All,

Are these vacuum readings good for an original stock 1972 Olds 350 2bbl with 69K?


1. At idle and warmed up my readings with a direct connection from the rear intake manifold to my gauge are 17" steady at idle and approximately 18.5" around 2000 rpm.

Wide open it drops off dramatically.

2. When I tee into the distributors vacuum advance and to the rear intake manifold connector it reads the same as #1 at idle and 2000 rpm. Is this ok?

3. When I tee into the front of the intake manifold and trans vacuum modulator I get 17.5" at idle.



Please let me know.
Thanks

 

How does the car run ? If it runs fine don't be to concerned about your vacuum readings
my car when I got it ran crappy with a lot of shake rattle and roll!.After I plugged all the vacuum leaks that I had I used my gauge to set the timing and adjust the idle screws ,and
I watched the gauge to get the most vaccum at idle .Your vacuum will drop off as the RPM rises .Just make sure that any potential leaks are plugged and or fixed .A big fix for me was changing the carb base gasket ,made a BIG difference.My readings are the about the same as yours but I only measure mine from the same port on the carb all the time.Hope this helps .

 

manny

I have a 72 w stock 350 4bbl motor and what I assume to be 140k (hey maybe its 40K IDK)

my vacuum reading is about the same at idle as yours. My gauge shows this to indicate late ignition timing. I would hope for a higher number closer to 20" w a stock motor in good condition

I need to check my t chain as its an unknown to me maybe my chain is loose causing the lower than expected vac reading

I did just do a comp test on my engine and got 130 +- on all but 1 cylinder. the one was 110 so I dont think my engine is in too bad a shape

 

Well with the gauge I have when its in the green range(18-22hg) you probably have all the vacuum you can get from the engine but small adjustments on the carb idle can yield a wee bit more ,you can adjust your timing a bit to achieve a higher reading also ,the gauge will tell you what is good plus the engine sound will change too ,smooth out especially if you do have a late timing reading.A small vac leak (or multiple leaks )will affect your reading quite a bit.

 

Elevation has some effect, don't know where you are at though. I get around 15-17" @ 5000 ft.

 

I'd replace the Vacuum advance canister, as your idle should go way up when plugged in!
Makes a HUGE difference in performance and mileage!

 

FYI reading the vacuum signal is a great way to dial in or tune your engine. Set the timing and air/fuel screws on the carb to obtain the highest possible vacuum reading while at curb idle. First make sure you don’t have any vacuum leaks. The carb, vac advance, power brake booster hose etc. can all be sources for leaks so verify it’s all tight or you will just be band aiding it which causes crappy mileage and poor performance.

 

x2 on the vacuum gauge it's a multipurpose and useful tuning aid ,short of having a timing light and other tuning gear.

 

Just saw this link on another thread seems like a helpful page

http://www.earlycuda.org/tech/vacuum2.htm

 

Thanks!!!

 

My 71 Cutlass Supreme:
Stock 350 4bbl, 8.5:1 CR, 23k miles, Compression Test all 150 psi range
Vacuum: at idle 750-800 rpm (park) I see 17.5. This value will change a bit due to local weather conditions.
My attachment point is the fitting behind the carburetor on the Intake where the Air breather connects to. When I use a “T” fitting with the Air breather attached, I will see a 0.5 lower setting due to normal leakage from the vacuum temperature switch on the air breather.