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Old 09-05-2016, 06:25 PM   #1
davepl
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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.
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Old 09-05-2016, 06:41 PM   #2
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Smile 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
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Old 09-05-2016, 06:59 PM   #3
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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.
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Old 09-05-2016, 07:03 PM   #4
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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
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Old 09-05-2016, 07:15 PM   #5
davepl
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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.
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Old 09-05-2016, 07:25 PM   #6
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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?
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Old 09-05-2016, 07:36 PM   #7
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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
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Old 09-05-2016, 07:40 PM   #8
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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?
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Old 09-05-2016, 07:45 PM   #9
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Re: Idle vacuum - why only 10 inches?

Quote:
Originally Posted by geezer#99 View Post
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?
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.
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Old 09-05-2016, 08:05 PM   #10
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Re: Idle vacuum - why only 10 inches?

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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.
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.
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Old 09-05-2016, 08:08 PM   #11
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Re: Idle vacuum - why only 10 inches?

Plus one on getting a second vacuum gauge to double check the one you're using.
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Old 09-05-2016, 08:21 PM   #12
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Re: Idle vacuum - why only 10 inches?

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

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Old 09-05-2016, 08:32 PM   #13
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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!
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Old 09-06-2016, 02:11 AM   #14
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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....
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Old 09-06-2016, 03:19 AM   #15
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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.
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Old 09-06-2016, 09:12 AM   #16
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Re: Idle vacuum - why only 10 inches?

Quote:
Originally Posted by davepl View Post
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.
Probably true. Have you talked to him about the low manifold vacuum?
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Old 09-06-2016, 12:51 PM   #17
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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.

Quote:
Probably true. Have you talked to him about the low manifold vacuum?
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.
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Old 09-06-2016, 07:09 PM   #18
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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.
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Old 09-06-2016, 08:21 PM   #19
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Re: Idle vacuum - why only 10 inches?

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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.
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?
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Old 09-06-2016, 11:08 PM   #20
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Re: Idle vacuum - why only 10 inches?

Sounds like the Cam sprocket and timing chain have jumped a link at this point.
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Old 09-07-2016, 04:01 AM   #21
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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
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Old 09-07-2016, 06:37 AM   #22
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Re: Idle vacuum - why only 10 inches?

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Originally Posted by toolboxchev View Post
Sounds like the Cam sprocket and timing chain have jumped a link at this point.
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.
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Old 09-07-2016, 08:13 AM   #23
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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.
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Old 09-07-2016, 01:44 PM   #24
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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.
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Old 09-07-2016, 04:38 PM   #25
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Re: Idle vacuum - why only 10 inches?

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Originally Posted by davepl View Post
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.
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?
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