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...

Bad lifter.

Also realize that installed spring heights open and closed pressures are somewhat cam specific. too high of a valve lift with too short a spring with high seat pressure will wipe out a lobe.

Also check for a bent pushrod, bad lifter combination of both. You could also, instead of the forementioned BS rhetoric I stated, A wrist pin knock or a bad valvespring allowing the valve to impact the piston top.

Thanks for the ideas. I did check all the push rods and they are fine. Might be a stupid question, but how do I check each lifter to see if they are good or not?

Also, not sure if this means anything, but the heads are double springed. Not sure why the machine shop did this but nonetheless that's how they are. Is it possible that there is too much spring pressure and are double springs necesarry?

I was wondering if you didn't have enough spring pressure. That's a really high lift cam and the springs may be too wimpy to get the valves closed fast enough at higher RPM.

I would bet good money that the cam eccentric is incorrectly installed.

Ok, so far I understand everything you guys are suggesting except the post above. I have to ask, what is the cam eccentric???

That's the thing that the fuel pump lever rides on.

That's the round piece on the front of the cam that drives the fuel pump.

could be the vacuume advance or the fuel pump spring is loose (not enough tension on the cam eccentric). this could cause it to slap on the eccentric. my engine is similar the lifters seem to be pumped but then one or two chatters slightly then disappear. i have comp plus cam set with 3 springs. i cant figure it out. what is your compression? could be too high.

68 400 h.o 4 spd to go!!

Even without looking and listening, I'll put money on it. One reason is the narrow, specific rpm range of the knock points to this as the problem because the harmonics--sping float/whatever vernacular--in the pump spring would come into play at a specific rpm.

The main reason is that incorrectly installed eccentrics seem to be a common problem, even with "professionals" (professional means that a person makes money at it, not any level of skill or compenticy.)

Out of the at least dozen times of being into the chain/gear, I have almost installed it incorrectly a few times. Strong mechanical aptitude and attention to detail will catch the problem before torquing the cam bolt, as I have caught myself getting ready to do the dumb stuff. (Yeah, I'm guilty of the dumb stuff too. As an old teenager, I had a mopar eccentric fall off a couple of hundered miles after the repair. Loose bolt, installed incorrectly--whatever. After it falls apart, does it really matter why?)

The eccentric has a tang that rests in a recess of the cam gear. If you study it and dry set it, the eccentric will fall into place and hold tightly with just a tad of hand pressure without bolting it on. It will have a caddywampus look in relation to the cam gear.

Do not start the engine again. If the eccentric falls off, there is a good chance that it will drop into the chain and the bolt will fall into the oil pan. All this can net total engine destruction. If someone can get pictues of the cam gear and eccentric, I'll try to point out the way it installs. Of course this involves pulling the cover.

Again, I'll bet money on this one. Any takers?

By the way, I haven't seen everything, but I have never seen a pontiac head/valve that doesn't have double springs.

I just happened to have a shot of mine.

I will pull the cover maybe tomorrow and take a look. This could very well be the problem. Remember, I am not a mechanic by a long shot so it is very possible that I screwed this up... Thanks a ton for the leads... I'll keep you posted!

With Christmas and all, I have yet to look into the leads you all have provided. However, I will be doing so soon...... Thanks again!

I just wanted to give props to Amervo, you were right on with your diagnosis of my engine knock. The builder found the cam ecentric exactly as you predicted. My cam was sliding front to back by a good 1/4" all due to the cam esentric thing impoperly installed. So anyone who changes a cam keep this in mind. My builder said this is only a Pontiac issue and is a known mistake people make when changing a cam...

For future reference, you should break in the cam with the single valve spring and then go to the dual. Obviously this was not your problem, but somehting to keep in mind for next time.

Thanks for the feedback. I cannot sling numbers, statistics, or select components, but I'm pretty good at figuring out what makes it tick, what's wrong and/or what it takes to fix it. Do you think that being a professional mechanic for 17 years has something to do with it, or it's from disecting Pontiac V8 engines for 30 years?

It's a common mistake, and it often takes some fishing to find the location. Anyone who claims it just falls in place and you bolt it up doesn't have much Pontiac V8 engine experaince. You see the results of simply setting it in place and bolting it up.

Merv, you da man. I remember you pointing that cam eccentric out on my old and new engines when you visited here. Remember the oversized fuel pump someone forced me to buy off Ebay?

Merv - when you're good, you're good! So... what does that expert ear tell you about my car?