Topic: 1974 cb750 oil pressure gague erratic

[fingersofskill]

I have owned this for 5 years now...Almost at 30k miles, oil changed at beginning of each season.
Did the same this year about a month ago, have only ridden to and from work and a few other short rides... Each ride at about 15-30 min.
Yesterday, IG was in the 90's and I rode to the beach
About 3/4 way there, I heard an abnormal amount of engine tick :-(
I looked down at the oil pressure gauge on right side of engine and saw it fluttering a million times a second between 0 and 40, occasionally spiking higher.
Since day 1, at idle the gague is at 0.
Once she is going 35+ mph, the gague  rises and holds steady at 40.
She has normal/common weeping on left side, nothing major.
My plan was to put some seafoam in the oil and run her a bit, then change the oil again.

Any thoughts on what else to look at/for?
Don't wanna take her to a shop if I don't have to :-)

Thanks in advance for any thoughts/ideas on this

[fingersofskill]

Going to get the seafoam... any last min thoughts? Bueller?

[calj737]

Not sure I'd add the SeaFoam. Your clutch is a tad particular about the oil and SeaFoam may not do it much good. More likely, your gauge has caught some debris on the underside. Have you removed it and inspected it?

Also, if the temps are that high, it may be time to use a heavier oil.

Your comment about normal/common oil weeping should not be accepted as "normal". While all too common, it is certainly not normal and is something you should address. And check the oil level while you're inspecting the gauge...

[70CB750]

What Cal says, check the gauge, others reported debris made the gauge show low numbers.

But since you heard abnormal level of valve train noise, it is more likely to be oil pump problem.  With a bit of luck solveable by replacing o-rings.

What weight oil are you running?  I use 15W40 for winter riding and 20w50 for summers, mind you VA gets hotter than New England.  But neither oil will show 0 on the oil gauge, ever.

[Old Scrambler]

Drop your pump and do a rebuild...............If your valves are clattering, your cam tower is starving............THAT'S EXPENSIVE

Oil pressure means just that, ALWAYS POSITIVE, and should be higher than 40psi, or your asking for trouble. Could be a simple o-ring failure.

[lrutt]

My gauge will read 40+ at idle and hit 80 going down the road, and that's when it's warm. Cold is higher.

Definately drop that sump and check it out.

[Bodi]

Being realistic, that sounds bad. Where is the gauge getting the measurement, from the switch port?
A gauge can be pretty wack and go wild dealing with pulsating pressure but the trochoidal pump should deliver fairly constant pressure. If it is pulsing that much, it wants a looking at. Pulsing pressure could be oil starvation but the oil tank would be obviously empty: a badly clogged sump pickup screen can do that, though.
- Badly worn main bearings will drop the system pressure.
- If someone assembles an engine without the valvegear flow restriction orifices, system pressure is quite low.

Gauges are cheap so I would try a new one first, and see if it agrees with the current one.

Zero pressure at idle is not good, but not immediately fatal. Engine loads are mild at idle and minimal oil will stop bearing and valvegear wear for a while. Not forever.
40psi at running rpm on a cold engine is not enough. You should have 60psi with the pump's pressure relief valve limiting it there. On a hot engine with thinned oil, 40psi is not too bad.
I worry when idle pressure on a hot engine is below 5psi: that's enough to keep everything wet, all that's needed.
If the gauge readings you describe are truly your system pressure, drop the pan and remove the pump. Check for a terribly clogged pickup screen or stuck relief valve first, then look inside the pump if those are OK. Check clearances and end cap seal condition. Replace the pump-engine O-ring seals at the least. There's no easy bench test for an oil pump beyond "it pumps" so if it looks good... you have some digging to do to find the cause of your pressure issues.

[fingersofskill]

Thanks for all the useful info!

I did seafoam the engine, ran her a short bit then changed the oil and filter.
Clutching is no different than before... I did an hour or more of reading about peoples experience with seafoam on these 750 fours, tried to educate myself as much as I can before doing it.

I used Shell Rotella T6 5w/40, read much here in this forum about this being used
I am back to where I was... 0 at idle (which has been the case since day1 of me owning her. didnt know any better, thought it was normal) and anything over 3k rpm she holds steady at 40.

The extra clattering noise I had heard that day has not returned, YAY

Yes, was a hot day out... we dont have that many up north here...

Soooo... Next, I will attempt to douche the gauge out, just to make sure that there is nothing clogged.
But I suspect Bodi is correct, I am looking at some pump repair.
Will get a rebuild kit from forum member Elan, tho sadly he is moving and not shipping till sometime in June
(He moving to Boston, I am just north in NH, make it into Boston often, maybe I can stop in and just pick one up?)
Wish me luck!

Thanks again for your ideas!

[70CB750]

5w40 is way too light oil. You should run 20w50 in summer.

[ekpent]

No one asked but does it still have a functioning stock oil light working in the dummy light cluster ?

[fingersofskill]

Expent, Great question... YES, the stock oil light is functioning, the best I can tell... it is on when I turn on the key and goes off once started

[fingersofskill]

I have ordered a oil pump rebuild kit from Elan