Hope this is not the case as mentioned above but several years ago I tried out a used boat that done the exact same thing. It would idle all day and wouldn't get on plane at all. We took it back to the gentleman's house and he had a compression check ran on it the next day and one cylinder had zero compression. Hopefully his will be a fuel issue and nothing major. I wouldn't buy a used boat today without running a compression check first. They don't make a cheap powerhead.