I've been playing around with it in the river - it's better now (set much richer) but not perfect - only can get about 3/4 throttle before it bogs down again. There's only the one mixture needle - no bottom end setting. BTW, the cooling system works at idle (just!) so thats not a problem.
Even at 3/4 throttle, the boat moves like there's no tomorrow! It doesn't look like a speedboat but it certainly goes very quickly. Looks like 2 months of hard work has created something worthy. It's still not perfect though (there's a split in the keel that needs solving with some fibre-glass and resin - the hull was leaking earlier!
I'll post some pictures when it's all done - so far, so good, though!