Jeff, before you give yourself an ulcer there are items that can sound like an engine knock but are not.

How's the hot and cold oil pressure?
Is the car standard or auto?
Does it do it on startup or all the time?
 

A legit knock is there all the time and changes frequency with RPM.  Be sure its not a lifter.  Be sure its not detonation. 

You can change the main bearings and rod bearings both with the engine in the car, but you have to ask why it went bad to start?  Chances are new bearings are just going to do the same thing if the clearances are wrong or the oil pressure is too low.  BTW, I've never seen or heard of a main bearing causing a knock, rod bearings do that in my experience. 

Do what 351MooseStang said.  Pulling the wires will usually find a rod knock.

Did you assemble the engine yourself?  Checked all clearances?

If you are certain the engine was built properly, sometimes an overuse of RTV can cause a little blob of my favorite stuff to find its way to the oil pan, then to the oil pickup, then the pump, and can even SOMEHOW make its way to the crankshaft internal passages, where the pressure and centrifugal force of rotation throws it towards a rod bearing oil hole in the crank.  And then it plugs the oil supply.  That is one method of failure.  This usually results in a crank needing to be reground.  The rod usually survives if you stop running the engine as soon as you hear the problem.

My 351W with the reduced base circle roller cam would make a noise sometimes that sounded like a ballpeen hammer on an anvil.  It came and went and was attributed to roller rockers or cam related.  Teardowns found NOTHING and the engine still lives on.

I had a 351 I built once that developed a rod knock.  Never found out why.  I know it was done correctly.  Sometimes, parts just fail for no apparent reason.  Had to grind the crank and install a new rod bearing.

Hope you can figure it out and the fix is not too bad.  Be sure to let us know.

Do you have Aluminum valve covers?
6sally6

351MooseStang wrote:

Both of the above suggestions are good ones. I had the two-piece pump eccentric crap out on mine years ago, pulled it and went electric pump. On the valve covers I run the old M/T covers with Crane roller rockers and had to add 1/8" spacers because the covers would eventually settle on the gaskets, allowing a few of the rockers to barely contact the covers. It was fine at first but the sound scared the crap outta me later on because it made a hell of a racket.

Good luck!

Yep, and that's why I check clearance on valve covers with clay, just like checking oil pan clearance.  I had to reengineer the breather baffle on my 331 to clear the rockers.  That kind of stuff is always easiest when the engine is still on the stand. 
 

First, seat of the pants isn't accurate.  You learn this when you race.  Runs that "feel" slow are typically your fastest, and runs that "feel" fast are typically awful.  Why?  Simple, slow is smooth and smooth is fast.  Sounds, chaos, things happening when you don't think they should (seems early, etc.) make a car feel fast, because its out of control.  When you and the car are in sync and your mind is reacting properly it isn't out of control, so it doesn't "feel" fast, but it is.

Second, could a carb go lean at 5,500RPM?  Sure.  It's pretty simple really, either its supplying enough fuel or its isn't.  Fueling requirements change as RPM rises because you have to fill the cylinders more in a given amount of time.  On acceleration, going for max power you are typically going to want an A/F ratio in the 12.5:1 range, and if the metering is wrong and it is going lean above 5,500 then it needs to be recalibrated.

Third, there is really no guesswork with an Edlebrock carb.  You need one of their tuning kits for your model carb.  That kit contains a booklet that has a chart that will show you what changes to make to enrich or lean the mixture under various conditions.  In your instance I would look to enrich the power portion of the fuel curve one step and see what happens.  This will typically mean swapping the metering rods, but go by the chart.  If it's not enough the chart will likely tell you two steps will involve swapping the jets and going back to the stock metering rods.  But I would advise going one step at a time.  There's nothing wrong with going too far and having to go back to the previous calibration, in fact I'd recommend it, but if you jump steps you can go too far the other way without realizing it.  The danger there is that a rich condition and a lean condition often "feel" the same.  If you jump over ideal and go from lean to rich you will operate under the mistaken belief that you haven't fixed the issue and continue to enrich the carb trying to solve a problem that you actually created. 

My $0.02, what you are describing is a surge, and a surge indicates a lean condition.  BUT, it also indicates that you are close to proper calibration, just a tad lean, so I'd move up one step in the power calibration and I'll bet it solves it. 
 

Springs need to be stiff enough to not bounce. They do that when a lumpy cam is installed. Stiffer springs held greatly butt......not too sstiff. A little trial/error helps. Play with different metering rods too. Real simple and NO spilled gas. Jet changes are pretty rare but same results...no spilled gas. On line instructions helped me more than the book.
6sally6

M1Mustang69 wrote:

TKO thats brilliant, thanks....

I know nothing of Edel Carbs and just have very little exposure to the Chart.....I did/want a starting point......

So 1 up the metering Rods and test I shall do.......what do the springs do and how do they do what they do; also, would the springs affect 5,500 and higher?

I tested just about every step up spring in the kit, but found literally no difference so I just went back to the stock ones.  That said, I have a stock hydraulic A code cam in my 289.  What I've heard is that you take a vacuum reading on a warm engine and use half that reading to determine the springs to use.  The color corresponds to a number, like orange = 5, etc.  As 6sally6 said, the springs have to keep the rods down at idle or idle fueling gets erratic because the rods move from the idle position into the transition position, adding fuel. 

I would get your cruise and power settings right first, then if and only if it still has a stumble off idle consider playing with the springs. 

The rods have a couple of design elements that effect fueling.  The diameter of the rod affects fuel flow at various points, and the taper or transition from one diameter to the other also effect the rate of enrichment. 

if you go to page 4 of tips and tricks on this forum, John Ha has a spreadsheet posted, Pretty thorough.

Edelbrock has a scatter plot that shows you what rods and/or jets do.  It will move rich or lean in 4% increments.  If this works as an attachment it will show you what I mean.  THIS IS NOT for the carb you have though; its for my 1403.  This is in the booklet that comes with the Edelbrock calibration kit for your carb, and is also easily found on Google if you type in something like "Edelbrock (carb model number) tuning".