Vacuum loss during acceleration issue

[Chuck Step-a-side]

I have just installed a used 350 crate motor (about 2000 miles on it) and rebuilt TH350 in my '84 Chevy Stepside. I have also installed a new Edelbrock manifold and carburetor to the 350 motor. The motor right turns over and idles like it should. While driving the transmission won't shift into second gear until around 35 to 45 mph and the gas is sluggish when pressing the pedal. Has anyone experience this or may know what my issue is. And as usual, thanks you for your help.

[Irish_Alley]

where do you have the vacuum hose to the transmission connected? and where do you have the vacuum hose for the vacuum advance connected to? they both should be connected to manifold vacuum. the vacuum advance doesnt have to be but for best performance it should

[Chuck Step-a-side]

The vacuum advance is on the carburetor (left port) and the transmission and interior vacuum line is on the manifold. I forgot to mention one very critical part - the manifold vacuum drops down to 3 psi while increasing the engine rpm which explains the transmission shifting.

[Irish_Alley]

i dont know why but i didnt even see your title to the post lol. im not really knowledgeable on vacuum issues. have to wait till the others respond

[VileZambonie]

The vacuum advance is on the carburetor (left port) and the transmission and interior vacuum line is on the manifold. I forgot to mention one very critical part - the manifold vacuum drops down to 3 psi while increasing the engine rpm which explains the transmission shifting.

So you are reading a positive pressure when you open the throttle? I question what type of gauge you are using. You should be using a vacuum gauge inHg (inches of Mercury).

Ensure you have vacuum present at the vacuum modulator on the transmission with the engine idling for starters.

[Chuck Step-a-side]

Yes, I use the Mighty Vac vacuum gauge and my readings should have been in Hg (inches of Mercury) not psi.

[75gmck25]

The carburetor has manifold and ported vacuum ports.  Are you sure you don't have the carburetor's ported vacuum port open so that it creates a vacuum leak as you come off idle?   

Bruce

[Chuck Step-a-side]

I have the carburetor's back manifold port going to the brake booster and front right ported vacuum port going to Vacuum advance and the front left port plugged. The manifold has a Tee with one port going to interior/canister vacuum and the other port going to the transmission modulator.

[VileZambonie]

Did you verify the vacuum signal at the modulator? Detent cable properly adjusted?