Topic: Oil tank drains on startup (wet sumping on cold start)

[Plzhalpimlost]

Hello, all! I’ve read all that I can about oil draining from the tank into the sump - it seems everyone who has “wet sumping” trouble has the issue when the bike is stored, due to a leaky check valve in the pump. This is not my issue.

Here’s my issue, as a start-up timeline:
Bike is cold and hasn’t been started that day : oil tank dipstick reads 3/4 full. Good.
Start bike up: within 30 seconds, all oil from tank is drained out. Dipstick dry, oil pressure still good.
Bike continues to warm up with dry dipstick.
Oil temp reaches about 100f and oil in tank begins to touch dipstick again.
5-10 mins later, bike warming up in the garage still, oil temp at about 125: oil is 1/4 to 1/2 full on dipstick.
I go ride, oil gets nice and hot, and reads 3/4, as it should.
I return home and shut off the bike. Oil dipstick still reads 3/4.
The next day/week, the timeline repeats.

The oil pump was rebuilt from Cycle-X, and Ken was kind enough to send me a new rubber stopper (even though he already replaced it during the rebuild) to see if that would fix it. I installed it, and it did not solve the issue.

Any thoughts here? Thank you in advance!
Jacob

[rotortiller]

I wonder if the sump screen is partially plugged up? It's obviously not pulling from the sump when the oil is thick but is fine when it thins out from heat. Mine drops some like yours on start up but to a much smaller extent. It's almost like the pressure pump is slightly more efficient than the scavenge pump when cold.

[HondaMan]

I've experienced 3 things that can cause this, 4 if you count the hose issues (outside the pump itself).
1. Inefficient scavenge pump. This can be caused by: poor/damaged rotors, too much clearance between the rotors and the pump (either the cover or the OD of the outer rotor to the housing), or the shaft seal between the pump halves being installed backward (easy to do). This last item will push cold oil from the tank into the scavenge side of the pump because the female side of the seal is facing the low-pressure, not the high-pressure side, so cold, thicker oil forces thru easily and breaks the scavenge suction until the oil viscosity drops to where the seal can hold against it.
2. Stuck (usually open) pressure relief valve. This is the one with the end cap and the spring inside, pressuring the piston that you can see thru the hole that feeds oil to the oil filter housing. Sometimes the ID of the long tunnel in that casting warps on these pumps, making it stick. You can hone it round again if you have a small hone, and it gets fixed.
3. Incorrect (too small thickness) O-rings in the hose fittings at the right side of the engine, or in the 3 that fit over the dowels that locate the oil pump. In the sandcast 750 this was a problem early on which got all the bikes recalled in the summer of 1969, when we had to drain the oil and drop the pan, replace the O-rings with 2.4x15 or 2.5x15 O-rings (which became the norm afterward) on the pump and hoses, install new oil and gaskets, and officially tell the owner that "Honda is sorry for the trouble". It never became a Service Bulletin, though. Today I use 2.5mm thick O-rings in all 5 places.

[PeWe]

My K6 wet sumped when I bought it. It had done 17000km back then.
A chock when checking oil level. Called my biker friend/ license mentor and colleague.
Got answer, typical for CB 750. Just run it and look after a 20 minute ride.
Never oil warning.

I have noticed that when bike is idling  a few minutes on sidestand, tank can drain causing a flashing oil warning.
On both my bikes. Both have fresh pumps that keep oil in tank.

I have also seen level in tank decrease when started cold, level increase after running for a while.

[Tracksnblades1]

Sounds like your running straight 50weight..?

[rotortiller]

Good to know PeWe as I sometimes thought that my bike might be a bit different from other units harmlessly doing the same as yours. My oil tank comes up to a normal level after the first mile or so. I always check the oil level immediately after shutdown. Like anything I suspect differing recovery times somewhat normal .

[PeWe]

Sounds like your running straight 50weight..?

20W-50 bike oil
Compare how our bikes behaves is a very good thing with this forum.

[Tracksnblades1]

Sounds like your running straight 50weight..?

20W-50 bike oil
Compare how our bikes behaves is a very good thing with this forum.

Indeed.

Straight 50 weight causes the same cold low oil tank level after start ups..

[Don R]

 Let us know which thing fixes it.

[PeWe]

Your CB750's  oil level do not change like that?

Oil tank full since last ride.
 (I like to have oil to top mark on dipstick when warm after a ride.)

Next start:
Oil and engine cold, room temp.
Level in tank will go lower the first 5 minutes if you look into tank.
Back to full after ca 20 minutes ride.

My bikes work fine like this
Both have oil pressure gauges to look at too.

[BenelliSEI]

PeWe…… most of mine do the same. I check them after 20-30 minutes, and when I’m done. I’m convinced this is why so many get overfilled.

I do like my small oil pressure gauge, on the end of the main oil gallery. As long as it’s pointing straight ahead (50-70 psi) whenever I glance down, I’m happy!

[Tracksnblades1]

No it’s not a conspiracy..

It’s well known cold thick oil takes longer to drain down and run back to the sump . Wet or dry..
Big engines do the same. Once the engine oil temp is normal it readily flows…

Just like cold honey it’ll pile up before it starts to flow. Then your toast heats it up, thins it out, and it’s a race
to get it ate before it runs over the edge…🤪

SAE oil weight used to be determined at 212*f (100c). And W stands for “winter”.
If you want to know how your 20w-50 flows at 35*f stick it in the fridge for a couple days.
It’ll pile up before it readily flows too.

[PeWe]

PeWe…… most of mine do the same. I check them after 20-30 minutes, and when I’m done. I’m convinced this is why so many get overfilled.

I do like my small oil pressure gauge, on the end of the main oil gallery. As long as it’s pointing straight ahead (50-70 psi) whenever I glance down, I’m happy!

That feels like normal behavior of a CB750

Why explained by Tracksnblades1. Cold thicker oil vs warmer.

CB750 need the 20W-50 oil by design.

Tank volume might be designed by Honda to handle this.

[Plzhalpimlost]

Sounds like your running straight 50weight..?

I’m running 20-50w, but the problem existed with 10-40w as well.

[Plzhalpimlost]

PeWe…… most of mine do the same. I check them after 20-30 minutes, and when I’m done. I’m convinced this is why so many get overfilled.

I do like my small oil pressure gauge, on the end of the main oil gallery. As long as it’s pointing straight ahead (50-70 psi) whenever I glance down, I’m happy!

That feels like normal behavior of a CB750

Why explained by Tracksnblades1. Cold thicker oil vs warmer.

CB750 need the 20W-50 oil by design.

Tank volume might be designed by Honda to handle this.

I hear you guys, but this bike is so close to “perfect” that I don’t feel like giving up and living with this quirk for the rest of my life. It’s annoying to have to wait as long as I do, or to stress out that I’m going to lose oil pressure. I’ve got a new oil pump on the way and will report back once installed.

[PeWe]

Very interesting to see if it will change.
I think this is seen by guys that look into the oil tank when warming up the engine and the first 10-20 minutes ride.
Warm up bike leaning on side stand is not good...

[willbird]

Very interesting to see if it will change.
I think this is seen by guys that look into the oil tank when warming up the engine and the first 10-20 minutes ride.
Warm up bike leaning on side stand is not good...

Overall I don't think anything good comes from having the bike on the side stand. I only use mine to hop off and then put the bike on the center stand.

Bill