Problem is when I take off my car will drastically deaccelerate almost to a stop. This happens 1st through 3rd gear. A little background first. I bought it 1 yr ago and it drove fine. Until recently the master cylinder got stuck and I almost crashed it . Good thing I geared down to slow it. I was new to parts and couldn't find 69 master cylinder and brake booster so I bought some for a 72 camaro and it works fine now( stopping ). Then a week later I drove it again and then that's when the drastic deaccelerating happened. I figured it was the fuel pump.. I replaces that... still happened , so I them took the demon 850 mechanical carb and rebuilt it. And it is still doing the same thing. I replaced spark plugs to msd to match the msd ignition. Still the same. Could it be the clutch? I do see a leak there. I noticed the muncie is a m20... so I checked and it didn't have any gear oil in it. So I put gl4 gear oil in it amd it still does it... what am I missing?

 

First thing we need to do is define the problem better. Is this an engine or brake issue. Your wording is messing me up you can not take off and deaccelerate at the same time. Are you saying you launch the car it goes for a few seconds then it stops pulling or does it feel like someone hits the brakes? If it feels like someone hits the brakes does how does it act if you push the clutch. How does it act if you do the same thing with you foot on the gas. All a clutch can do is slip. This would not feel like a deacceleration as much as a lack of acceleration. It would also be worse at launch and grab more as the car starts to move.

The most common this would be what we call a hesitation or some call a bog. Car goes then falls flat maybe even stalls but most of the time it just drops so much power it feels like the motor just shut off. If that is the case the next question is how does it run if you slowly accelerate? Do you still fall flat or does it get up to full power eventually?

Does your 69 have drum or rotors brakes on the front. If it has drums or its a Z28 with they upgraded brakes then the 72 master cylinder is a problem just not the problem you are having right now but one you will have in an emergency stop.

 

Sounds like it might be falling on its face due to lack of fuel?
Double check the float level (see if you can blow air into the cab with your lips empty)
Check the fuel pump pressure with a gauge
Manually check the fuel pump volume output
If there is a pinhole in the line to the gas tank the fuel pump will suck air and not pump fuel

 

Found out brake booster was pushing too much causing brakes to grab when coasting in neutral. It wasn't much but when the car ran for a good 5 minutes and the booster got warm it would cause the car to jump/brake. Made adjustments , good to go now. Much appreciated guys