Just put in a rebuilt 350 in my blazer, and when I get on the highway and speed up the acceleration slows down considerably once i hit 45mph. What could it be? I'm thinking timing is off, could that be it?

 

timing is def a good place to start.

 

I am having a very similar problem with a K5 I just put a 305 in. It's impossible to see the timing marks. Dang thing runs great until about 35-45, then acts like a govenor is cutting power.

 

is the exhaust emission valve on the pass side open? is it at the exhaust manifold collector

 

I checked the valve in the exaust manifold before installing the engine to make sure it was normally open, because I removed all of the vacuum stuff. It seemed like a pretty strong spring holds that open. So, I think it is open.

 

is your choke sticking?

 

I have a smooth running 305 with edelbrock carb and intake. I have great acceleration and power at low speeds, but at 4th gear power just drops completely and I can barely make it to 65.