2003 Yamaha 90hp four stroke

I have recently purchased a used 90hp Yam 4stroke. The clock says just under 300 hours. After installing the motor and initial start it seemed fine. The next time I started the motor I got a fair bit of blue smoke at start up and it then cleared up. Ran a few gallons through it and it ran fine. Next start up it smoked again. That run was about 5 hours. Started the next day and this time no smoke. Started it again later that afternoon and just a hint of smoke. Relating it to a car engine where the valves are almost upright, where oil can seep into the valve stems/seals, but this is an upright 4 cylinder and the valves appear to be vertically positioned. Going to try and see if it smokes or not after shut down is made with motor trimmed up and then down (like dogs tail). I want to see if that makes a difference. Any ideas? My first thoughts were valve seals/guides but now I'm thinking sticking or even broken oil ring?

Can you do a leak down test?

what do you mean by "leak down test"?

It's like a compression test except each cylinder/piston is positioned top dead center (I believe) and then pressure is applied into the cylinder through the spark plug hole and you determine if any of the cylinders are leaking beyond what is allowed by specs. And if they are you are able to determine where the leak is occurring (ie...intake valve, exhaust valve, or cylinders around piston rings). You need a mechanic that is experienced to perform this as it isn't as simple as a compression test.

Here is something you can do to start. Perform a dry compression test and then a wet compression test. If the results are different then your rings are bad. 4 stroke engines don't smoke. If they do that means the rings are worn and the valves are not seating properly. In both cases they allow oil to enter the combustion chamber over night. Therefore causes smoke on start up. The simplest thing you can do before buying any engine is to do a compression test.

Thanks for the advise.......I thought about "leak down" after I sent the question and kind of figured it out......I wasn't familiar with that term so it took me a bit.......I'm a good helper but not quite a mechanic......I'll pull it out of the water in the next few weeks and see what I can find.

Thanks.......I'll let you know what I find in a couple of weeks.