Topic: Help! - CB750F Oil Pressure

[01Thomas]

Hi
My rebuilt CB750F motor won't generate oil pressure, not on the kick starter, not on the electric starter and not when idling for 30 seconds with the red light staying on.

Here's what I've done and found out:
1. There isn't any oil getting to the oil filter which is the where the pumped oil goes after being discharged from the oil pump.
2. The tunnel from the oil pump discharge to the oil filter is clear.
3. Going backward along the oil flow diagram, the oil in the tank does leave the oil tank via the innermost oil hose and enters the motor at the rear-most of the two hose connections. When I remove the oil pump from the crankcase the oil flows freely out of the crankcases' rearmost of the two orifices.
4. If I lie under the bike with a torch in one hand and pushing the starter button with the other hand I can see that the oil pump sprocket is being spun, therefore the oil pump must be rotating.
5. The oil pump is in good condition with the rotors having minimal pitting and meeting the clearances listed in the manual. The O-rings have been renewed but not the internal seal.
6. If I dribble some oil into the oil pumps inlet port and rotate the oil pump's drive gear by hand (in either direction) it doesn't go anywhere. I would have thought this oil would be transported through the pump to the discharge port. What could prevent this from happening?

Any comments or suggestions?
Thomas

[emlupi]

I believe that you would have to fully submerge the pump in oil in order to get a good prime and move oil through it. Correct me if I’m wrong?

[slikwilli420]

You need to totally purge all of the air from the oil pump. It needs to be submerged in oil and rotated back and forth until all the air burps out. If you ran 30 seconds without oil to the valvetrain, you will likely have destroyed the whole top end.

[johans]

Yes , submerge the entire pump and rotate in both directions until no bubbles but if you rebuilt the motor and used assembly lube on the cam, ect you will not destroy the top end in 30 seconds . Could not hurt at this point to get a turkey baster and oil the cam through the valve tappet covers for some piece of mind. I also take out the plug where you install your oil pressure gauge and oil that galley way. Also , pull your spark plugs until you get oil to the top. Good luck.

[70CB750]

When i change oil, i crank the engine  for 50 seconds before the oil pressure builds up - and thats with oil pump full of oil - at least to some extent.

[HondaMan]

This is common following a total purge of oil in these pumps, and when heavier (and cold) oil is used in the startup tank.

After I rebuild these, here is how I reprime the pumps:
1. Set the pump in at least 4" deep oil  and rotate the gear BOTH DIRECTIONS many times until the bubble stop rising from the ports. Do this with 10w40 oil.
2. [With a pan to catch the dribbles...] Install the pump back into the engine, with the oil hoses already attached to the engine (this presupposes your bike is assembled) to preclude bubble re-entry to the intake tract.
3. Set an electric heater right close to the oil hoses (you might want to remove the bike's side cover) and heat up the hoses and oil tank. Make sure you have at least 2 quarts of 10w40 oil in the tank, don't try it with 20w50 in this weather or it will be very hard to prime (after oil pressure is restored, fill the tank up with 20w50 and call this your 'break-in oil' batch, change at 500 miles).
4. Remove the sparkplugs.
5. After it gets warmed up well, spin the engine in 5-8 second bursts with the electric starter until it fills the oil filter and head: you will see the oil light go out and hear the cranking slow a bit when it finally grabs the oil. Then remove the #4 exhaust tappet cover, shine a flashlight down in next to the valve spring, and you will see oil dribbling down inside when it reaches the top well enough to start.

A side note: running the engine 30 seconds with no oil may well have damaged, even destroyed, the main bearings. I have seen this happen many, many times (and just sent a crankshaft to one of our members in Peru because of this same situation, and have 2 more engines in my garage in this condition right now). So, if in the end you find that you cannot get oil pressure above 40 PSI (hot, at 2500 RPM) then presume you must change the main bearings at the least, and possibly have to polish the crankshaft and replace the bearings (with Black shells, most likely). Just a thing to think about...

[01Thomas]

OK, Thanks everyone. This is the about the 5th 750 Four motor I've assembled and I've never had this issue before. I'll try the oil pump purging trick and hope that sorts it out.

The engine was assembled with assembly lubricant. I hope that the 30 seconds at idle didn't damage the bearings but if it did it's too late to worry about that now.

[01Thomas]

Hi
After spending all afternoon on this engine, it now delivers decent oil pressure and there is good flow to the cylinder head.

Here's a summary of how it went:
No amount of priming in a bucket of oil made the slightest difference, so I stripped the oil pump rotor section again and removed the large (46mm) o-ring on the pressure side of the pump. After putting it back together again she pumped oil immediately when turning the pump by hand. I re-installed the pump and it's all working the way it should.


Now about that O-ring:
When checking the oil pump as part of the engine rebuild I had replaced the 2 large o-rings, taking the sizes from this document (attached, page 6, item 4). These didn't allow the pump cover screws to close the pump properly and it struck me that the pump might have internal leakage as a result. Removing the o-rings and allowing the machined-in spigots to seal the unit has reduced the internal leakage to the point where the pump actually pumps.

I assume the listed size of the o-ring is wrong and should be 46x1mm. I'll see if I can get some in that size and fit them when I do my next oil change.

I'll sleep much better tonight!

[HondaMan]

When you removed the O-rings the second time, were they (either one) kinked? This would have let air in during the suction at startup, breaking the prime.

In most cases where a pump I am working on won't prime at all, it is due to the seal between the 2 chambers being hard (or even broken/split). This lets the air from the empty crankcase (scavenge side of the pump) side of the pump leak into the pressure side and break the suction, too. The seal is still available from Honda (tricky to change!) in the GL1000 and GL1100 pump parts' listings, same size as in the 750. It's a custom seal with a metal frame.

[PeWe]

I have used this kit. No problems with pressure. I hope my last build (k2) will get pressure quick when I'll try to rotate engine within a month. Engine completed end of 2017, pump primed and filled with oil that flowed well.

https://www.ebay.com/itm/Honda-CB750-Oil-Pump-Rebuild-Kit-SOHC-O-Ring-Stopper-Seal/162442240391?fits=Make%3AHonda&epid=892069741&hash=item25d24fe187:g:BO4AAOSwvoxb7K3g:rk:1:pf:1&frcectupt=true

[johans]

Attn : O' Oracle "Hondaman"  , You mentioned -   "A side note: running the engine 30 seconds with no oil may well have damaged, even destroyed, the main bearings."   Do you find this to be true with the bearing being installed with assembly lube ?   or do you use motor oil when you assemble this area?


ps . The vitalmotorparts rebuild kit is great , my current build had the oil level in the tank dropping when sitting for a  period of time , now it is constant if sitting a while.

[PeWe]

One more thing when priming the oil pump.
The relief valve need to be opened too and let air out just for sure, only working with the sprocket to pump out bubbles might not be enough. Also non return valve.
I got the recommendation from an experienced CB750 mechanic I know as well as finding it in a post  http://forums.sohc4.net/index.php/topic,168975.msg1959844.html#msg1959844

[johans]

Great advice , makes sense.

[HondaMan]

Attn : O' Oracle "Hondaman"  , You mentioned -   "A side note: running the engine 30 seconds with no oil may well have damaged, even destroyed, the main bearings."   Do you find this to be true with the bearing being installed with assembly lube ?   or do you use motor oil when you assemble this area?


ps . The vitalmotorparts rebuild kit is great , my current build had the oil level in the tank dropping when sitting for a  period of time , now it is constant if sitting a while.

With assembly lube, I think it depends on which lube was used. They vary a LOT in effectiveness. In my last 2 car engines (Ford straight 6-200 (3.3l for the metric guys)  and a 351 cu. in. V-8 ) I used moly-based lube, knowing it would take quite a while to wet them. I sold the big car a couple years later, but the straight 6 went over 448,000 miles with no issues nor low oil PSI struggles of any kind.

In the bikes: moly assembly lube is pretty thick stuff and can actually bind the electric starter in the 750, so I use 20w50 oil instead. But, I also make dead sure I have oil all the way to the head, pooling, before I ever start them.

I have recently had 2 engines in with spun cranks and/or bearings. One was lubed by hand with 10w40 oil and then started with an unprimed and empty pump: it stopped running 45 seconds later (said the owner) and when I got it was seized tight. It spun both the crank bearings and the crank journals, is currently awaiting another crank (in my garage). The other one was lubed with a light (non-moly) assembly lube and started without a primed pump (but it was still wetted from previous use, and just reinstalled) and run until the owner 'thought the filter should be full, but still had the oil light on', and it spun the bearings, but the crank was only scratched. He estimated it was running about 20-30 seconds. I just had the journals turned until clean again, and it will make it OK with the Black bearings, coming in at just over 0.0010" clearance at startup (this will become about 0.0015" at 1000 miles or so).

So, those are the numbers I've been working with most recently.

[01Thomas]

Hi
Sorry for the late response but yes, they had some kinks when I removed them. I should have mentioned that.

When you removed the O-rings the second time, were they (either one) kinked? This would have let air in during the suction at startup, breaking the prime.

In most cases where a pump I am working on won't prime at all, it is due to the seal between the 2 chambers being hard (or even broken/split). This lets the air from the empty crankcase (scavenge side of the pump) side of the pump leak into the pressure side and break the suction, too. The seal is still available from Honda (tricky to change!) in the GL1000 and GL1100 pump parts' listings, same size as in the 750. It's a custom seal with a metal frame.