfer slots; NOT connected to an intake runner,) and pull the PCV out of it, but get your thumb over the hole pronto before she stalls. Then slide your thumb over to expose some hole. Notice the idle quality gets better, and she gets an increase in rpm.. Find the optimum hole size for idle quality, and then if the idle-speed gets too high,then retard the timing some. Repeat as often as necessary, until you have the timing down to a reasonable number (14* for a stick, and maybe 18/20 for an automatic); while simultaneously getting down to 750/800 rpm. Fiddle with the hole size under your thumb for best quality.
Now,without moving your thumb; yank that hose off the port and shut off the engine.Ok now the hole size you have exposed is the correct size for this combo. Your job is to translate it somehow into the carb and PCV system.
So calculate the area of that exposure and divide it by 4. The PCV will take 1/4, and each primary throttle valve will take 1/4, and that leaves 1/4 for fine-tuning; this is your starting point.
By field tests, I came up with the 2 of 3/32 holes.
After you get this base-lined, THEN you can fine tune the T-Slot exposure to trimmer adjustment.
Here's how I do that;
If you find the trimmers like to be screwed too far from the midpoint, then change the "curb-idle adjustment" say 1/4 turn atta time, and put the trimmers back to mid position . Repeat as necessary.
The timing has to be stable. I choose springs that keep the timing stable to at least 200rpm over the target.
If your exhaust is still watering your eyes, your hole size is probably too large or the timing is not enough.
Here's how you know when the t-slot and trimmers are set right; just retard the timing until the engine stalls. If it keeps on running down to 600 rpm or 5*advance before it stalls, you got her close to perfect. Start it up and put the timing back.

In year 2000, I had almost that exact combo;367 cubes at 11.3Scr, with aluminum Eddies, and the 292/292/108 cam; but with an AirGap and Holley 750DP.
I closed up the secondaries tight, and drilled holes in the primaries to give the engine the air it was craving. I started with 18* idle-timing, about 800 idle-rpm, and 2 of 1/16" holes, one in each blade. I run a jobber,factory 318 PCV.
My car has a manual trans and with this tune the power pulses at low speed were so strong the car would start bucking to the point I had to de-clutch.
So I decreased the Idle-timing to 16 then 14. And the idle-speed went down too low. So I drilled those holes a bit bigger, I think I ended up at 1/8" which turned out to be too big. So I soldered those holes closed, moved over and drilled 3/32" holes.
At 14*advance and the transfer exposure set a little taller than wide, the idle came in at 750rpm with vacuum between 10 and 9. With that cam, I ran a 10.5PV and IIRC 72/78 jets. The idle vacuum will depend on the installed centerline, and your altitude; I am at 930ft.
That cam is long gone; it was just too big for me.
Transfer Port exposure will be very important.
 
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Wow AJ! That's a lot to absorb. I'm going to take some time to mull it over. I know I need to look the carb over and see what I did to it when the engine last ran 10 years ago.
There's a common theme that you and YR have, concerning transition slots.
Thank you for your time, effort and interest!
 
YR has more experience than me so if ever I might say something that contradicts him, chose his advice over mine, anytime every time. There are several others perhaps many, that have more experience than have I.
But I'm more verbose,they say,lol.
 
For experiment, set the timing at idle by advancing/ adjusting the timing for BEST VACUUM. Then see if it kicks on the starter (Thanks, Crackedback, we used to do that when we were "Chevy" kids, LOL) Then take your light and see where it is.
 
This is where I'm going to leave it for now, because the car needs LOTS of other work before it's driveable:

I tried it with the plate holes open again, it runs better with them plugged, and the secondaries cracked. The holes are 9/64, probably too big.

I took 7* out of the dist., it has 22 initial now, 35 total. It would probably like even more initial. (BTW, no vac advance.)

The idle is stable now, much better. Thanks all!

When it's driveable, I'll tune further (If I can figure out AJ's theories, lol.) It has a 727 w/ manual shift and a 3500ish convertor that I'm sure will change things when I put it in gear. (No driveshaft, see below.)

Now, I have a 9" w/ ladder bars and coil overs, front floor pans, and subframe connectors to do. I have all of that stuff. I also need to get a roll bar, no cash right now.

All of the running gear came out of a D150 "Pro Street" pickup that I bought totaled. It's been fun so far, can't wait to drive it! I'm guessing it's 1000lbs. lighter than the D150, and that ran well!
 
Cool! I too will soon get my 360 with a 292 cam broke in. Up top a modded 850 thermoquad which used to be on a drag 440 cuda so i was told. It does seem livelier than my previous thermoquad which was much leaner running. Just moving the throttle the squirters will throw a much bigger stream than my lean thermo.
 
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