1200 rpm idle

[lmdpth@lmd.ericsson.se (LMD/T/TB Peter Thorsen)]

My 1500 spitfire will not consistently idle below 1200 rpm.
Can this be caused by a leaking inlet gasket?
How can I test if the gasket is leaking? I read somewhere that
you can spray a certain fluid around the manifold with the
engine running and a leaking gasket would cause this fluid
to be sucked into the engine and cause misfiring or at least
a change in rpm. I just don't remember what fluid this was or
where I read about it. Any ideas?

 - Peter