Ulf Witasp

Gentlemen,
I have to start with telling you I'm new to Oldsmobiles, I have only been dealing with Mopars before.
Bought this 70 Cutlass Supreme that's supposed to be a real SX, but I don't know for sure. Never mind. It has a numbers matching 455 with "the best flowing heads" as I read somewhere. I can't find my note with the casting number. I have no clue of the internals but it got to have a mechanical cam from how it sounds. An runs
I put new headers on, set timing to 36-37 fully advanced but I had (or have) some vacuum issue. It has an Eddie intake with the spread bore flange so I had to find an adapter for the carb. All good but still no good. I did all the usual troubleshooting, without digging too deep, but can't find out what's wrong. When I start it could it has 5-6 Hg vacuum, then after driving 5-10 minutes the power brakes start working and then i have around 10-12 and it idles too high (900 rpm?). I have tested different carbs, right now I run a Holley vac secondary. Put in a 3.5 power valve so it runs rich before it warms up and vacuum closes the valve.
When it warms up it runs like a dream. If I just roll 10-15 mph and floors it on drive it shifts into third gear with the 275's spinning until I let of. The gearing seems to be around 4.11 and of course that helps, but still
What could cause an engine to go from 5 to 12 Hg vacuum just because it warms up?
Thankful for any ideas!

442Harv

I would give it a compression test, cold then do it hot, and see what it says.

Ulf Witasp

So you think the rings can seal better when hot, or are you thinking valves?

442Harv

A leak down test can tell you that.

BillK

Almost sounds like you might have a vacuum leak. What adapter did you use for the carb ? Usually on the Edelbrock spread bore manifolds all you need with a Holley carb is the thin plate to keep from having a vacuum leak.

https://www.edelbrock.com/square-bor...-0-gaNycGzNDNE

Ulf Witasp

A vucuum leak yes, but what kind vacuum leak would go away when the engine warms up? That's what I wonder...

No I used a one like this:
https://www.google.com/url?sa=i&url=...AAAAAdAAAAABAE

Duh

With the amount of details you left out it can be anything.
What do you consider cold?
What carburetor?
How is your distributor set up?
Are you using a choke?
Does the car have any OEM emissions on it?
Details......

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Ulf Witasp

Cold - room temperature until engine gets as warm as it normally gets after 5 to 10 mins of normal driving
3310, 750
20 deg at idle, 37 fully advanced (w/o vacuum connected). Too slow curve, fully advanced first at around 2,800. Haven't had time to do anything about it
Tried to make that Holley choke work properly but gave up and stripped it open. The low vac signal won't close the PW anyway so that works a little like a choky
No emission gizmos

Duh

1. When an engine is not at operating temperature you have a less efficient burn. This will cause a lower signal (vacuum) especially at idle.
2. A power valve is not the same as a choke. A power valve has no effect at idle. The power valve only comes into play when you start to pull from the booster. A bad power valve diaphragm will cause an overly rich/ flooding condition.
3. A choke causes an increased signal to all circuits. This is how it richens the mixture. It also allows more vacuum to be seen.

Without being able to see the car in person my opinion is that it's most likely not a vacuum leak. For it to leak that much you would find it easy. I'm thinking your carburetor is the culprit. It just sounds like a bad tune up. Plus you don't know what cam is in the motor, that can make this condition worse. I'd check for an overly rich condition.