I've got a 68 that recently lost power. I tried searching the forum for related topics, but I'm not sure what it is I'm looking for. Here's the issue:

Driving home yesterday, all of a sudden I could feel a loss in power. I was doing about 40 mph. She still ran but when I would step on the accelerator, there was a noticeable lag. She would hesitate and then go, but it was still diminished. The exhaust no longer sounded smooth, but struggling and loping, like maybe one of the spark plugs fouled but those were all replaced two months ago.

I've checked the timing and it looks okay. The vacuum lines to the carburetor and distributor look fine as well as the one to the transmission. When she's in gear, when I first accelerate there is a strange hiss, which sounds like a vacuum leak, but I can't find where it's coming from. There's also a strange, intermittent chirp coming from somewhere, barely audible, but I've never heard it before.

The engine is a ZZ4 350 crate motor with about 60K on it in front of the classic TH400. Transmission is full of perfectly pink fluid and was rebuilt a few years ago; she doesn't have an issue shifting gears.

Any help would be much appreciated, this is my daily driver.

 

Im fairly new to trannys so be chill, but does yours have a vacuum ?modulater i think it is? To control it?

 

The vacuum modulator looks fine. There's also a coolant leak on the back of the motor (blown head gasket? intake manifold gasket?) but if that was the case, wouldn't I have trouble all the time, and not just in gear? She seems to be missing as well but I'm not able to take her apart just yet.

 

Get the leak fixed. Soon....now. Double check the vacum hoses and timing after the leak is fixed. When your car is in gear it is under a load and will respond differently, even if you are sitting still.

 

My vote is a bad intake manifold gasket.