I a have freshly rebuilt 425 in an F-85 Cutlass that my son and I did a few months ago. There are few hours on the motor as the car wasn't drivable until recently. Under load (steep hills) or when I put my foot into it, the valves become very noisy (rattling sound). The heads were rebuilt and converted to comp cams roller rockers with guide plates etc. Distributor is an MSD plug and play. Looking for thoughts on how to quite this down. Do the valves need to be adjusted once they break in, bad gas, timing, all of the above or something else? Thoughts are appreciated.

 

Sounds like detonation. When you installed the cam , did you degree it in?
With an aftermarket cam you need about 16/18* at 900 rpm then 36/38* at 2500 for a total.
May be to lean, what carb and what jet size? Need all the info you have.
Gene

 

Here's the rebuild specs on the 1967 425:

Crank - Steel Forged/Balanced
Pistons - Egge 10.25:1
Rings - Federal Mogul Sealed Power (file gapped)
Heads - C with valve job but no porting
Valve Train and springs - Comp Cams Mag Rods, Mag Lifters, Mag Rockers, Guide Plates, BeeHive Springs
Cam - Mondello JM 20-22
Cloyes Double Roller Timing Chain
Intake - Edelbock Performer
Carb - Edelbrock 750 CFM (factory jets)
Exhaust Manifold - Hooker Comp Headers

Cam was degreed and came out spot on.

 

Back off the timing or add race gas. The better gasoline will be MUCH more fun!