Stock SBC 350 Crate Motor valve chatter under load
Check to see if the rotor is pointing to #1 sparkplug wire on the distributor cap when it is at TDC or between 12*-18* BTDC. Do this with a breaker bar and socket by hand not with the starter motor. If you are using the rev limiter set it at 5500 rpms. What are the settings on the electronic mechanical advance curve? Are you using vacuum advance also? Is there a vacuum line used from the carburetor to this distributor? I was wondering about this because the directions say if vacuum is not wanted simply do not connect the vacuum port. Post back the settings. Another possibility is you have some bad gas (water in the gas). To a half of tank of gas add 1 bottle of Chevron Fuel System Cleaner that treats 20 gallons of gas and a bottle of STP water remover. Drive it around about 20 miles to see if there is a improvement. If it is detonation try putting about 5 gallons of 93 octane to what is in the tank now. Try for a half and half mixture to the amount of gas you have. This should stop that. Post back the results.