Idle vacuum - why only 10 inches?
 

My truck only makes about 10 inches of Hg for vacuum at idle. That's a fairly low idle of about 600. Pop it into gear and the idle falls to maybe 500, and the vacuum drops to about 7. Much more than that and you're into the power valve I assume!

It runs pretty well. It doesn't stall (barely dips at all) when the AC cuts in, and there's no kicker solenoid on these, so it has to manage without a bump in idle speed.

There are no leaks that I know of, and inducing a leak by unplugging a hose makes it run noticeably worse (which I don't think would happen if there were another large leak). Also it runs worse with the PCV blocked, so it wants that additional air. All that leads me to believe no leak, anyway.

But is 10" too low? What's normal for a stock truck with a very-close-to-stock cam in a 402?

Where would I start chasing to improve it? I tried more timing, without success, and mixture screws can worsen but not improve it.

A higher idle might help, but it's not like it's 16 at 1000. I'd like 15 at 750, but haven't had any luck getting it.

Like I said, it runs OK. If the gauge said 16 I'd pat myself on the back, in fact, but it doesn't, and I don't know why it's low. Maybe it'd run even better! Gauge is a large tune-up style one, quite accurate so far as I can tell and compare.

Re: Idle vacuum - why only 10 inches?
 

Dave
Is the motor new/rebuilt and in good condition?....good compression?
If so, what is your timing at? and otherwise, is the vehicle in good tune?...ie: good power, good response...no hesitation etc.


all Good
Coley

Re: Idle vacuum - why only 10 inches?
 

Brand new motor, made 330hp on the dyno against a rated 310hp, so seems spot on. No problems to speak of. Timing is about 10 BTDC at idle.

Re: Idle vacuum - why only 10 inches?
 

normal is any where from 18 - 21 20 being usual best . 10 is way too low unless your running a monster cam . id have to say something carb related if you adjust stuff and seems to be no effect . normaly I set timing to the book then adjust the carb air flow then idle double check the air flow then adjust timing to best vac . you say powervalve so I'm assuming holley . they are a pain in the ... to set and get to stay there . id bet good money its carb

Re: Idle vacuum - why only 10 inches?
 

Q-Jet, freshly rebuilt by Carb Connection in Kirkland.

If I had a spare carb to throw on, I would, but unfortunately I don't.

Re: Idle vacuum - why only 10 inches?
 

Did you degree in the cam when installed?
How deep do you have your lifter preload?
Is tdc #1 verified with the balancer/timing tab?

Re: Idle vacuum - why only 10 inches?
 

q jets don't have powervalve I'm unframiliar with the carb shop are they close enough to drive to id bet they would help adjust it to your motor

Re: Idle vacuum - why only 10 inches?
 

That's really weird. Even if the cam is a step above stock. I'd expect approx 18" idle vacuum. 10" vacuum would typically indicate a vacuum leak, but you'd have hesitation on acceleration, and the engine would probably diesel when you shut it off.

Are you sure the vacuum gauge is reasonably accurate?

What do the plugs look like?

Re: Idle vacuum - why only 10 inches?
 

Ah, yes. Too much lifter preload could cause valves not to fully seat, which could cause low manifold vacuum. Pretty easy to do if you're not familiar with the procedure and/or there's no oil in the lifters.

Re: Idle vacuum - why only 10 inches?
 

It "don't mean much", but it was actually built by a retired Winston Cup (NASCAR) engine builder who retired from North Carolina up to my area. And he's been an engine builder and machinist for decades. So I figure he knows how to set the valves.

And I watched.

They also smoke-tested the motor with a Snapon machine that injects smoke and basically does a leak test, and it was one of the tightest they've tested. But that only rules out a leak from intake to crankcase/valley, not bad lash.

If it were the valves, it'd show up on a leakdown test too, would it not? I'd have to check a couple of different cylinders to be sure.

Re: Idle vacuum - why only 10 inches?
 

Plus one on getting a second vacuum gauge to double check the one you're using.

Re: Idle vacuum - why only 10 inches?
 

I've always gone off of this chart for simple stuff. Sorry for the large image.

http://www.onallcylinders.com/wp-con...-reading-1.png

Re: Idle vacuum - why only 10 inches?
 

I ordered a new large-face (2.5") vacuum gauge to make sure it's not just the gauge, as suggested.

The needle is quite steady, so I don't think it's a valve issue, unless they were all the same. Otherwise the needle would bounce around I think.

I'll follow up when I have more data!

Re: Idle vacuum - why only 10 inches?
 

My guess would be not enough base timing and you have the throttle blades open to far to compensate. Try increasing the base timing to about 14 deg and then shut down the throttle to get your idle back down that will bring back your vacuum....

Re: Idle vacuum - why only 10 inches?
 

Try bumping up your timing to 14 initial. I just did my 350 after being stubborn for years and setting at 10 btdc. Now my motor purrs like a kitten, and I have more vacuum on a 100k bottem end. Also a noticeable increase in throttle response and mileage. It had to be repeated to me multiple times that cheap S*** ethanol needs the timing more than the real stuff.

Vacuum is at 20, now when warm. about 15 cold., sticking fricken valves. 100 k on the bottom end.

I have an Eddy carb and they are extremely easy to balance out with the vac gauge.

Re: Idle vacuum - why only 10 inches?
 

Probably true. Have you talked to him about the low manifold vacuum?

Re: Idle vacuum - why only 10 inches?
 

I tried plugging the vacuum advance into manifold vacuum, cheap an easy way of adding a bunch of initial timing, which I confirmed with the light (about 14-16). But it didn't improve the idle speed nor vacuum. So the added timing seemed to do very little.

Only in passing, because there's no problem other than the number on the gauge. But I'll find out what he uses for lifted pre-load, if it's anywhere near tight (like more than 1/2 turn past zero) I'll chase that lead a little further.

Re: Idle vacuum - why only 10 inches?
 

I know you tried to add some timing. Here is what I would do. I have found over the years that timing marks are not always correct. I would not hook up the timing light right now, I would hook up the vacuum gauge to manifold vacuum, disconnect the vacuum to the distributor, start the engine and start advancing the distributor and watch the vacuum gauge. You may have to lower the idle. I think the vacuum will come up. When you turn off the engine see how it restarts, if it starts slow you would have to retard the distributor some.

Re: Idle vacuum - why only 10 inches?
 

Tried that just now to no avail. Actually had to advance the plug wires by one to get "more" advance (just based on the distributor hitting the intake), but it turns out more advance doesn't help. If you add much more than I already had it starts to lose vacuum and stumble, so I just put it at the fastest idle and it still cranks OK, left it like that for now.

I think I got it up to 11". Still not enough... my new gauge will be here in a day or two, but it's still low... I might have 13" and the gauge is a little off, but it's not far off (because I've used 3 gauges on it now, all close).

What's next?

Re: Idle vacuum - why only 10 inches?
 

Sounds like the Cam sprocket and timing chain have jumped a link at this point.

Re: Idle vacuum - why only 10 inches?
 

Hey Dave just a shot in the dark, does your new transmission have a vacuum operated shift modulator? I just had an issue with my turbo 400 where the vacuum hose had a leak, but I'm not sure if that even applies here since I don't remember what kind of transmission you put in

Re: Idle vacuum - why only 10 inches?
 

Maybe it didn't jump a tooth but the builder installed it with a timing offset.

OP, did your builder use adjustable timing sprockets? You might want to ask him if he did and what advance/retard he put the cam at. It seems weird to me that you can advance the timing and not get the rpm to increase.

Re: Idle vacuum - why only 10 inches?
 

Next I would adjust all the valves with the engine running. Just so you will know how they are.

Re: Idle vacuum - why only 10 inches?
 

Man, I'd hate to think the cam could be off. The motor is VERY responsive down low and very wheezy above 3000, but like I said it ran well on the dyno.

Re: Idle vacuum - why only 10 inches?
 

I'm no professional engine builder but those symptoms sounds like an advanced cam by your statement above. Strong power down low and running out of steam above 3000 isn't normal. Is it possible the builder installed the cam advanced a few degrees to help the low end power for you?

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.