Topic: Low Oil Pressure when Warmed up.1978 CB750F SS.

[Xxthegonzxx]

Hey Guys,

I just completely rebuilt my engine and noticed something strange. When I start up bike the oil pressure is normal. Usually around 45-60PSI when idling. Once it gets warmed up the oil pressure drops to 5 PSI then when I take it out for a ride it drops to 0. My oil light has come one once or twice and I eventually turn it off right away.

Yesterday I took it out for a longer ride and noticed that the oil pressure dropped significantly when fully warmed up. 0 PSI at 50 MPH and then my light comes on. I end up pulling over the side of the road and it eventually stalls out. I try to turn it on again after a minute or so but nothing. I can hear the starter cranking but no go. While I'm stopped I get off and check everything. I notice that the top of the engine is getting oil. After about 15-20 minutes the engine cools off and I start it up again fine. The pressure gauge still shows 0 PSI but there is no oil light on (good thing?) I open up the oil tank and check it as the bike is running, and I notice that there is oil flowing. I try to ride it home, which is about 15 minutes away, and as I get near my exit the bike is struggling to keep speed and rpms up with a wide open throttle. It finally stalls as I'm exiting and going 30 mph and I push it about a quarter-mile home.

I'm thinking my low oil pressure could be the oil pump. I've checked the clearances before and they've been up to spec but I have not checked the oil return valve. Could something plug that up and cause this? The reason I think this is happening is because the top of the engine is getting oil, but maybe the bottom/transmission isn't? I've also heard that there might be an issue with the crankshaft bearings but I've replaced them with new ones with the rebuild. I just bought a new oil pressure gauge since the old mechanical one I have might be off/broken. Any ideas on where to start troubleshooting? I'll replace the oil pressure gauge and report back. I'm hoping it's just a problem with the oil pump. I don't mind taking the oil pan off but if I have to open up the engine again I'm going to be pissed!

[70CB750]

What was the plastigauge reading on those new crankshaft bearings?

I would start with new oil pressure gauge just like you - but since the oil light comes on (thats about 7psi) I would not have high hopes.

[Xxthegonzxx]

Crank bearing shells were replaced with all new greens as that's the color that matched the previous ones. I don't have the exact measurements at the moment but I remember that they all where within spec. I'll start out with the new oil pressure gauge and see if maybe it's off. More than likely it will be my oil pump as the last time I changed the oil I noticed excessive gunk in the oil pan from when the engine was opened up (I might have done some woodworking next to the engine and some bits of wood dust might have gotten in that thing).

[70CB750]

Did you replace oil pump o-rings?  It could be as simple as that.

[Jerry Rxman Griffin aka MuthaF'er]

Drop your oil pan cover and see if you have cloth, etc debris covering your filter screen. Cloth rags shed and it ends up in the pan.

[flybox1]

Drop your oil pan cover and see if you have cloth, etc debris covering your filter screen. Cloth rags shed and it ends up in the pan.

+1
sure glad i dropped the pan for the first oil change 
I diligently covered the cases the next time i had them open.

[Don R]

Drop it again and have a peek. Unless your header is blocking it that's an easy job. I found a gob of chewed o rings in a motor, the cylinder sleeve o rings were hanging down and must have gone through the trans.

[Ace]

What oil pressure gauge did you purchase and can you post a pic of it?

I take it that you have checked your oil level and it's good.

Not changing this into an oil thread, what viscosity of oil are you using? Mine at start up for 15W-50 is about 70-75 psi. Warmed up after 10 mins drops to 10 psi. New engine including oil rings in pump and near gear shifter.

[Xxthegonzxx]

Did you replace oil pump o-rings?  It could be as simple as that.

Oil pump O rings have been replaced.

[Xxthegonzxx]

What oil pressure gauge did you purchase and can you post a pic of it?

I take it that you have checked your oil level and it's good.

Not changing this into an oil thread, what viscosity of oil are you using? Mine at start up for 15W-50 is about 70-75 psi. Warmed up after 10 mins drops to 10 psi. New engine including oil rings in pump and near gear shifter.

I bought some Royal Purple 20w-50 oil. That's what I've been using for the past 6 months.

[Xxthegonzxx]

I bought this gauge and it's being shipped to me now. I'll update this thread with what I find. I'm hoping it's just an oil pump issue. I'd hate to have the bearings worn again.

http://www.dimecitycycles.com/vintage-cafe-racer-caferacer-bobber-brat-chopper-custom-motorcycle-engine-parts-oil-pressure-gauge-psi-49-6460.html

[scottly]

but I have not checked the oil return valve.

There are two valves in the oil pump. The return, or check valve keeps the oil from draining out of the tank into the sump when the engine isn't running. There is also a spring-loaded oil pressure relief valve, which controls the maximum oil pressure. If the valve sticks open, or it's spring is weak, oil pressure will be dumped away from the bearings into the sump.

[Ace]

This is my oil pressure after 10-12 mins of idling. The oil pumps in these bikes are pretty good considering the punishment they take. I took the oil pressure gauge off in the end. Even warmed up 40 mph pressure was around 40 psi. Good enough for me. My issues turned out to be clutch plates. I thought it was oil pressure issue.

If you plasti gauged your bearing shells and are within spec, no oil leaks and oil pump O-rings replaced, I'd pull the oil pump apart if it hasn't been, replace the 2 big O-rings 46mm I think and possibly the seal in the middle of it if cracked. Check tolerances and all good then you know it's not your pump. Gauge possibly, oil light coming on under low revs, yeah my F2 did that on the odd occasion. Didn't put an oil pressure gauge on the F2, if the oil light doesn't come on while riding then I assume all good. May you have a dodgy sensor or wiring loom?

[Xxthegonzxx]

Alright Guys/Gals,

I think I might have found the problem, at least in theory. I won't know for sure until I take everything apart this weekend. Now bare with my thought process here. I've been doing some research online and found this site that explains how a cb750 oil pump works:

http://cb750k2.honda4.nl/lubrication-system/lubrication-system.html

I read this passage:

1. Oil pump
The oil pump is a rotor type trochoid pump mounted on the bottom of the crankcase and driven from the primary shaft through the kick gear. It consists of both the delivery and scavenge pumps, a leak stopper valve and a relief valve. (Fig. 3-4) (Oil leak stopper valve)

I did notice that the bike was very difficult to kick when I stalled out. I was trying to kick the bike back up to start it and it was SO hard to kick that it wouldn't take. I ended up pressing the starter but it wouldn't want to budge either. I had to wait for the engine to cool down finally to try again and it started back up. No oil light on upon start up but no oil pressure as wel...very strange. I'm going to pull it all apart this weekend and get back with an update.

Edit:

I also noticed that the last time I reassembled the oil pump, turning the gear was REALLY hard by hand.

[Xxthegonzxx]

Just an update:

Received the new gauge from DCC and still getting 0 PSI after a 5 min ride around the block. The PSI shoots up as soon as I hammer on the throttle but once it idles it sits at 0 or doesn't even register on the new oil pressure gauge. I also thought about mentioning one thing I noticed before. When I stalled out the one day I took it for a longer ride I noticed that the transmission was running rough. I could hear the gears clinking and rolling as I coasted off the road. It sounded like there was no lubrication.

Now if you think about the way the oil pump works during engine operation, it uses the the oil pressure to  open the oil leak stopper valve to maintain oil flow. This valve is possibly clogged or plugged. Unfortunately I did not have a chance this weekend to take it all apart as I was helping my brother change out his brake pads. I'm going to dissassemble the oil pump sometime within the next 2 weeks and report back with results. Stay tuned.

[Ace]

Did you install the oil filter and spring in the correct order?

[Xxthegonzxx]

Thanks for your patience guys. Just an update:

I dropped the oil pan this weekend and found bits of metal flakes in it. A considerable amount if you ask me. I'm sure they're ferrous metals as they all seem to stick to my magnet but any more experienced wrenches please chime in. I took several pictures of the oil pan and metal bits so please feel free to let me know your thoughts.

I also had the chance to take the oil pump completely apart and didn't notice anything out of the ordinary. The "rotors" inside the pump on the scavenge side had minor gauges. Didn't have my gauge with me so I couldn't measure but they all looked well enough to perform the job of syphoning oil. I am going to purchase some new filters and gaskets but for now the bike sits in the garage.

To conclude, I'm thinking something deeper might be going on here. I might have to tear down the engine yet AGAIN. I really hope this isn't what needs to be done as I'm trying to think of everything else that could be causing this loss of pressure. Point me in the right direction guys!

[Don R]

Stop running it, it's not going to get better. When it gets tight it is siezing up from lack of lube. From the look of the metal you've enther pinched a ring and it's eating a cylinder or the top end is starving for oil and grinding it's self up. Maybe all of the above. An engine will not survive long without oil pressure.

[Ace]

Time for some open heart surgery to find the problem. Keep us posted.

[Xxthegonzxx]

Time for some open heart surgery to find the problem. Keep us posted.

I think you're right Ace. That's what I was afraid of hearing but it sounds like it's gotta be done. I'll tear it down within the next weeks and keep you guys posted. Thanks again everyone.


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[Jerry Rxman Griffin aka MuthaF'er]

That's a nasty pile of ....... in your oil pan. I'd cut my loses and consider it dead before it becomes more fatal. Last time I saw chunks that big I was reminded of tequila.