![]() |
Register or Log In To remove these advertisements. |
|
|
![]() |
|
Thread Tools | Display Modes |
|
![]() |
#1 |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
|
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.
__________________
1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible |
![]() |
![]() |
![]() |
#2 |
Registered User
![]() Join Date: Oct 2011
Location: Victoria, B.C
Posts: 3,794
|
![]()
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
__________________
....for some men, there is experience, skill and effort....for the others...there is visa and UPS LOL 1966 Chevy 1/2 ton (Florida- Red/white) 1972 Chevy 1/2 ton (California- Blue/white) 2005 Chevy Silverado HD2500/Duramax 2000 Dodge Ram 1500 |
![]() |
![]() |
![]() |
#3 |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
|
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.
__________________
1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible |
![]() |
![]() |
![]() |
#4 |
Registered User
Join Date: Jun 2015
Location: @
Posts: 890
|
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
|
![]() |
![]() |
![]() |
#5 |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
|
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.
__________________
1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible |
![]() |
![]() |
![]() |
#6 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,594
|
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? |
![]() |
![]() |
![]() |
#7 |
Senior Member
![]() Join Date: Jul 2000
Location: North Texas
Posts: 3,484
|
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.
__________________
Mike 1969 C10 LWB -- owned for 35 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- sold 1981 C10 Silverado LWB -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 24 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then! |
![]() |
![]() |
![]() |
#8 | |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
|
Re: Idle vacuum - why only 10 inches?
Quote:
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.
__________________
1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible |
|
![]() |
![]() |
![]() |
#9 |
Senior Member
![]() Join Date: Jul 2000
Location: North Texas
Posts: 3,484
|
Re: Idle vacuum - why only 10 inches?
Probably true. Have you talked to him about the low manifold vacuum?
__________________
Mike 1969 C10 LWB -- owned for 35 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- sold 1981 C10 Silverado LWB -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 24 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then! |
![]() |
![]() |
![]() |
#10 |
Registered User
Join Date: Jun 2015
Location: @
Posts: 890
|
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
|
![]() |
![]() |
![]() |
#11 |
Senior Member
![]() Join Date: Jul 2000
Location: North Texas
Posts: 3,484
|
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?
__________________
Mike 1969 C10 LWB -- owned for 35 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- sold 1981 C10 Silverado LWB -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 24 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then! |
![]() |
![]() |
![]() |
#12 |
Registered User
Join Date: Sep 2016
Location: Vancouver BC
Posts: 784
|
Re: Idle vacuum - why only 10 inches?
Plus one on getting a second vacuum gauge to double check the one you're using.
|
![]() |
![]() |
![]() |
#13 |
Registered User
Join Date: Feb 2012
Location: Lynd, Minnesota
Posts: 201
|
Re: Idle vacuum - why only 10 inches?
I've always gone off of this chart for simple stuff. Sorry for the large image.
![]()
__________________
Semi professional heavy equipment operator Amateur mechanic |
![]() |
![]() |
![]() |
#14 |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
|
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!
__________________
1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible |
![]() |
![]() |
![]() |
#15 |
Registered User
Join Date: Mar 2016
Location: Rugby.UK
Posts: 42
|
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....
|
![]() |
![]() |
![]() |
#16 |
Registered User
![]() Join Date: Jul 2015
Location: 2nd left past the stump on a dirt road.
Posts: 2,629
|
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. |
![]() |
![]() |
![]() |
#17 | |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
|
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:
__________________
1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible |
|
![]() |
![]() |
![]() |
#18 |
Registered User
Join Date: Apr 2009
Location: Lowell AR
Posts: 360
|
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.
__________________
http://67-72chevytrucks.com/vboard/s...d.php?t=698292 |
![]() |
![]() |
![]() |
#19 | |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
|
Re: Idle vacuum - why only 10 inches?
Quote:
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?
__________________
1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible |
|
![]() |
![]() |
![]() |
#20 |
Registered User
![]() Join Date: Jul 2015
Location: 2nd left past the stump on a dirt road.
Posts: 2,629
|
Re: Idle vacuum - why only 10 inches?
Sounds like the Cam sprocket and timing chain have jumped a link at this point.
|
![]() |
![]() |
![]() |
#21 | |
Registered User
Join Date: Nov 2012
Location: Mexico, NY
Posts: 559
|
Re: Idle vacuum - why only 10 inches?
Quote:
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.
__________________
'84 K10, SM465, NP208, 3.42 Gears, 33x12.5-15 GM 4 bolt 880 roller block, 062 Vortec Heads Comp Cams XR258HR-10, MSD Street Fighter HEI Edelbrock: Performer Intake 2116, 1405 Carb Long tube headers, 2.5" duals to Flow Master Super 40's Vortec Engine Build |
|
![]() |
![]() |
![]() |
#22 |
Registered User
Join Date: Apr 2009
Location: Lowell AR
Posts: 360
|
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.
__________________
http://67-72chevytrucks.com/vboard/s...d.php?t=698292 |
![]() |
![]() |
![]() |
#23 |
Registered User
Join Date: Nov 2015
Location: Colfax California
Posts: 1,644
|
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
|
![]() |
![]() |
![]() |
#24 |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
|
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.
__________________
1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible |
![]() |
![]() |
![]() |
#25 |
laying low
![]() Join Date: Mar 2001
Location: Searcy, Ark. USA
Posts: 13,200
|
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?
__________________
Boog 69 Chevy stepside, 358/T350, 4.11 posi, 4.5/4 drop, rallys, poboy driver primer is finer 91 Chevy sportside, Tahoe, Yukon & GMC Crewcab All GM..'nuff said. I stand for the flag and kneel at the cross |
![]() |
![]() |
![]() |
Bookmarks |
Thread Tools | |
Display Modes | |
|
|