When Hugh takes David out in the boat, he speeds up and cuts the engines so they can talk. As soon as he turns off the key, the boat stops dead in the water with no drift whatsoever. It's almost as if it were anchored, though one was not seen used.
The MGB allegedly has a bad carburetor, and David repairs it with a spare. The MGB actually has two carburetors, as clearly seen in the film (later US models had only one so this was feasible until that point), and they are not identical, so it wouldn't be possible to know which is bad without diagnosing the car first. Also, when he first starts it, the air cleaner assembly is already reinstalled. He'd likely start it, get it warmed up, then adjust the carbs before reinstalling the air cleaner assembly.