Topic: 1978 CB750K — Post 836 OIL PRESSURE - Seems bad :(

[PeWe]

I check oil level after at least a 30min ride to get correct level. Add if not at top marking. 5 mm over no problem.

I check oil level now and then during rides,  quickly done when refilling fuel.

[SKTP]

That usually indicates the by pass valve is opening on the pump, and then closing again. How high does it go initially? What weight oil are you using?

I bought a rebuilt oil pump from cycleX. I has very few miles on it at this point. However, my old pump and this new on both have been allowing the oil from th tank to go down into the sump fairly quickly when sitting. Like usually the side tank read empty in about a week.

I am using BelRay 20w50 (recommended by HoindaMan) but I think that I will try 10x40 and see how that works.

After initial warmup like I am talk 3-5 minutes, the stick reads within the range...after 20 mintues or so it is above the range by about 1/2 and inch or so.

[PeWe]

Thinner oil will not help the pressure in a CB750
Remove the oil cooler, run stock setup as Hondaman wrote.

Check if the oil hose for oil up to the tank is OK inside. Are they old Honda or aftermarket or home made a hydraulic shop has crimped?

I guess you already have checked inside oil pan for debris that can block the strainer where oil up to the tank will pass.

[SKTP]

My mechanic friend who has been helping me believes that the thick oil and the HD studs are cuasing the oil to get up in the head and is not getting back down to the pump fast enough...has anyone experienced this?

He has been working on CB750 for decades and thought the Belray 20x50 oil felt really strange.

[BenelliSEI]

My mechanic friend who has been helping me believes that the thick oil and the HD studs are cuasing the oil to get up in the head and is not getting back down to the pump fast enough...has anyone experienced this?

He has been working on CB750 for decades and thought the Belray 20x50 oil felt really strange.

If that’s what’s happening the tank should be bone dry when your pressure drops to zero?

[Alan F.]

Wouldn't any excess oil in the head just find it's way down the cam chain tunnel?

[BenelliSEI]

Wouldn't any excess oil in the head just find it's way down the cam chain tunnel?

….good point.

[PeWe]

"....thick oil and the HD studs are causing the oil to get up in the head and is not getting back down to the pump fast enough..."

1.)
This has been up in another thread where one guy had much oil inside valve cover, oil passed thru guides and caused smoke thru the pipes.
That time small oil return holes in head gasket that slowed down the oil flow from top.
Nothing about low level of oil in tank.
Opened up the oil return holes in gasket helped. It was an MLS gasket if I remember correctly and could be used again.

I have drilled my CB750 heads to ensure better flow up to cam. HD studs 8mm thick and holes in head were down to 8.5mm in one head made me to drill the 2 holes for oil up to cam with a 9.5mm drill.
All others 9.0mm.
Ported heads so a few holes on inlet side were a thrill to drill. How much material left? It went well.

This started when one head was tricky to remove. Head nuts had sunken down, pressed the alu around the HD studs very tight.
I had to have a heavy nylon hammer to bang the exhaust spigots to make head free.  I let my nearby shop mill all 16 sunken holes so they all were parallel with mating surface and +18mm wide for the washer.

I did not have any oil pressure or sudden low oil level then. Only loose nuts caused oily spark plugs.

2.)
BUT, I have had sudden low oil levels.
I saw level at half on dipstick after ca 25 minutes ride. I thought it will raise.

After another 20 minutes even lower.
I had to return to home, stopping more and more often to see how level just sank. I stopped at a good place made for stops were road had a long pocket. Level had then went under the dipstick.

Called my wife and explained how she could take the car and drive to me. Explained were in the garage she should grab 2 bottles if oil. Spectro HD 20-50 mineral.
I filled tank with both just for sure to have oil all the way home, ca 60km.

I instructed her to watch if bike smoked. I accelerated hard up to  160kmh and throttled off, followed by open throttle. No smoke.

Engine consumed oil so head off. The 5mm valve kit with bronze guides  did not last long. The guides had  horrible oval holes at top. That head got iron guides and new valves with std valve stems.

3.)
I had after that got a sudden low oil level again. I usually make sure that oil level is at max.
I saw level to be just under half during a ride 40-60 km from home. Level did not raise more so back home where I checked oil level and found it at max.

That happened a few times so I replaced oil pump.

It happened again during a ride I had to interrupt, back home to see level to be max again.

I remembered that I had replaced the oil hoses earlier, just before these low oil level events. Not cheap hoses from CMSNL.

I replaced those and mounted a set of hoses a guy in Germany had made. He  fixed a few sets after I had shipped old to him.
My K2 has used them since restore and never had an issue. Over 10.000km then, now +16.000km.

So the new set on. No problem seen so far, 3000km. I swapped the earlier pump back since it was restored with new seals, rotors etc.

So today the thought is oil hoses.
Can oil return hose be too thick?

Interesting to see if oil level can be low again despite some revving.

These events did not trigger low oil pressure lamp.

I have earlier got low pressure, blinking lamp when bike had idled when parked, leaning on side stand.

It happened first on my K2, saw almost no oil in tank.
Added a little more to later see level was too high. Had to remove the same amount I had added.

Later same thing on my K6. Realized that let engine warm up a few minutes parked on side stand is not a good idea.
No problem when let bike stand up.

20W-50 oil is not thick for a CB750. It is designed for it.  Air cooled one part of it.
I have read a  Porsche forum where use of 10-40 instead of correct 20-50 caused expensive repairs. Too low oil pressure and short lived guides.

I searched for oil temperatures when we had a hot summer. 120*C in tank.
Air cooled Porsche forums must have the answers was my idea.
120*C no problem. Shorter intervals, good synth lives longer than mineral oils.

Modern liquid cooled engines with tighter clearances another thing.
Cars using 0-30 and even thinner!

[HondaMan]

My mechanic friend who has been helping me believes that the thick oil and the HD studs are cuasing the oil to get up in the head and is not getting back down to the pump fast enough...has anyone experienced this?

He has been working on CB750 for decades and thought the Belray 20x50 oil felt really strange.

That's nonsense. These bikes were made to run 20w50 oil and the opening from the head to the bottom end is 1"x4" wide.

[HondaMan]

"....thick oil and the HD studs are causing the oil to get up in the head and is not getting back down to the pump fast enough..."

1.)
This has been up in another thread where one guy had much oil inside valve cover, oil passed thru guides and caused smoke thru the pipes.
That time small oil return holes in head gasket that slowed down the oil flow from top.
Nothing about low level of oil in tank.
Opened up the oil return holes in gasket helped. It was an MLS gasket if I remember correctly and could be used again.

I have drilled my CB750 heads to ensure better flow up to cam. HD studs 8mm thick and holes in head were down to 8.5mm in one head made me to drill the 2 holes for oil up to cam with a 9.5mm drill.
All others 9.0mm.
Ported heads so a few holes on inlet side were a thrill to drill. How much material left? It went well.

This started when one head was tricky to remove. Head nuts had sunken down, pressed the alu around the HD studs very tight.
I had to have a heavy nylon hammer to bang the exhaust spigots to make head free.  I let my nearby shop mill all 16 sunken holes so they all were parallel with mating surface and +18mm wide for the washer.

I did not have any oil pressure or sudden low oil level then. Only loose nuts caused oily spark plugs.

2.)
BUT, I have had sudden low oil levels.
I saw level at half on dipstick after ca 25 minutes ride. I thought it will raise.

After another 20 minutes even lower.
I had to return to home, stopping more and more often to see how level just sank. I stopped at a good place made for stops were road had a long pocket. Level had then went under the dipstick.

Called my wife and explained how she could take the car and drive to me. Explained were in the garage she should grab 2 bottles if oil. Spectro HD 20-50 mineral.
I filled tank with both just for sure to have oil all the way home, ca 60km.

I instructed her to watch if bike smoked. I accelerated hard up to  160kmh and throttled off, followed by open throttle. No smoke.

Engine consumed oil so head off. The 5mm valve kit with bronze guides  did not last long. The guides had  horrible oval holes at top. That head got iron guides and new valves with std valve stems.

3.)
I had after that got a sudden low oil level again. I usually make sure that oil level is at max.
I saw level to be just under half during a ride 40-60 km from home. Level did not raise more so back home where I checked oil level and found it at max.

That happened a few times so I replaced oil pump.

It happened again during a ride I had to interrupt, back home to see level to be max again.

I remembered that I had replaced the oil hoses earlier, just before these low oil level events. Not cheap hoses from CMSNL.

I replaced those and mounted a set of hoses a guy in Germany had made. He  fixed a few sets after I had shipped old to him.
My K2 has used them since restore and never had an issue. Over 10.000km then, now +16.000km.

So the new set on. No problem seen so far, 3000km. I swapped the earlier pump back since it was restored with new seals, rotors etc.

So today the thought is oil hoses.
Can oil return hose be too thick?

Interesting to see if oil level can be low again despite some revving.

These events did not trigger low oil pressure lamp.

I have earlier got low pressure, blinking lamp when bike had idled when parked, leaning on side stand.

It happened first on my K2, saw almost no oil in tank.
Added a little more to later see level was too high. Had to remove the same amount I had added.

Later same thing on my K6. Realized that let engine warm up a few minutes parked on side stand is not a good idea.
No problem when let bike stand up.

20W-50 oil is not thick for a CB750. It is designed for it.  Air cooled one part of it.
I have read a  Porsche forum where use of 10-40 instead of correct 20-50 caused expensive repairs. Too low oil pressure and short lived guides.

Your oil pressure journey reminds me of some old oil hoses I saw around 2000-2002 or so: the liner inside the hoses had cracked and split from age and the flap-shaped liner would pull closed across the suction hose's interior when hot, pinching off the oil supply. When this happens to the return-side hose, the low pressure side of the pump only makes about 7 PSI, not enough to "flop" the flap over, and the oil would divert elsewhere (most likely to the transmission) instead of back to the tank.

[SKTP]

Now I need to figure out which is which on my bike

[HondaMan]

Now I need to figure out which is which on my bike

Just a bit of experience (not advice) from my roadracing days: installing the oil cooler with the hoses on the bottom side instead of on top will ensure faster oil delivery at startup and fewer loss-pressure episodes, or "bubbles", in the supply. The arguments always ensued that with hoses on top the oil in the cooler would stay there when the engine was off (mostly true), while having them on the bottom ensured no bubbles would form in the cooler while running (also true). In the end, making the coolers lower than the camshaft always worked better in raced SOHC4 bikes we saw. In touring, it didn't seem to matter.

In the end, if lifetime was to be the measuring instrument, the ones with the hoses on the bottoms yielded much longer [racing] engine life, seldom needing teardowns for oil-related wear issues during the season - only during the winter, and my own never once needed any parts replacements for oil-related issues. At first, it had hoses on the bottom of the cooler, until I discovered someone else's magic trick of using the Ford power steering oil cooler instead, in between the tank and engine, behind the right footpeg. For roadracing with fairings, that was Nirvana.

All of this presumes, though, that the oil check valve in the oil pump works 100%. Without that, all bets are off!

[BenelliSEI]

Given your problem, why add variables? I’d dispense with the cooler and make sure a stock set of hoses from the tank to the engine are perfect and not collapsing or blocked.

[Honda Hansel]

Im with John on that one .Remove all the non OEM and check hoses


Check the screen on the oil pump.Is it a OEM or aftermarket                                                                        thinking the scavenge oil pump might not be able to pull the oil thru the screen on the pump and there for the tank goes empty at high RPM
How is the return flow in the tank at 3000 RPM look inside with flash light
there should be a lot of oil returning to the tank

My two bits

Honda Hansel   

[scottly]

My mechanic friend who has been helping me believes that the thick oil and the HD studs are cuasing the oil to get up in the head and is not getting back down to the pump fast enough...has anyone experienced this?

It's easy to tell if this is happening: pull an exhaust tappet cover and see if oil pours out.

[SKTP]

Life has been busy...I finally got the cooler off the bike and ran it for 15 minutes on the center stand with a fan on the headers...no oil pressure issues. I will test drive this weekend...thank you for all of the replies

[BenelliSEI]

Life has been busy...I finally got the cooler off the bike and ran it for 15 minutes on the center stand with a fan on the headers...no oil pressure issues. I will test drive this weekend...thank you for all of the replies

Sounds good, I hope your ride is a success!

[SKTP]

I removed the oil cooler, and after last weekend's test (with no oil cooler) I was feeling optimistic. The stick in the oil tank showed half full. The test last weekend was on the CENTER STAND. Today it was on the side stand.

The bike started right up and I watched the oil pressure gauge intently. It was at 60-70ish initially but after about a minute it had a drop to near zero, oil but no oil-pressure gauge light..It built back up to 60 and had a couple fluctuations....then at about three-and-a-half minutes, complete drop to zero, light went on, I tuned the bike off...it is #$%*ed

Stock oil-lines, no oil-cooler, new Cycle-X rebuilt oil-pump. Very bummed. Taking it to a mechanic. I do not have time to take the motor out again.

[scottly]

Did you remove the tappet covers to see if the top of the head was full of oil??

[newday777]

Oil pump bad?

[Don R]

  My old 836 reminded me of this thread, I started it and warmed it up, 50-60 psi on the gauge, the oil in the tank was moving but right on the bottom of the dipstick low. I had a quart handy, poured in half and after a good ride back home it was a half qt. over.
  It's coming apart for chains anyway, so I'll do the pump kit that I have waiting.

[PeWe]

If oil level is good on center stand and during rides, everything is OK.

Both my CB750 does not like to idle on side stand for several minutes. Oil will not refill tank as it takes oil from it. Oil lamp will start to flash.
Checked tank inside and see oil pumping but not enough to fill the tank.

Both has restored oil pumps with new seal, o-rings plus new and better rotors.

The  pump must have an oil level high enough in pan to suck oil. When leaning that much it will be problem. That's what I have seen.

How much it will lean depends on side stand, if bent or not. Uneven surface can make bike to lean a lot. Wheels on higher grounds than side stand. (This is how a bike can be dropped when parking)

My K6 got new expensive aftermarket oil lines from CMSNL. The season after that strange sudden low oil in tank started to happen. I checked after over 30 minutes ride, less than half. It had full last time. Returned to home to find oil level at full.
After several times of sudden low oil level, I replaced the pump. That did not help.

I finally replaced the oil lines and the problem disappeared. Same type of oil lines my other bike (brown K2) have had since I built it.

 A steady oil level at full.
The other restored pump went back in again.

The latest good oil lines made by a guy in Germany.
Send the old ones to him and he will restore them in same lengths. Mine are good looking black, alu fittings look better than new.

My thoughts if an oil line from engine --->tank  can be too wide....  I have not made an autopsy yet

[Don R]

 This bike is lowered as it was a drag bike and that's the look I like, it has a shortened side stand and unfortunately no center stand, a PO removed the lower part of the mounts. I'll check it again when level, good tip, thanks.

[BenelliSEI]

Did you change those hoses? I think you have a suction line plugging up. You always have pressure on start up and then it disappears. Something is closing off…..

[HondaMan]

That just sucks! If we assume there is nothing wrong with your pump or the built in oil pressure relief valve, the only thing that can cause that sudden loss of pressure is no oil?

When the bike (and you) have totally cooled down, is there any oil left in the tank? Could the return line from the scavenge side of the pump to the tank be collapsed internally or twisted and kinked? If the engine is sucking oil faster than it gets returned to the tank, this could be a cause of zero pressure…….

About this: twice I have seen oil lines where the inside liner was cracked (these were OEM Honda lines, 40+ years old at the time) and when cold, the plastic liner would lay pretty flat, but when warmed up it was more flexible and would lift up and seal off the supply hose, causing instant loss of oil PSI. The trouble with trying to find this was: after a minute or two, starting the engine brought the oil PSI right up again, and the cycle repeated.

BUT - this was in the SUPPLY side of the oil to the pump. The pump was still scavenging whatever was in the engine, so the tank(s) remained largely full.

So, I'm wondering: if this scenario was happening in the RETURN line to the tank, blocking the flow back to the tank when the oil was hot, it might explain each of these scenarios you've posted? It would return oil to the tank when relatively cool, and the oil itself stays cool for about 10 minutes from a cold start. Then, if the internal hose lining got hot and "flipped up shut" like those other two 750s did, it would stop the oil from returning to the tank when hot. Then when cold, the cycle would repeat.

Can you post a picture of the oil hoses?

Oh, and BTW: the return-to-tank oil pressure is very low, like 5 PSI tops. It doesn't take much to stop it.