Topic: oil pressure light comes on after 20 minutes or so

[setdog]

1976 cb750 k5

My oil light comes on after approx 20 minutes of rridingg.
Upon start the light goes out, stays out, but then pops on after nicely
Warm.

What might cause this aside from a bad sensor?
And how can I reassure myself that oil is flowing?
Can I just open the valve tappets and look?

I don't have an exterior gauge for this, and while I could purchase a new sensor
I want to know if there is way to make sure lubric action is okay.

[Duanob]

Start with the obvious, Check your oil level. Check for bad wiring under the chain cover. Pull the pan and make sure your oil intake screen isn't clogged. Do a fresh oil change and see if that changes anything.

Do you have any leakage on the ground under the chain cover? Could be worn out o-rings on your oil pump start sucking air and leaking oil when hot. Until you can sort out whether its a bad sensor or actual oil pressure loss I wouldn't run it!

[setdog]

Oil level looks good. Using 10/40.
Pulled the pump last week, screen is mint.
Changed oil then too.
No oil leakage on the ground, some common seepage around a Couple tappets
Or head gasket, not enough to set any alarms bells IMO.

Now the pump gaskets...I did not change but I think I'll go ahead and re gasket the pump,

There seems to be plenty of oil flowing, oil tank gets plenty warm so any blockage in that area
Is probably minimal but inclined to think its okay.

[iron_worker]

Your pump is likely worn and not producing pressure like it once did.

You should verify what your actual oil pressure is with a mechanical gauge set up at one of the oil gallery ports. If your oil pressure really is low then definitely don't run it like that.

IW

[dave500]

try another pressure sender unit,they are cheap.

[Duke McDukiedook]

try another pressure sender unit,they are cheap.

Cheaper than a pump if it is the problem.

[74750k4]

Easy to install a pressure gauge at the end of the oil galley in the plug cap. Nice addition to your idiot light as well.

[Bob Wessner]

+1 on the gauge. An added benefit is you get to see how low the oil pressure can go when you are idling at a light in 90+ degree temps. 

[Prospect]

Do you notice any changes in shifting or clutch. I had the same problem and it turned out to be a worn pump. The clutch basket wasn't getting enough oil and shifting was difficult. If your shifting is ok then possibly the oil sensor needs replacing.

[Jerry Rxman Griffin aka MuthaF'er]

Try 20w50

[70CB750]

How high is the idle?  With Mobil Delvac on a warm day, the oil light would come on around 900rpms, yes it was bit low for an idle.

[lucky]

How many miles on it?
IF it is over 30 years old little bits of rubber can come off of the cam chain guide rollers and get clogged up inside the oil passages of the crankshaft.
If you found noting in the oil pump screen you are probably ok.

If it was me I would buy a new oil pressure sending unit and see what results you get if any.

If the bike has more than say 70,000 miles, the oil pump gears could be worn and you can measure those.

[bollingball]

Oil level looks good. Using 10/40.
Pulled the pump last week, screen is mint.
Changed oil then too.
No oil leakage on the ground, some common seepage around a Couple tappets
Or head gasket, not enough to set any alarms bells IMO.

Now the pump gaskets...I did not change but I think I'll go ahead and re gasket the pump,

There seems to be plenty of oil flowing, oil tank gets plenty warm so any blockage in that area
Is probably minimal but inclined to think its okay.

Have you changed those three o-rings by the metal collars 15x2.5mm There is also one o-ring under the shifter cover. It has been said that these four o-rings will help maintain constant pressure. part 14
http://www.cmsnl.com/honda-cb750k6-four-usa_model14380/partslist/E13.html#results
Ken

[bollingball]

Here is the walk through on these o-rings.
http://forums.sohc4.net/index.php?topic=78643.0
Ken