Triumph Repair: Spitfire 1500 valve rattle, fuel air mixture, piston pin

Question
I have a 77 Spit 1500, electronic ignition, weber carb, and OD. The problem I am having is valve rattle. It seems no matter what I set the timing to the valve rattle. This only happens at cruise throttle setting. In other words when its under stress such as climbing a hill or loading the engine the valves don't rattle. But when I get to cruising speed of about 65mph and just hold the throttle enough to maintain speed the valves rattle pretty bad. If I increase throttle they quit and if I left of gas they quit. I have set the timing from 2 degrees ATDC to 10 degress BTDC. I have run regular gas through Super and it makes no difference. I am just making an educated guess that my valves my be getting weak and allowing the valves to float. Oh this mainly happens at rpms of 3000 or more.  Any help would be appreciated.

Answer
Hi Mark,
The misnamed rattle called "valve rattle" is detonation which is uncontrolled explosions rather then the burning of a fuel/air mixture in the combustion chamber.
The fact that your changing of the grade of fuel and changing of ignition timing and couple that with the fact that added throttle makes it quit is an indication that your noise is not detonation.
I also doubt it has anything to do with the valves.
Just from what you tell me I would guess that you have either piston slap, a piston pin problem or possibly excess carbon in the combustion chamber that is being struck by the piston.
I doubt it is big end rod bearings as decel usually makes the noise louder as does RPM. Plus you would note a loss of oil pressure.
Since load changes the noise you need to look at everything that is under load. Valves don't know if you are under load or not. Pistons, rods, crank, flywheel, clutch etc do know when you apply or don't apply a load. You should find an experienced mechanic to listen to it before you decide what action to take. Don't tell him my guess until you hear his evaluation. Do tell him about the fuel change and ignition changes.
Howard