Re: Idle vacuum - why only 10 inches?
 

I talked to him about it today, and he said:

- It's unlikely it'd run as well as it did/does with the cam off by a full tooth
- Near impossible with a brand new double-roller that it'd jump

So, if it's off, it was installed wrong. And if it were installed wrong, it'd be a lot worse off than it is. Or so the theory goes.

While the responsive-down-low does sound like it, if the cam were advanced, though, I'd expect strong idle vacuum. If it were late, I'd expect it reduced.

I really am not in the mood to pull it apart and degree the cam just to chase a number when there are no other symptoms (but I verified with a new gauge today that it's only got 11").

I also disconnected the vacuum booster and accessories today to rule them out as a leak source, no change.

My cam specs follow - it's more than stock, but not big or wild I wouldn't say:

Intake: 214
Exhaust: 218
Lift: .496
Int Cntr: 110
Ex Cntr: 120
Overlap: 66

Re: Idle vacuum - why only 10 inches?
 

I went out and checked compression, thinking if valve timing were off messing up idle, it should mess up cranking compression too, but I got a full 160psi which I think is probably about right for 9:1.

Does that point anyone in any direction?

Re: Idle vacuum - why only 10 inches?
 

Just out of curiosity, did you happen to compare your vacuum readings to the vacuum chart Dieselhollic provided?

Re: Idle vacuum - why only 10 inches?
 

I did, but low steady vacuum just comes up as late valve timing, which I don't know how to check short of degreeing the cam, and I don't want to pull the motor apart like that if I don't really have to. If I could confirm it had to be fixed, I'd go for it, but it's a lot of work to rule out a possibility...

Re: Idle vacuum - why only 10 inches?
 

What carb?
What's your timing curve like?
Have you confirmed #1tdc with the timing tab?

Re: Idle vacuum - why only 10 inches?
 

Original Q-jet, rebuilt by a Q-jet shop. The only "wait a minute" moment I can think of on the carb is when they were dynoing it they wanted to increase the rear jet size, did not have one, and so they drilled it out to the specified size. It fixed the fuel curve but could a secondary jet "leak" extra fuel or something? It's a long shot, I'm just trying to include all of the info.

Curve is stock, but I'm running 12-15 degrees initial to get as much vacuum as possible. But at this point I'm not even looking at the timing tab, just maximizing vacuum as best I can by increasing advance until more's not better.

Not sure what you mean by confirming #1 TDC - like center it with a piston stop? In what scenario would it be wrong, mis-ground gearset or cam? I wonder if the timing tab has the accuracy/resolution to show me. I guess even 2 degrees off would raise suspicion, but let me know what you'd like me to check.

Re: Idle vacuum - why only 10 inches?
 

I've seen secondary throttle plates stuck open before. That drops your vacuum.
So which is it? 12 or 15 initial. Accuracy is the key. That's why you confirm your timing tab with a piston stop.
http://www.crankshaftcoalition.com/w...op_dead_center

Some cams were ground advanced or retarded 4-6 degrees.
If you had any fuel leaking it would speed up your idle. Likely not a problem right now.

Can you screw either mix screw in all the way without the motor stumbling and wanting to quit?
Where's your vac advance hooked to?

Re: Idle vacuum - why only 10 inches?
 

Geezer, would I be able to see the secondary blades well enough on a Q-jet with the air doors open to know if they're misadjusted? Or will I have to remove the carb for that?

Both mixture screws, if turned in, will cause the engine to sputter and die eventually. What I don't get is why more air speeds it up but less fuel does not.

You'll have to defend that one. What do we care if it's 12 or 15? It's not 4, which is the tuneup spec, so anything other than 4 is "wrong", no matter how accurate you are about it. Doesn't matter to me if it's 12 or 15, so if it matters to you, you know something I don't, and you should share...

I've got to think on it some more, but because the timing tab is based off the CRANK gear, it's not even clear to me what I could confirm other than the accuracy of the timing tab, which as noted, I presently don't care about. I can't diagnose anything about the valvetrain with a piston stop and a timing tab, at least I don't think I can, but I could be missing something.

Vacuum advance is where it's supposed to be, but I tried doing it wrong out of the gate to make sure that wouldn't solve it (of course connecting it to manifold vacuum kicked in a bunch of idle advance, but it didn't help, or I would have been done). So it's back to ported. Long story short, it maxes vacuum around 12-15 degrees as noted, whether you get there via the vacuum can or static adjustment of base timing.

The only other thing I can think of is that even though the well screws were epoxied, I still think it drains fuel, as you have to crank for a while to fill the bowls when it's sat for a week. It also eventually overheats at idle no matter the weather, but it takes 30 minutes of idling to do it... it just creeps.

Re: Idle vacuum - why only 10 inches?
 

What did you vacuum read when opening and closing the throttle rapidly? Needle pulls down to zero? ...Indicates rings. Just looking at the Dieselhollic chart for other possible answers.

I also have low vacuum and It pulls down to zero, but I have good compression and no blue /oil smoke out the exhaust. So I question the chart.

Re: Idle vacuum - why only 10 inches?
 

Try tightening each intake manifold mounting bolts.

Re: Idle vacuum - why only 10 inches?
 

I'm going through the same thing with my small block right now. I hope you figure it out, and can help other's (me) to eventually solve this issue. I'm sure I'll figure it out eventually too, but I'd rather not tear the motor out or apart if I don't have to. It's pretty hard to drive it right now though. It ends up idling down to low in gear, and has almost no added boost for the power brakes. I'm sure that you must also be having brake issues?

Re: Idle vacuum - why only 10 inches?
 

Camshaft being off will show up in cranking compression and your's looks fine.

Just something weird, I once had a similar problem and it ended up being a crack in between the heat passage and intake runner of the intake manifold (cast iron).

I spent a lot of time diagnosing that engine only to fix it when I replaced the manifold. (crack found later)

Re: Idle vacuum - why only 10 inches?
 

What about trying a smoke machine just to absolutely rule out even a small leak or leaks? I know nothing about it accept that it works, is that hard to come by or a PITA to do?

Re: Idle vacuum - why only 10 inches?
 

They actually did do a smoke machine test on this one, but it was into the valley/crankcase to diagnose a cam gallery plug leak. That part, the crankcase, was perfectly sealed and tight, no leaks (other than the plug).

Can they use it to smoke out the other direction, like intake-to-exhaust-crossover leak, or intake to atmosphere?

Re: Idle vacuum - why only 10 inches?
 

http://www.therangerstation.com/Maga...acuumLeaks.htm

Take a look at this web page. It has about the same vacuum diagnostic images that Dieselholic posted, except the images are animated.

Please take a close look at that first section on finding vacuum leaks.
They offer several methods for checking for vacuum leaks. Of those, the preferred method for me is spraying carb cleaner.


It should be noted that unless your eye balls have been calibrated specifically for checking for vacuum leaks, your results from just looking will be generally very unreliable. :lol: :lol:

They say normal vacuum should be 17-21 and are showing 20 in.Hg.
Your signature says Redmond, WA and Google says that is 43 ft above sea level.
If any engine is going to get 20 in., it is going to be that close to Sea Level.

Re: Idle vacuum - why only 10 inches?
 

Your cam numbers don't mathematically add up

what is the intake duration and the exhaust duration and the lobe separation angle (LSA)

Re: Idle vacuum - why only 10 inches?
 

Here is the actual cam card. It's supposed to be a direct replacement for #3883986, which has an LSA of 115. This is effectively the same cam as the 427/390hp engines. That was a pretty same motor, though lumpier than the 335hp I guess.

I didn't have him go look for his notes, but he degrees the cam on every build and so would have done so on this one too. That's why I'm not too worried about valve timing, hoping he would have caught any mistake in that step. Anything's possible though.

Re: Idle vacuum - why only 10 inches?
 

That lobe separation thing has me a bit confused also.

Re: Idle vacuum - why only 10 inches?
 

It's not (110 + 120) / 2 == 115?

Re: Idle vacuum - why only 10 inches?
 

it is a fairly stock grind. I would bump the idle up to 675 and bump up the ignition timing and see what happens. If the performance isn't t here Id worry about it but if it running as you want then leave it alone

Re: Idle vacuum - why only 10 inches?
 

I thought they are both supposed to have the same lobe separation.????

Anyone one this???

Re: Idle vacuum - why only 10 inches?
 

LSA is the distance between the peaks of the intake and exhaust - so you take the two of them and add them, divide by two, and that gives you the difference, which is the separation.

You can google "lobe separation angle" and get a similar description (below). Since a tighter LSA means the lobes are closer together, they overlap more. That's why a wider LSA gives you a better quality idle, but less power up top.

“The lobe separation angle is the angle in camshaft degrees between the maximum lift points, or centerlines, of the intake and exhaust lobes,” says Eric Bolander of Erson Cams. “It affects the amount of valve overlap; that is the brief period of time when both the intake and exhaust valves are open."

Re: Idle vacuum - why only 10 inches?
 

I am thinking something internal has got a micro crack in it.

Have you done vac tests both cold and extremely warm? A simply test may reveal something.

Re: Idle vacuum - why only 10 inches?
 

are your power brakes working? bad vac gauge? Interesting, and frustrating, dilemma... Interested to see how this works out Dave, hope you figure this out soon - this kinda stuff is maddening! All indications are the motor is running just fine. Fall off above 3500 rpm is a "bit" low for a bbc, but not by much - most of the grunt is in the lower rpm range up to and maybe slightly above that neighborhood. When initially reading I assumed your Winston cup guy stuck a monster cam in it, but clearly he didn't. Are you experiencing the symptoms of low vac? (thus the pb question)

Re: Idle vacuum - why only 10 inches?
 

Have you ruled out any leakage between the intake and carb? Are you mounting a square bore carb to a spread bore manifold or visa versa? The reason I ask. I'm also chasing low vacuum and I discovered that the PO installed a square bore to spead bore adapter upside down and that I my vacuum leak may be coming from the missmatch or lack mating surfaces. I ordered a new plate specifically for SQ B to SB configuration. I'll let you know if it cures my vacuum issue.