Yamaha 140 BET 4 cyls 140 HP outboard, premix, 6 x 3 months seasons insalt water (aprox 300 hours). Off seasons I run it weekly for 15 minutes with the garden hose and muffs.<br /><br />Last year it started overheating past 4000 RPMs, sometimes: pressure would climb up to 20 psi and suddenly start to drop to 2 or 4 psi. Took it to idle and restarted and sometimes it would stay OK at around 16 psi, others it would go down again to 2 or 4 psi. Apart from that, never a problem. <br /><br />Spark plugs and lower unit oil changed twice a year. Always started (except for a bad battery once which cured it for good). Impeller changed once every 2 years, and greasing points relubed yearly.<br /><br />Last week I changed the impeller, head gaskets, cylinder zincs, Tstats, and removed some sand and salt from head cover, around cylinders, Tstat housings and everywhere I could reach with a wire or hose. Flushed it several times and to the ocean. Ran this weekend OK upto 4,000 RPMs no problem. At idle in less than 10 seconds pressure climbs to 6 or 8 psi. At 4000 RPM pressure upto 16 psi, after 10 seconds or maybe 20, pressure slides down to 2 or 4 psi and temperature starts to rise. I think it used to go higher to approximately 20 psi before. Tell tale shows a good??? stream of water. Tried replicating this in neutral with no success and the water pressure will rise above 20 psi at over 4500 RPMS in neutral and stays there. Been able to replicate the problem reliably this weekend while in gear. My idea is that the poppet valve spring is weak (salt water, 6 years) and running at speed creates a vaccum which aids the poppet valve opening. Since this vacuum is not present at idle, I cannot replicate this in neutral.<br /><br />I will try to open the poppet valve with water pressure and a gage to see what readings I get.<br /><br />Can anyone confirm these ideas for me?<br /><br />Can anyone confirm how the poppet valve should be placed: long end towards the engine or short end towards the engine?<br /><br />Thanks
