I only seem to have about 14" of vacuum at idle on my 73 350 with #7 heads.

I've checked compression and I have around 145 psi on all 8 cylinders. (it takes 4 or 5 revolutions to climb to 145 in steps - I had the throttle closed and only pulled one plug at a time, so starter wasn't spinning super fast)
I have adjustable rockers and I have triple checked the valve adjustment. They're super touchy, but I'm pretty sure they're all spot on.

I don't see any open vacuum ports or any potential leak areas. I've basically pulled each hose off one at a time and plugged the hole with my finger and the vacuum doesn't rise.

I tried idle mixture screws - not really changing from 2.5t out to 4 t out... doesn't seem to change anything much.
Any ideas of things I should check?

 

First the compression needs to be checked with the throttle fully open. What is your timing set to and what is your idle rpm?

 

Is the carb snug on the intake? Don't over-tighten, you'll warp it.
Have you checked the vacuum advance canister for proper function, and does it hold a vacuum?

-Loosen the distributor hold-down just loose enough to rotate it.
-Hook up your vacuum gauge to a direct intake vacuum source.
-At curb idle (6-750ish), start twisting the distributor and adjusting the A/F screws to achieve the highest possible vacuum reading. Report back the results. Is that vacuum reading fairly steady?

 

I will try these things inclkuding repeating the compression test. My timing is around 15 deg. and it idles pretty nice there around 850 rpm - no run on or ping... I'll mess with it

I have the progression ignition setup so I can mess around with timing easily.

I'll also try propane enrichment around the carb base. It looks good and I actually had one "only barely snug" mounting bolt but it made no difference firming that up

I will report back!

 

What carburetor are you running?

 

If you have a 1973 350 with #7 heads, it obviously isn't stock. What else has been done to it? What cam? What distributor? What intake and carb? Is there a reason why you are asking? Does the engine not produce enough vacuum for the power brake booster?

 

From what I've read, normal vacuum at idle is 15 to 22. Yours is just outside of this, but not enough that I'd worry about it. As Mr. Padavano says, are you having vacuum-related problems, or are you just looking for something that isn't broken to fix?

 

ok... Joe asked all the right questions.
I just dug into the cam specs and it says 16" at 1000 rpm. I swear I had a read a higher number when I bought this edelbrock 3712 cam, but that may have been a comp one I was considering. I have a Holley Street dominator intake. Carb is stock qjet. distributor is Progression Ignition one.

I was having poor braking and maybe slightly hard pedal. Last night, I changed the "age unknown" front brake hoses and it's noticeably better. I can still occasionally lock the back wheels under hard stops but that might be another thread after some prodding around. Thanks all. Still some stuff for me to check - I'm still going to double check the carb to intake gasket and fitment.

 

That's a really mild cam. Wouldn't expect it to even have that low of vacuum to be honest, though, if your compression is actually 145, that's fairly low (not broken engine low, just less compression than I've run on any of my builds). Could be a bad brake booster. Could be a leak around the carb or intake. Check all the vacuum lines for cracks, especially at the connections. Make sure timing and air/fuel mixture is correct.

 

I have that cam and unless your block was decked or the heads were milled, you around 8.5 to 1 with Felpro head gaskets. I am currently at 160 to 175 psi with 9.5+ to 1 with that cam. I ran the Melling version of that cam with 8.5 to 1 in a chebby truck, it was 140 to 155 psi. I am pretty sure my vacuum was higher, I want to say 18 or 19 inches at idle. You could try adding timing at idle, I would try usually run around 30 at 1000 rpm, may improve vacuum enough. Usually a bone stock Qjet will be lean at idle with any upgrades and should boost vacuum higher.

 

I'll mess with it - I'll put a gauge on the ported vac port too and see where I am on idle... I might be compensating for too little advance with too much idle stop screw...
I'll retest compression with plugs out and throttle open

 

I agree withe above posts about that being a very mild cam (specs show 204º @ .050" lift) and it should generate around 19" vacuum.

One of my previous rebuilds had a 204º cam, and it generated around 20" vacuum. My present 217º cam generated around 15-16" vacuum, which made the power brakes marginal (hard pedal). I rectified that by increasing the initial advance from 10º to 18º and changing the vacuum advance from ported (no vacuum advance at idle) to manifold (full vacuum advance at idle), and I limited the vacuum advance to around 10º with a home made advance stop. Since you have the Progression setup, this should be extremely easy to replicate.

With all that said, there is something amiss if you have 16" vacuum with that 204º camshaft. You need to find out what is going on with that, and then you won't need to use the band-aids mentioned above.

 

I think I found it.... I took the carb back off to look at the base gasket, and hey, there's a hole in the plenum floor of this holley intake. I googled and it's something to do with egr. Even though egr is blocked off on this intake, and always has been AFAIK, some threads online say it's a vacuum leak to one runner. I don't understand but I cleaned it all up and marine-tex'd it shut as they said to do.
now I have a solid 16" at 1000 rpm like the cam specs say. Brake pedal is much better. Idle is much smoother.

And... now the idle mixture screws actually do something noticeable

 

You might try bringing that idle down to 750 rpm.

 

Depends, if that is in Park, it should put it around 750 rpm in gear, about perfect IMHO.

 

My idle is not set to 1000 rpm

 

Sweet !

Hystat how does she feel, healthy wheelspin on a dead punch ?

 

one wheel peel. Has 2.73 gear and 245 width tires so not a tire burner per se. I have one of the 3.23 posi 12 bolt with 10 bolt cover unicorns in the garage I will swap in when my cargo coils get here.
I rechecked compression properly. All eight between 152 and 158.
car is nice cruiser now.

 

Nice, as I understand it those are very good numbers for a post 70 350 with a mild build.

Good gear for both ends of performance. My 66 442 came that way, car felt great and it was only a 2 speed Jetaway.

 

Not really a unicorn. It’s what my car currently has installed, and there are two more O-Type rearends stored behind the garage.

 

for 73-77 a-body they're pretty rare. Like there's not much evidence GM made them for this application.
I think this one came from a 75 grand am
also, type O were a 10 bolt ring gear i think. This is 12