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Cb750 dreaded no oil pressure


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  • Cb750 dreaded no oil pressure

    I just picked up a 1977 cb750 with a 836 big bore and cam to put in my amen savior frame. I have read every thread on no oil pressure I can find and still cannot get oil pressure. The previous owner told me the motor had been ran and had heat cycles but can not confirm. I have pulled the pump 6 times and tried different methods to prime it including letting it soak for 24 hours, and packing with vaseline. I then bought a rebuild kit and completely disassembled and inspected with no issues found and tight tolerances. Primed and reinstalled in the bike and ran the starter off and on for close to 4-5 minutes adding oil in the head, filter, and where the oil pressure gauge goes. No luck. I then put a spare oil pump on the bike and did the same thing with no luck. I finally started up the bike 5 different times running it 3-4 seconds each and still no oil pressure. I have verified oil is flowing to the pump and I know the scavenge side is working as when I let the bike sit for a couple days it will wet sump and as soon as I run the starter the oil fills the tank again. I am out of ideas and want to have her on the road when summer comes around so any other ideas are appreciated!

  • #2
    for '77, a K and a F model were available - know which one that engine is?
    Checked the oil pressure relief spring?
    Got an oil pressure gauge to adapt for measuring the oil pressure?

    The Oil Pump Rebuild Kit, Complete.
    "Pressure relief spring.
    Important to keep oil pressure up instead of dumping off into the crankcase.
    Note: Stock pressure relief springs are known to dump oil too soon and show low pressures when springs get old and in racing applications.
    Many people would add shims to the springs and boost the pressures a bit when needed...
    Our kits also have the discontinued OEM center oil seal that the other kits are missing..."
    Click image for larger version

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    Looked here?

    Click image for larger version

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    1977 pump diagrams, pdf:

    Click image for larger version

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    includes a link to this:
    The SOHC/4 Technical Documentation Library
    The SOHC/4 Technical Documentation Library is a service of the SOHC/4 Owners Club.


    • #3
      I think I would pull the oil filter and run it a few seconds to see if the problem was the pump or the rest of the oil system.


      • #4
        Thanks for the reply’s. It is a 1977 f model out of a super sport. I used that same rebuild kit except without the strainer screen so everything including the spring was inspected cleaned and replaced. Maybe I will try adding a shim to the spring, we’re they just using a washer of some sort as the shim? I pulled the oil filter and was turning it over with the starter before the pump rebuild and nothing was coming out. Post rebuild when I went to pull the filter there was some oil in the filter but I believed it to be from filling where an oil pressure gauge goes by the points cover with oil and it working it’s way back to the filter. I do have a pressure gauge on it and it reads nothing as well as I have taken the gauge off and ran it/ turned over with the starter and nothing comes out of the hole.


        • #5
          look here:

          Topic: Help! - CB750F Oil Pressure

          check oil seal between feed and return rotors, full immerse pump for priming, ensure body is tight to mounting flange, not held off due to 1 of the 3 locating dowels.

          are you sure the oil pump drive gear is actually turning the pump rotors...not sheared drive gear pin?


          • #6
            Click image for larger version

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            I pulled the drain pan and turned the motor over with the starter and all of the oil when I turn it over is draining from above the pump where the picture shows. Is that normal?


            • #7
              look closely at the oil flow image above in post #2; oil goes through the feed rotor, then to the oil should be coming out of the oil filter feed hole. go to the link.
              missing one of the collars (part #9)?

              ...Maybe I will try adding a shim to the spring, we’re they just using a washer of some sort as the shim?
              No, that is referring to the oil pressure relief valve - if the oil pressure is low, the spring may be weak, so shims (washer) is added to stiffen the spring so more oil pressure is required to open the pressure relief valve.

              Possibly the one of the two valves, the oil pressure relief valve, is stuck open.

              Some good info here on the two valves in your pump:

              2016 Topic: Oil flow to engine

              "There are two valves there, detailed in the lower right corner of the diagram above, and here:
              "leak stopper valve"
              Click image for larger version

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              The valve (#10) under the cap that is secured with two hex bolts (#13) is the "leak stopper valve", this prevents oil from running through the pump under the force of gravity, when the bike is not running.
              It should pop open under light pressure once the bike starts running, I don't know if there's a spec on this but someone else here may know (elan?). This valve has a piston (#10) with a rubber face (#11), held closed by a spring (#12).

              The other valve (#5) is under the round cap (#7) with the hex shape cast into it. This is an all-metal plunger (#5) with a stiffer spring (#6) than the other valve, this is the pressure relief valve, it opens when the oil pressure gets too high and relieves pressure by dumping into the sump.
              pressure relief valve
              Click image for larger version

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              It's possible that your leak stopper valve is stuck closed, or your pressure relief valve is stuck open."
              "...found a problem with the exploded view of the pump. In this picture which is in the shop manual and the Chiltons the valve (piston) which is #10 in the drawing looks to be all the same diameter. It is in fact a smaller diameter on one side. The smaller diameter goes into the bore first. This is the side where the seal sets. The larger diameter is on the outside where the spring sets. I don't know what happens if this is installed backwards but I thought it could be a problem."

              Last edited by TriNortchopz; 03-21-2023, 7:08 AM. Reason: added images and details of pump valves, + shim info


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