Background: The engine is a 350 that was running ok before I pulled it from the donor truck. It came with a edlebrock manifold, holley carb and idled and ran like it had an aftermarket cam. I put it in my 64 and got it started tonight. Previous owner had the timing off by a couple of teeth (I didn't check the position of the wires when I pulled the cap)I reset the distributor and it started right up. I didn't do anything else to the engine except clean it up, new valve cover gaskets, switched over the waterpump, etc. Engine was supposedly low mileage rebuild.

Problem: When it started it was knocking really bad, sounded like denotation in a number of cylinders. Running it for a few minutes made it better but there is still definately a denotation on the 1-7 bank. It doesn't sound like a rod or main bearing noise. Adjusting the timings doesn't seem to help. I did drive it briefly to make sure the new donor trans was working. It accelerates quickly (wasn't ready for the increase from the old 283, it broke the tires loose off idle with a jab of the throttle)and runs good. Couldn't tell if the noise went away at higher RPM due to exhaust noise.

Carb is running rich and when trying to get it started the first time before fixing the timing is did backfire so I may need to replace the powervalve (it stumbles a little off idle). Also, the gas is lowest octane that was in the tank when the 283 went south.

I tried checking compression but my guage is not reading correctly (it read 60# at all cylinders but when I checked it against 120# on my compressor it only registered 45#)I interpolated and assumed that the 60 equated to about 150-60#. It doesn't run like its low on compression.

Any ideas will be appreciated, I don't understand where the denotation has come from.

Update: talked to a contractor that also races. He suggested that the motor may be higher compression and problem was the low-octane fuel in the tank. Also said he had a similar problem it found that the timing and carb set up was really touchy. I drained the gas tank, but in 5 gallons of 92 octane, checked the dwell (ok) and fired it up. Started running much better, no detonation. Fiddled with the timing but my light is junk and just ended up getting it close by ear. Listened closer and found that the noise is coming from the carb throat at idle to about 1800 rpm. Could this be timing or cam overlap resonating back through the carb? Probably didn't notice it before with the air filter on. Took it out for a drive and is runs really strong but hesitates and stumbles at idle or low rpm crusing. Work fine at full throttle! Carb id as 3910 which is 4150 780 CFM replacement for 67 Z28 302 or 375-396. Looks too new to be original but date code is 2921 (292 day year 71, 81, 91 or 01)so maybe it is NOS replacement. I need to figure out if it is going to be too much carb for cruising around town.