Ok techs, here is my question. I have a 68 400 block that was completely rebuilt 3 years ago. I've put maybe 6k miles on it since the rebuild. This past summer, late in the season I began to notice a loss of power. Keep in mind I am no mechanic. So, the first thing I did was remove the valve covers. I instantly noticed that one of the rockers had little to NO movement. I shut the motor off and instantly knew I had a bad cam.

The cam that was in the car was Comp Cam w/ a .495 lift. Not sure on the duration etc. The block was bored .30 over and all new bearings, rods, pistons etc. were installed. I also had the heads completely redone w/screw in studs etc. Hardened seats new valves etc.

So, I just decided to replace the cam. The cam did in fact have one lobe wiped out (3rd one from the firewall). Now keep in mind the only sign of anything wrong was a loss of power. No knock, no noise etc. Anyway, I have never done a cam before but thought I'd give it a go. I pulled it apart and installed a new Isky brand, Ram Air I spec. cam w/ a .512 lift (when I had the motor rebuilt I asked for Ram Air I specs on the entire motor but went with the .495 lift instead of the .512 lift).

I installed the cam 100% according to the directions etc. I then did the break in exactly as specified etc. All seemed fine. Then took the car for a ride and she was running excellent!

On the thrird test run I noticed what sounded like a knock. However, it does not sound like a rod knock as it sounds like it is coming from the top end. During the install of the cam, I made 100% certain that I never over tightened the rockers nuts as I was TERRIFIED of over tightening etc. I even have since gone back and removed the push rods and checked them for a bent one and found nothing wrong.

This so called "knock" is ONLY present at about 2500 RPMs and is not there at idle, or if you exceed the 2500 RPMs is dissappears. Now all total, since the cam install I have only driven the car maybe 5 miles and have only let it run for a few momonets at a time. I also did SEVERAL oil changes, before and after break in as per the instructions. The oil has never nor does not currently show ANY signs of contamination.

Also, I did change the cam and lifters, they came as a set. I used the cam lube exactly as directed as well. Keep in mind, this "knock" was NOT there prior to the cam change. Any suggestions and or ideas would be much appreciated. I am at the point where I am not sure what to check, what to do or what could even have gone wrong??????? Thanks in advance for any help you may have to offer...