Topic: 73 CB750K3 no oil pressure

[Mark K]

1973 CB750. Engine rebuild, engine on stand. Oil pump rebuilt with new springs/hardware and reinstalled in engine.

Attempted oil pressure test today. Using battery power from my truck, engine turns over briskly. Ran the started for <30 seconds each time with a minute rest time on starter. Attempted 5-6 times, no reading on pressure gauge.

Items I checked after 1st attempt;

Oil hoses checked for blockages before I installed on engine, both completely clear.
Oil tank inlet and outlet ports are not clogged.
Pressure gauge works, checked with low air pressure, got a reading.
Removed oil pump, submerged oil pump in oil while rotating the oil pump gear. Oil moving in and out of ports.
Without draining pump, reinstalled pump, three collars and o-rings installed.
Oil pump gear meshing and turning with internal gear inside of case when turning engine over with ratchet and socket while oil pan off.
Reinstalled pan.
2nd oil pressure test attempt same results - no oil pressure.

Any and all assistance appreciated.

Thanks in advance.

Mark K.

[PeWe]

You need to get all air out of the pump when completely submerged in oil.

Release the pressure releave plug and rotate pump in both directions until you can feel either hole will suck hard when finger is blocking the hole. ( Both holes the hoses are connected to)

When all air is out tighten that plug again. Not too hard since the alu is thin.

Very thin oil make this priming easier. The right 20W-50 oil will quickly enter when pump is working

[Mark K]

Thanks PeWe. Not knowing any better, I used 10W-30, probably too thick, huh?

I searched the forums and did find a thread concerning same issue. A suggestion was to pour some oil down the cyclinder head drain holes to get oil into the passages.

[scottly]

Pouring oil into the drain holes in the head won't help; the oil will end up in the sump. The pressure pump gets oil from the tank, not the sump.
Since you are doing this on a test stand, please post a pic of how you are supporting the tank on the stand.
It is not uncommon to take a long time to build pressure on a "dry" motor. The pressure pump does not provide a large volume of oil, so it takes time to fill the oil galleys and filter housing. Were the pressure relief valve or oil stopper(?) valve messed with?

[Mark K]

Okay, so no oil in the drain holes.

I know the picture isn't the greatest, but the oil tank is connected to the engine by the two hoses and held up by a coat hanger hooked on a metal pole.

I rebuilt the oil pump and replaced the o-rings. Rotor clearances were within spec, and yes the relief and stopper valves were taken apart and reassembled.

I appreciate your help Scottly.

[PeWe]

You need to rotate the sprocket and blick one of the holes with finger. You'll feel which hole that get underpressure when rotating in a certain rotation.

Rotate in other direction, block the other hole. Bubbbles will come out, if you release the 14mm plug, more bubbles that are stuck and blocking flow.

[Mark K]

PeWe,

I'll have to take the oil pump back out of the engine and try what you are saying.

Is the 14mm cap the Relief Spring Cap?

Thanks PeWe.

[PeWe]

Yes, that cap. I primed 2 pumps a few years ago.
I had to release that to get out all air.
I had to take out pump, since no pressure after 30 sek starter without plugs, my K2 build when finished.

Be careful to not pinch the 3 metal collars inside o-rings.

Rather easy to take it out when bike parked on sidestand.
Stock pipes for 3-4 off, pan off after oil is drained.
Remove pump and prime it carefully, pump back.
I was laying on the floor on my back to see how pump went back on, parallel to not damage any of the 3 collars.

I have bought some extra collars after my pump fixes.

I replaced that pump later on with a pump with new rotors.
My other bike got a refurbished pump with new rotors too. That bike on sidestand too.
Parked on center stand must work too, maybe easier.

[Mark K]

Thanks PeWe,

Engine is currently on an engine stand, so fairly simple to remove oil pan and pump. I hope to work on this weekend. I'll post results here. Thanks again for your help.

Mark K

[BenelliSEI]

Mark.... when you remove the cap on the pressure relief valve, pull the spring out and make sure the piston moves freely in the bore hole...... Nice setup, should all work.

[Mark K]

Thanks for the advice, I'll check that when I pull the pump back out. Hope to get to it on Friday unless the wife has me putting up Christmas deco.

[Mark K]

Mark.... when you remove the cap on the pressure relief valve, pull the spring out and make sure the piston moves freely in the bore hole...... Nice setup, should all work.

Removed oil pump, removed pressure relief valve cap and piston, piston moved freely. Reinstalled piston and spring, except cap, submerged pump and rotated the gear - lots of air bubbles. Reinstalled cap and put oil pump into engine. Setup tank, lines, and pressure gauge. Took twice with the starter turning to get oil pressure and oil to the head. Success!

Thanks to all of you who gave advice to help me, I really appreciate it!

[BenelliSEI]

Well done.

[MauiK3]

Also, when pondering these motors, have a copy of Hondaman's book handy, it's really helpful with this type of problem.

[PeWe]

Oil pump is a nightmare to get working quick enough before crank bearings start to wear.
Assembly lube give som extra lube and running without oil pressure.
I have used Red Line assembly lube. Recent jobs with cylinder, pistons and cam swaps the more fluid version.

Next engine build will get Lucas oil assembly lube. It is tackier and will hopefully stick to the bearings for a longer period of time when engine might not be started that soon.

I had no oil pump issues after rebuilds in the 80's. Back then I flushed pump with naphta so it had a lighter fluid mix of naphta and oil. Air bubbles could evacuate easier than using new 20W-50 oil in a refurbished and cleaned pump.

[bryanj]

Used to use STP oil additive on shells for cars years back

[PeWe]

A friend that restores/repairs engines and bikes has used STP too, for many years.

[newday777]

Used to use STP oil additive on shells for cars years back

"However, it is not designed for vehicles with wet clutches"
Quote from

https://carfromjapan.com/article/car-maintenance/is-stp-oil-good/

[bryanj]

True, but would the tiny amount on shells give a problem, who knows, i have also used coppaslip on shells before now when an engine is going to stand before running, turns the oil a lovely colour

[HondaMan]

I've used STP, Redline, 50w and 90w oil on the bearings: it seems that so long as you put something there that won't drain away, it works fine.

[Mark K]

I used Lucas assembly lube for all bearing races and anything else metal to metal when reassembling the transmission and engine. Also used 20w-50 when priming the oil pump the 2nd time. Now the trick is going to be how to get the engine off the engine stand so I can put it back into the frame.

[RAFster122s]

Seen the C shaped engine installer arms some guys have used in their bikes? Need a hoist to use them.

[PeWe]

Seen the C shaped engine installer arms some guys have used in their bikes? Need a hoist to use them.

I need a small crane in my garage when taking out the engine off my brown CB750 K2, install it in another K2 rolling chassi when ready.

After that lift an engine from bench into the former brown K2. I'll see when that will happen

I know a guy with a V shaped lifting device hanging in chains from the ceiling where he has an I beam mounted all over the garage.
His garage was properly built when he bought the old house!

[bryanj]

All we had to fit engines in the 70's was a knackered back

[Mark K]

I plan on using Dino's method from his Hack a week video #25. I built the wood jig, but since I didn't have a 2x4 four feet long, I used one 40" long. Too short! Have to get a longer one tomorrow. To get the engine off the stand, I'm going to turn the engine upside down and then lower the stand so it's laying on its side, loosen the bolts and pivot the engine onto my motorcycle jack. At least in my mind's eye that's how I'm going to accomplish it. 😉 Going to enlist my sons help in lifting engine (w/Dino's jig) into frame.