Topic: 1970 cb750 oil pressure drop

[RayWrangler]

ok so i did a frame up resto on a 1970 k0

when i start the bike i get 60psi which i think is normal. after about 5min it drops to 40psi. i ride to the pub or around town. now about 15min into the rides it drops to about 0-5psi. way way to low for me.

i pull over wait 45min. start the bike and were back up to 60psi. than it drops faster, back down to 0 in about 10min.

i have fresh oil.
i have replaced the o ring seals twice in the past month thinking it was that.
oil pump looks fine


other info

when i did the top end, i put new gaskets and clean pan on, than i test road it, dropped the oil pan, and was a bunch of gross slug. cleanned all that out, dropped the pump. cleaned it. primed it and put back in. road the bike thought all was well. yet again dropped the pan an no slug, but changed to orings and did the pump again. remounted and thats were i am now.

what else could cause crappy low oil pressure?

[Elan]

Does the red light also come on? Wondering if its a gauge problem.

Hopefullly not a main bearing

[brandEn]

I think this is pretty common. Once the oil gets warm and thins out a bit the pressure drops. If your not getting an oil light and the guage pressure raises when blipping the throttle and is up say around 20psi when at cruise, your good.

[brandEn]

The main bearings are clam shell bearings and there are 5 of them or 10 halves....
Rod bearings there are 4 of them or 8 halves.

I seriously doubt you have main bearing issues. You would have a very loud knock if you did. Don't ask me how I know... ha-ha
If your oil light comes on then you may have some concern. Those external gauges can be very inaccurate if its a cheap one and I have personally gone through 3 now on my bike and had symptoms very similar to yours before I ponied up and bought a good one.

[Elan]

yeah if your oil light is not coming on, and you know it works, then you are getting at least 5-7 psi at minimum.

[Black 750K8]

+1 on changing the gauge. Also there is a o-ring under the shifter cover that can screw up the pressure along with the 3 on the pump. All 4 of them must be the correct size. All of this is in Hondamans book. I have 40# at idle with hot engine. Check with a new gauge first make sure you are being told the truth. Are you using 20w50?
Hondamans book should be one of your top 5 tools.

EDIT I meant to type 20#

[70CB750]

What Elan said.

My K0 is over 60 when cold, than fully warm it goes down to 20psi in idle.  That goes for Mobil Delvac 15W40, my winter oil.

I even got down to 10 on hot days when I was running synthetic mobil1 15W50.

Of course, my gauge was not calibrated against verified pressure, the numbers could be completely wrong.

[JWExperience]

If your pressure light functions and is not coming on I would not sweat it...most likely a gauge issue. These bikes commonly register around 5psi at idle so even a little error in the gauge would show very little pressure. I think if you had no pressure and ran it as long as you did you would have started to notice a knock or other issues by now.