Honda CB750K Oil Pressure

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

Motorcycle Classics tech expert Keith Fellenstein

Editor’s note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.

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.

The sound and the fury: celebrate the machines that changed the world!

Motorcycle Classics JulAug 16Motorcycle Classics is America's premier magazine for collectors and enthusiasts, dreamers and restorers, newcomers and life long motorheads who love the sound and the beauty of classic bikes. Every issue  delivers exciting and evocative articles and photographs of the most brilliant, unusual and popular motorcycles ever made!

Save Even More Money with our RALLY-RATE plan!

Pay now with a credit card and take advantage of our RALLY-RATE automatic renewal savings plan. You save an additional $5.00 and get 6 issues of Motorcycle Classics for only $29.95 (USA only).

Or, Bill Me Later and I'll pay just $34.95 for a one year subscription!

Facebook Pinterest Instagram YouTube Twitter