I use engin oil, and a light weight 3in one oil. I then fill both with
engin oil, then fine tune it with 3in one. Putting the 3 in on in the
dashpot which takes the longet to fall. That way both carbs have equal fall
and rise times, and so are extreamly well balanced.
If your oil is too heavy but your carbs are otherwise perfectly in tune you
will notice a hesitation when you slam you foot down that is relieve by
easing up a little. On the other hand too thin an oil and you will find
that trying to maintain a smooth 3000 rpm in second will end in the car
bunny hopping down the road.
These test should be done with the engin realy warm, i.e. driven 10 or so
miles. So that the dashpots have had chance to reach normal working
temperature.
A synthetic oil i.e. 10w50 will have the effect of keeping the viscosity
consten over temperature ranges so that the dashpots are not over dampened
when cold.
ps. isn't 10w30 a bit thin for a Spitfire engin, shouldn't you be using
10w40, 10w50 or so to save you bottom ends. I use 15w40 in mine, but am
going to change to 10w50 when I get the valve guides done!
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