Just bought a Yamaha 70ETLK on E-bay (no giggling please). Seller claimed he was a marine mechanic with 30 yrs experience, and his feedback was excellent. In his ad, stated the outboard had a rebuilt powerhead with new Wiseco pistons .020 over, new water pump, bearings, etc. I removed the plugs, checked the bores, and verified the new pistons. After mounting, took for it's first run. Overheated within 1 minute of running at 3/4 throttle and water pressure dropped to 5 PSI, so assume the impeller is slightly cooked. Removed the thermostat at the landing, ran again without an overheat issue. I'm going to replace the thermostat and impeller, but wonder if there's any heating issues with this family of engines that might be an issue.
