GS 750 oil pressure

[andyp62]

Hi all
i am having problems with the lubrication on my 1977 GS750B, 8 valve engine, the oil light doesn't go off so i removed the clutch and oil pump, sump and strainer, measured the pump as instructed and found everything to be within spec, changed the filter, new oil, turned the engine over, still the light is on so i removed the oilway plug from behind the barrels and there is no oil there at all.Any ideas where to go from here very much appreciated, the drive to the pump was fine, no signs of wear, just not sure what to look at next.
thanks in anticipation
Andy

[hampshirehog]

Have you checked that the oil light feed isn't earthing out or that the switch isn't stuck in the open i.e. earthing position? Every oil pressure issue I've come across on a GS is actually an electrical issue.

[Nessism]

Agree with checking the electrical system. The sensor switch is a simple metal plunger. Then the engine is off the plunger touches the engine which provides a ground path for the light to turn on. After the engine starts oil flow pushes the plunger off it's seat which interrupts the ground path and the light turns off.

[Grimly]

Might be the pump has lost prime. When you assembled it, did you do it greasily or bone dry?
I'm amazed the number of bikes that sit for years and start up with oil pressure appearing within a few seconds, so the oil pump on these is fairly good at self-priming, but that also depends on the supply to it being a) unblocked and b)air tight.
Might also be a good idea to drop the sump plate and check the pickup strainer - mine was nearly blocked with old gasket and clutch particles.

[andyp62]

Have you checked that the oil light feed isn't earthing out or that the switch isn't stuck in the open i.e. earthing position? Every oil pressure issue I've come across on a GS is actually an electrical issue.

i will take it off and check, thanks

[andyp62]

i removed the sump and checked the strainer, it was pretty clear but cleaned it aswell

[Brendan W]

Disconnect the oil pressure switch, motor stopped. Does it go off now?
If yes, take the oil switch off, reconnect the wire and push the plunger with the switch body in contact with the case.
The light should go out when you push the plunger in.
If memory serves you can turn the crank by hand with the plugs out and oil should appear in the pressure switch hole.

[andyp62]

the pump was oiled before re assembly, i have taken the pressure switch off and turning the engine over oil comes out of the hole where the switch was fitted but nothing is coming out of the blanking plug behind the barrels which i thought there would be

[Brendan W]

Does the light go out when you disconnect the pressure switch?

[andyp62]

Yes it does, I turned the engine over after disconnecting the wire with the oilway blank removed from the gallery behind the barrel but no oil came out

[Brendan W]

Is the switch plunger with the disc on the end free to move.
If it can't lift clear of the deck when the oil arrives then the wire remains grounded and the light stays on.
Is the insulator between the plunger and the switch housing still ok?
With the switch off the bike check for no continuity between the wire terminal and the switch housing.
If the plunger is not isolated from the housing the oil light will remain on irrespective of the plunger movement.

[andyp62]

hi
the plunger was seized solid, a new switch has now sorted the issue, is it ok to start the engine with the cam cover removed to check oil is reaching the top end?

[Rich82GS750TZ]

Maybe OK to crank the engine for a couple of turns of the cams but I wouldn't go starting the engine. Could make a mess. Take the spark plug caps off.

[hampshirehog]

Yep, it makes a helluva mess if you leave the cover off!

[Brendan W]

hi
the plunger was seized solid, a new switch has now sorted the issue, is it ok to start the engine with the cam cover removed to check oil is reaching the top end?

Like they guys said, messy. Once the oil is up in the main gallery and putting the light off it's a straight run to the top using the holes for the outer rear headbolts. It's hard to see what could prevent that apart from overuse of liquid gasket. Wouldn't be the first time either