Honda generator help please!

[spikey78]

Ive got this pretty clunky old 2.5kw Honda genny that's a bit of a donkey to be honest but just about does the job - anyway as far as I can work out it should have some sort of automatic idle control so that when you put an electrical load on it the revs stay about the same. Mine appears to have the mechanism but it doesn't operate and so I have to manually wind up the idle speed.
Anyone know how its supposed to work? I haven't really looked at it that closely and I expect its pretty simple but any info that could be gleaned beforehand would be useful! Thanks