I have got two of the first 300's from early 04. (Z300TURC).. I bought them used in 07 with 49 hrs on them. They have had some issues... Yamaha has provided various modifications during the warranty. Just before the end of warranty in late 08, an aluminum housing for the cooling on the back end of the port motor broke apart. It a somewhat known issue that can be found in various forums and not just with 300's. The corrosion issue is where a stainless spring sits directly against thin aluminum cast in the cooling water that is salt... so it corrodes. The break put salt water into the motor pan while I was running back home 20 miles off shore. The air intake is low and sucked up some salt air mist and fouled a few cylinders. It took 7 hours to putt in on the starboard motor!. The port motor ran a little rough, but the dealer said the power head was done. With a little persuasion, Yamaha covered the new power head under warranty. They tried to say it was lack of maintenance but it was pretty obviously an issue... so they agreed to cover the 10K in motor repairs... Thank you Yamaha!. While in the shop they upgraded the oil pumps to inject more oil in mid range use.... That sounded great also....and I appreciated that but they stuck me with fixing the corrosion issue on the starboard motor because it was not broken..... it was going to break so I fixed it so I did not break down 20 or more miles off shore again. They also stuck me with two new water pumps... but that many have been the dealer getting some extra cash.. not sure how that ended up being my cost... I was happy to shell out $800 for a fresh factory tech tuned motors that I could rely on.
Well... the new oil pumps inject a lot more oil now and foul the plugs when I went trolling (idling reduces the cylinders to two or four)... They did not foul before the warranty work!. I Had the experience of loosing a motor after a few hours on troll...Well, there is a fix for that. Troll for a while then pull up and burn the buildup off the plugs (2500rpm or more for 15 minutes) then troll again. Swap back and forth rather than troll with both and carry a few sets of new replacement plugs. A hassle... yes but then again it's 40 to 50K to re-power and I did get a deal on the used motors so I am still not really complaining too much.
As of last weekend, I have a pissing problem on the port engine... so I am back on-line to research that issue. Ever since the warranty work was done the pisser has been weaker than normal. This winter on the last fire up.... it was a dribble. So I am land locked until that gets fixed.
All in all.... I am a Yamaha fan. dirt bikes, street bikes, 4-wheelers and outboards. Over all my years of ownership I have had one on going issue and its the port side z300. I run up to 120 miles off shore so I rely on these motors to operate and for the most part they do.
I also have a 225 HPDI and it blew a power head last spring... It started running rough so I shut down, paddled back and took it in. No over heat, no over rev, no computer issues. They figured two oil injectors clogged. That motor was 6 months out of warranty! but Yamaha replaced the power head for free with out any persuasion.... the dealer even fixed my boat tailor lights...the bill was $0.00! I tried to take the dealer staff to lunch and they decline and said the motor should not have failed and I did not need to do anything but go back to fishing. How cool is that! (Mt Houston Marine, Houston Texas) I bought that motor used also with about 50 hours on it... It failed at 160 hours. Runs good now..
So I deal with a few issues.. is there a boat owner that does not have a few issues every now and then? I think most of the issue I have had with the 300's is the fact that they were new in 2004 and the product was not fully field tested. If the motors were purchased after 2006 they have more of an impression of reliability because the early motors had to go in several times for upgrades. In my research on issues, most of the complaints are pre 2007 posts.