Low vacuum on rebuilt 400

I've been getting a lot of helpful info on the forum since rebuilding the 400 in my "78 Trans Am ('72 motor), especially about vacuum which has always been a mystery. Before I rebuilt it the car was a monster, but had developed many problems. A worn out crank, rings gone, all the usual reasons for a rebuild.

Now that it's all fresh I'm right at the brink of having everything back right like it was, only "new and improved." The one final sticking point seems to be a low vacuum reading that is keeping it from screaming as before.

Even though I thourghly cleaned and rebuilt the quadrajet, I can barely get a vacuum gauge reading past 20, no matter how far I adjust the mixture screws. It should be close to 30 at idle. The car sat for nearly a year after the rebuild while I did body work, replaced floor pans, etc. I'm wondering if the delicate insides of the carb could have gotten blocked during the down time. As I understand it, even though I can get a smooth idle and hit high RPM easily, if there is blockage in the carb somewhere it will throw off shift points and compromise everything else.  

20", are you kidding? No V8 performance engine makes 20", even when they were brand new. Brand new cars with modern EFI do not make over 22". I have worked on hundreds of Pontiac's, and never seen even a stock motor make more then 15". You can't even buy power valves for carbs to run with over 15". Anything over 12" is excellent.  

umm, ok.  

Brazil ...    what are the measurement on the gauge you have????

in/mg
cm/hg
kpa

in/h20


i don't know where you are.. but most of us are used to the inches of mercury scale...

with a gauge that reads 0 to 30...

i would say that you might be reading the centimeters of mercury scale...

20 centimeters is equal to 8 inches of mercury...
25 centimeters is equal to 10 inches of mercury
30 centimeters is equal to 12 inches of mercury..



so.... lets go from here...

who double checked the timing marks on the timing gears?????  when it was assembled..

do you have a cam card??? that lists the cam number... lift , duration and overlap...

what kind of ignition???    HEI or points....     where is the timing set at??????  that makes a huge difference in engine vacuum...

when you rebuilt the carb... there is a screw in front of the primary rod hanger...   you would have to remove it with a pair of needle nose pliers...  it controls the primary mixture off idle..  down too far and the engine runs lean.. up too far and it runs rich..

if you adjusted that weird screw... you may have to readjust it...

post what you know...  

Thanks Waynep, I'm in Houston.

I use an Edelbrock vacuum gauge #8092,(0 to 30) and check vacuum with in/hg. The timing gear marks are right on the money - double, triple and quadruple checked. The cam is a Comp Cam (1.50:1 rockers) with .364" intake, .407" exhaust, and intake duration 274, exhaust duration 298 degrees.

Stock HEI ignition, timing set to 16 degrees advance. With these settings, my vacuum gauge (hooked to the manifold port) reads right on 20 in. With the mixture screws nearly all the way backed out it makes it to 21. The same test on my '79 Trans Am with a similar setup always reads 29, and has a rebuilt Q Jet that I know is not blocked. The weird primary mixture screw still has the "DONT DARE TOUCH THIS!" plug in it and is still set from the factory.

Thanks again for taking the time to go over this with me.

Brazil  

Brazil,
I'm with Flynryan on this one, I've never seen an engine pull higher than low 20's. However, that doesn't mean there is no problem. Have you tried another guage? Guages are delicate and a drop or bump can throw them out of calibration. Have you tried swapping carbs to see if that's really where your problem lies? What Comp Cams grind number are you using? The specs you mentioned don't match anything in their online listings that I could find.
Steve  
Trying to help... 'cause we don't all have to learn the hard way!

Ya most stock engines I have seen produce 18"-20" of vacuum.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

OK, thanks y'all, can't argue with majority vote. I'll give an update when I get it together.