Thierry,
I don't want to alarm you, But I would seriously look into
having the lower end of your engine looked at. You may have problems
with your crank bearings. the 1500cc Spit engine is notorious for
having problems with its bottom end, and part of your regular
maintenance should be replacing the main crank bearings every 80km.
Many years ago I drove a '79 Spitfire 1500 which started to make a
rattling noise under acceleration. I ignored it for too long and
ended up throwing a conn rod through the engine block. The fact that
your vibration occurs only with a warm engine (meaning the oil
pressure is lower) and while accelerating (higher stresses on the
engine) means to me you could have lower end problems.
It also may be as simple as a loose sparkplug wire. The vibrations
that occur from a misfire are more pronounced while the car is
accelerating under load.
Let us know what you find out
Karl Theis
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