Oil temps above 260 really do require an oil with an HTHS of at least 3.8, and better still 4.5.
240 is a fairly normal oil temp, a bit warm but it will work.My car has an oil temperature gauge in the gauge cluster. Even when going all out on the track the oil temperature gauge has never read about 240*F. Now, a caveat: the gauge is controlled by the engine computer (DME).
I'm thinking just cool it down.Oil temps above 260 really do require an oil with an HTHS of at least 3.8, and better still 4.5.
Although the Honda 390 is probably the best OPE engine ever made, when it's run hard, and above rated RPM, that is probably not the best choice. I lost 2 new Honda powered water pumps using the recommended 5W-30 oil. Like the OP, the engines ran hard, and the oil was not able to protect the connecting rod big end. Catastrophic failure was the result.Switch to a full synthetic 10w30.
One of my first cars was a '67 Chevrolet Corsa convertible. It had an air-cooled six cylinder 164 CID engine with four one-barrel carbs. It also had a head temperature gauge in the instrument cluster. I recall seeing 425 F when running 75 MPH on the highway for extended periods. The recommended oil was SAE 30.Oil temps above 260 really do require an oil with an HTHS of at least 3.8, and better still 4.5.
I just rebuilt the engine, I put the valves at almost zero lash as I expected the new tappets to wear a bit.Might be a good idea to check the shroud and cooling fins for obstructions as well as checking valve clearances.
I'm using synthetic 20w-50 oil.Switch to a full synthetic 10w30.
20w50 is thickI'm using synthetic 20w-50 oil.
I would use 10w-30 if I could get the oil temperature down.
Its a new rebuilt engine with around 10 hours on it, old cam new tappets.Have you looked into a valve adjustment
Possible a mechanical situationIts a new rebuilt engine with around 10 hours on it, old cam new tappets.
Zero lash is not good. Valve clearances will tend to tighten, not loosen over time. Valve seats receed faster than the cam and tappets wear.I just rebuilt the engine, I put the valves at almost zero lash as I expected the new tappets to wear a bit.
One of my first cars was a '67 Chevrolet Corsa convertible. It had an air-cooled six cylinder 164 CID engine with four one-barrel carbs. It also had a head temperature gauge in the instrument cluster. I recall seeing 425 F when running 75 MPH on the highway for extended periods. The recommended oil was SAE 30.