I purchased the 4100 used at 600 hours, now 1200. I have had no issues with the Kawasaki 25 engine at all. It is air cooled. It does not have the vertical stack muffler so it has a good bark to it. I have a new muffler on hand to install so maybe that will help. It has good power although I use it mostly with the scoop, pesticide sprayer, electric seeder, aerovator, and rock rake so I rarely run at full throttle. Consequently can't really comment on the fuel economy other than the first couple of years, prior to the 4231TD purchase, I used it with the 60" mower and 72" rough cut and it seemed fairly efficient. It's a bit cold natured when the temps fall below freezing, but has always cranked within 20-30 seconds. The rough cut does better attached to the 4231TD when the pastures start to toughen up in July & August. Both tractors have been good performers, but as you say they do need attention. On the other hand my wifes 7 year old Hustler Z / Kawasaki 20 seems to mow year after year with oil and filter changes. Just add fuel.