Non-Pcar Engine Vibration and Ignition Timing

[Hugh R]

Hi All:

Not a pcar, but my 1961 Aston Martin DB4. The Aston board isn't very active, so I thought I run this buy you knowledable chaps.

I've been getting a vibration in my trannie at about 3500 to 4000 rpms, and then the vibration disappears and I believe its the engine, it does it in the garage with car in gear and foot on the clutch, so trannie isn't moving. My ignition timing is at 28 degrees at 2500 rpm, (where the factory said in 1961 using any "Suitable grade of regular petrol of 100 octane or greater").

I can only get 91 Octane fuel in California. Granted that in 1961 100 Octane to the Brits meant Research or Method octane rating, not R+M/2, I'm wondering if its pinging so bad that its vibrating the trannie. If I back the spark advance off from 28 degrees at 2500 rpm to less than that, the car won't idle as the advance is then somewhere around zero.

Any suggestions? Aviation fuel or octane boost? or recurve the distributor advance. I believe recent rebuild with new pistons, rings and valves and guides increased the compression and is causing this vibration/pinging. The crank, front damper and flywheel were zero balanced along with the pistons and rods to about 5800 rpm by the machinist. Thanks for any thoughts.