Honda CB750K Oil Pressure

| 8/20/2014 10:39:00 AM

Motorcycle Classics tech expert Keith Fellenstein

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Honda oil pressure

Q: I have just finished rebuilding a 1970 Honda CB750K. It is my first attempt at rebuilding this type of engine. I have rebuilt other types with good success.

I now have about 480 miles on the engine and it is running great. It sounds good and doesn’t leak a drop. My problem is the oil pressure. Lately I find that after running the engine for about 30 minutes the oil pressure light comes on at an idle. I suspected the oil pressure switch. So I pulled it and have installed a pressure gauge. When cold the pressure starts at about 65lb. After running for about 15 minutes it drops to 60lb and remains there no matter how long I run it. — Daniel Pensyl/League City, Texas

A: According to the shop manual, the oil pressure bypass valve is set to 56.9psi at 4,000rpm for an engine temp of 176 degrees Fahrenheit. It looks like your engine has the correct oil pressure. An old rule of thumb was 10lb of pressure for every 1,000rpm, which makes your setup exactly right at about 6,000rpm. If your oil pressure light continues to light, it could be the switch that’s faulty. They’re still available for about $35. If you look for it on a parts schematic, it’s part no. 37240-P13-013 on the starter motor schematic. MC

11/11/2014 11:26:35 AM

I had a similar experience with my recent rebuild of a 1971 CB750-K1. Whenever the engine got fairly warm, the oil light would come on at idle. Revving the engine make it go out. I temporarily installed an oil pressure gauge and ran the engine up to operating temperature. Sure enough, I watched the oil pressure drop as the heat increased. It went as low as 30 PSI which was enough to convince me the problem lay within the pump itself. This engine had been totally rebuild (by me), but I used the original oil pump without disassembling it. New internal seals or replacement pumps are not available. The engine had previously suffered case damage from a broken drive chain, so I had replaced the case as a set when I rebuilt, never suspecting there might have been collateral damage to the pump. Once I identified the problem, I replaced the pump with a salvage unit, which tested fine on the bike. When I did the 'autopsy' on the original pump, I found the outer centrifuge rings were both badly scored. Most likely from debris from the case damage.