Topic: 1975 CB750 Oil Supply Issue

[John G]

In my previous post I was saying that gas was going into my oil over filling the oil bucket. Well it appears I have an oil supply issue. After running the engine for a while the return sucks the oil out of the engine but it doesn't appear to be suppling oil to the crank. I think on a cold initial start up it supplies oil and the when it gets hot it must stop the supply and the crank gets robbed of oil. Anyone have ant ideas where I should look? Thanks

[PeWe]

Oil hose from oil tank to engine is old and collapsing?

[newday777]

In my previous post I was saying that gas was going into my oil over filling the oil bucket. Well it appears I have an oil supply issue. After running the engine for a while the return sucks the oil out of the engine but it doesn't appear to be suppling oil to the crank. I think on a cold initial start up it supplies oil and the when it gets hot it must stop the supply and the crank gets robbed of oil. Anyone have ant ideas where I should look? Thanks

Further explain why you think you have an oil supply issue to the crank. What evidence are you going by?
 🤔

[BenelliSEI]

Suggest you install a temporary oil pressure gauge on the engine to really see what’s going on. Have you removed and reinstalled either oil line recently? It’s very easy to put a “twist” in a hose when tightening. That twist is just like closing the pipe…..

[John G]

I took the two oil lines off and checked them out. No obstruction and the oil tank is clean. Must be an oil pump problem perhaps.

[HondaMan]

Oil pump.
This often happens from 2 causes that I often see:
1. Something damaged the scavenge-side rotors in the oil pump, so it cannot pull the oil effectively enough from the crankcase to push it up to the tank.
2. The Seal between the 2 chambers of the oil pump has become hard (due to age, not from use) and it will not seal the pressure side from the scavenge side of the pump. This causes the pump itself to deprime the scavenge side.

[John G]

Thanks Honda Man but I think it would be the opposite side of the pump as the oil returns faster than it is supplied. Therefor the oil bucket looks like it's got too much oil in it. I also put a new seal in the oil pump.

[Don R]

 Have you checked the engine oil pressure with a gauge and does the idiot light work?
 I'm also thinking the top end would go dry before the crank. Is there oil at the rocker covers?
 I had a motor with the transmission hose fitting plugged once, you might check that and are the hoses correct on the back of the oil tank now?   
 

[willbird]

Thanks Honda Man but I think it would be the opposite side of the pump as the oil returns faster than it is supplied. Therefor the oil bucket looks like it's got too much oil in it. I also put a new seal in the oil pump.

IMHO the scavenge side of the pump has a job to return ALL of the oil to the tank, IMHO it cannot possibly return too much.

Bill

[BenelliSEI]

Thanks Honda Man but I think it would be the opposite side of the pump as the oil returns faster than it is supplied. Therefor the oil bucket looks like it's got too much oil in it. I also put a new seal in the oil pump.

IMHO the scavenge side of the pump has a job to return ALL of the oil to the tank, IMHO it cannot possibly return too much.

Bill

+1 to that. Install a temporary oil pressure gauge and see what’s really going on. Of no pressure, STOP running it and pull that pump.

[PeWe]

Oil level in tank to be checked after a 20-30 minutes ride.
That level to be at max on dipstick.

If engine does not take oil, no refill needed until next oil change.

I refill my bikes, K2 and K6 with 2.9L  at oil change.
(Or 3x 1 quart bottles)

A good ride, check level and adjust if needed.

If oil pump leak oil to sump when parked, it will take time to get level back. If refill oil now will end up in too high level.

My K6 was a wet sumper from the very beginning. It worked fine during 100.000 km when it got another pump I restored.

[madmtnmotors]

After running the engine for a while the return sucks the oil out of the engine but it doesn't appear to be suppling oil to the crank.

If the pump were failing to supply oil "to the crank" your valvetrain would suffer the most and likely be the first to start squealing as the cam and cam bearings (a.k.a. "cam towers") self destructed. If that didn't get your attention then the subsequent failure of the rod and main bearings would. If what you seem to think is actually true then I don't understand how the engine even continues to run...

How much oil have you physically added to the engine?

If it really bothers you then the only way to know for sure is to add an oil pressure gauge as has been suggested.

[HondaMan]

Thanks Honda Man but I think it would be the opposite side of the pump as the oil returns faster than it is supplied. Therefor the oil bucket looks like it's got too much oil in it. I also put a new seal in the oil pump.

The scavenge side of this pump flows 25% more oil than the supply (pressure) side can move from the tank. That's why the scavenge rotors are larger than the pressure rotors. This is normal in these designs, not just in motorcycle engines.

The other place where trouble like this can (and does) arise is inside the tunnel (casting) where the little rubber plug moves when oil pressure comes out of the pump. This is the check valve: it is designed to hold oil inside the oil filter and engine when the engines stops. The little rubber plug there fits inside a piston made of turned aluminum (precise size) and the ID of the tunnel is supposed to be 0.004" larger: some of these pump housings warp over time right on that part of the casting, and the plunger tends to stick slightly open. When that happens, the oil tank slowly drains into the crankcase while the engine is parked and the oil is warm.

A third place (but rarer than the others) is in the oil pump's relief valve. This is the right-angle casting that comes right off the pump at 90 degrees to where the above-mentioned plunger lives. It is also sometimes thought of as the "pressure regulator" plunger: its piston is also finely round, but the pump casting around it can warp and then the piston gets stuck slightly open: this, too, lets oil leak from the tank into the crankcase when the engine is stopped.

Most of the time, though, I find the teeth of the rotors are just chewed up from some hard debris that went thru the engine (especially in the K0-K3 engines). The most common source of this that I have seen is the little blade underneath the cam chain on the crankshaft: the chain gets too loose and the engine is decelerated sharply, throwing the loose loop to the front of the chain, and that slackness hits the top of the little blade and breaks it off. Then it gets ground up in the tranny gears and goes thru the oil scavenge pump, back to the tank, then back thru the pressure rotors to the filter where it finally gets trapped. But on the way, it chews up the tips of the rotors, after which they both leak, and produce low oil pressure when the engine is hot. Sometimes they just don't pick up enough oil in the scavenge side for lack of suction, leaving the crankcase too deep.

[John G]

Thanks again Honda Man. I'm embarrassed to admit this but it appears I put to much oil in to begin with. I have know idea how I managed that but there it is. I'm learning. You'd think a guy this old would know everything. Guess not. Hahaha

[HondaMan]

Thanks again Honda Man. I'm embarrassed to admit this but it appears I put to much oil in to begin with. I have know idea how I managed that but there it is. I'm learning. You'd think a guy this old would know everything. Guess not. Hahaha

I'm not going to tell you how many times I have done that...   

[Bodi]

Too much oil is common with the dry sump 750 after an oil change or if the level is topped up when found low. The other models are all wet sump, no tank - so what you measure on the dipstick is accurate.
There's a simple piston valve in the oil pump that stops oil from draining out of the tank down into the sump through the suction pump when engine is stopped, opened by scavenge pump suction... it has a pretty weak closing spring so that not much suction is needed. They often leak a bit from sticking open a hair or the rubber tip wearing out. So after sitting a while there's some oil in the sump. If it bothers you, the valve can be fixed but the pump has to come out. As long as there's some oil in the tank to supply the pressure pump on start it's no big deal, the scavenge pump will fill the tank pretty fast.
If that valve does leak - filling the tank to normal level after sitting a while means there's too much oil, by the amount in the sump. If that's a lot you get oil spewing from the tank on startup. Just a little and it will show over full on the tank level but if it isn't overflowing that's not a big problem. Check oil level just after a ride or a few minutes running after startup.
Unlike for a wet sump engine, too much oil won't be damaging anything... just making a mess if it overflows the tank. It can't leak at a huge rate through the pump so overnight or even a week won't be a problem but over a few months it can drain the tank.
Yes damaged rotor/outer pump parts will drop volume and pressure but these parts can take a lot of punishment before causing issues. I've looked at a lot of pumps and have never found one without pits and scars there (except new, and those have been unobtainable for decades). They still pump fine even with damage that looks pretty scary. Really worn out pumps are rare, just changing oil and filter regularly is the absolute most important thing you can do to have an engine that lasts. Filthy oil will wear the plain bearings and pump internals.
Yes try a pressure gauge either temporary or permanently mounted. On cold start idling you should get 60psi or so, when after a ride hot idling 10psi is not abnormally low. There is a relief valve that limits to ~60psi but cold thick oil will give higher pressure if revved because it can't flow through the relief valve fast enough to limit the pressure so well.

[PeWe]

I remember when I noticed very low oil level in my for me new CB750 K6 spring 1979.

I called my colleague that also helped me to get my driving license.
He answered, "no worries, typical CB750, oil level will be correct when riding it". He was right.
The pump must have been like that from the very beginning.

That engine got a better pump almost 40 years later keeping the oil level in tank

Thanks to this forum I knew better. Made sure the metal "piston" with rubber could glide in-out pump by ease.
It also got new rotors a forum member made together with seal and o-rings.
The thin gasket too.
There are new aftermarket gaskets that are thicker.

Good to know that 60 PSI is the good cold start number.
My K6 show that during cold start.
I had some thoughts about the crank bearings status since my K2 has shown closer to 80 than 60 PSI.

I need to check the pressure relieve spring on the K2 then. Switch back to an old stock spring if still 80 PSI with the new  oil it got just before winter sleep.

I'll might replace the entire engine
New test rides and brake-in needed. See if I can get the VM29's correct direct

[madmtnmotors]

Too much oil is common with the dry sump 750 after an oil change or if the level is topped up when found low. The other models are all wet sump, no tank - so what you measure on the dipstick is accurate.
There's a simple piston valve in the oil pump that stops oil from draining out of the tank down into the sump through the suction pump when engine is stopped, opened by scavenge pump suction... it has a pretty weak closing spring so that not much suction is needed. They often leak a bit from sticking open a hair or the rubber tip wearing out. So after sitting a while there's some oil in the sump. If it bothers you, the valve can be fixed but the pump has to come out. As long as there's some oil in the tank to supply the pressure pump on start it's no big deal, the scavenge pump will fill the tank pretty fast.
If that valve does leak - filling the tank to normal level after sitting a while means there's too much oil, by the amount in the sump. If that's a lot you get oil spewing from the tank on startup. Just a little and it will show over full on the tank level but if it isn't overflowing that's not a big problem. Check oil level just after a ride or a few minutes running after startup.
Unlike for a wet sump engine, too much oil won't be damaging anything... just making a mess if it overflows the tank. It can't leak at a huge rate through the pump so overnight or even a week won't be a problem but over a few months it can drain the tank.
Yes damaged rotor/outer pump parts will drop volume and pressure but these parts can take a lot of punishment before causing issues. I've looked at a lot of pumps and have never found one without pits and scars there (except new, and those have been unobtainable for decades). They still pump fine even with damage that looks pretty scary. Really worn out pumps are rare, just changing oil and filter regularly is the absolute most important thing you can do to have an engine that lasts. Filthy oil will wear the plain bearings and pump internals.
Yes try a pressure gauge either temporary or permanently mounted. On cold start idling you should get 60psi or so, when after a ride hot idling 10psi is not abnormally low. There is a relief valve that limits to ~60psi but cold thick oil will give higher pressure if revved because it can't flow through the relief valve fast enough to limit the pressure so well.

This is what led me to question the amount of oil that had been physically added to the engine since it sounded like a simple case of overfilling to me.

[willbird]

Thanks again Honda Man. I'm embarrassed to admit this but it appears I put to much oil in to begin with. I have know idea how I managed that but there it is. I'm learning. You'd think a guy this old would know everything. Guess not. Hahaha

I have fired up a couple 750 that I had never seen run and the PO never saw them run. The proper way would have been to drain everything and put in proper amount of oil (my older wiser self says) but we just added oil and fired them up. And of course they over flowed once they had run for a bit. Better safe than sorry tho :-).

Bill