Topic: K4 750 First Start Oil Not Returning to Tank

[pickleknuckles]

I think I know the answer to this one but I need confirmation and the rest of the internet has been no help.

Just finished cleaning up my '74 CB750 after 20yrs in storage. New everything but old pump, fresh oil. Starts fine. Fired the starter till the oil light went out. Then ran it for a bit on a hanging tank just to make sure it was good to go before vacuum sync. Runs really well just idling. Impatience got the best of me and I fired it till the oil light went out again just to hear it crank. Now no oil in the tank... I think that's normal. Do I just need to actually run it so the oil returns? I assume so seeing as there's no other way for it to pump back in. Any thoughts on how long?

Thanks.

[69cb750]

Oil tank level will decrease first time bike is started.
Peer into tank, if there is oil in tank bottom add more.
If tank is dry you have a problem.

[HondaMan]

If you had the usual 3.5 quarts of oil to start with a dry engine, it will 'lose' almost a quart in the startup, just below the dipstick. This stays in the engine and oil filter. Add about 1/2-3/4 quart more before running it again, though. Reason: if the oil pump went dry, chances are the 50-year-old oil seal between the 2 chambers is both hard enough and has enough wear clearance that it cannot get a vacuum on the suction side to send oil back to the tank. This will pump the tank fully dry and wreck the crankshaft and bearings. I have a black-painted K4 engine in the garage now that someone got impatient with and just started it up anyway - it needs a crank and all new bearings!

[bryanj]

If you mean the oil dissapeared overnight it has leaked through the pump into the engine which about 75% of the 750's do

[pickleknuckles]

If you mean the oil dissapeared overnight it has leaked through the pump into the engine which about 75% of the 750's do

No, leaking back. I cranked it just to hear it and watch the oil light go out. It's been 20 years. Lol.

If you had the usual 3.5 quarts of oil to start with a dry engine, it will 'lose almost a quart in the startup, just below the dipstick. This stays in the engine and oil filter. Add about 1/2-3/4 quart more before running it again, though. Reason: if the oil pump went dry, chances are the 50-year-old oil seal between the 2 chambers is both hard enough and has enough wear clearance that it cannot get a vacuum on the suction side to send oil back to the tank. This will pump the tank fully dry and wreck the crankshaft and bearings. I have a black-painted K4 engine in the garage now that someone got impatient with and just started it up anyway - it needs a crank and all new bearings!

There's definitely oil in the tank below the stick. I can see the rear tube on the underside flush w the oil level.

https://photos.app.goo.gl/6dcRT9GXMiuHKabP9

[rotortiller]

That picture looks like the oil level is flush with the pickup in the oil tank, not good as it will eventually suck air as hondaman said. I would expect the oil pressure light to come on when it does cavitate. If the system already took 3 liters of oil and the tank is basically empty you have a problem. I would not run the engine until the tank oil is at least several inches above the tank bottom(showing low on dip stick) and then keep an eye on it .

[pickleknuckles]

That picture looks like the oil level is flush with the pickup in the oil tank, not good as it will eventually suck air as hondaman said. I would expect the oil pressure light to come on when it does cavitate. If the system already took 3 liters of oil and the tank is basically empty you have a problem. I would not run the engine until the tank oil is at least several inches above the tank bottom(showing low on dip stick) and then keep an eye on it .

Cool. That's what I was trying to figure out. So it's lower than the filled-from-dry scenario Hondaman suggested?

I need to put a new pan gasket on anyway, so I'll just rebuild a pump and toss it in while I'm at it. I bought this bike in the early/mid '90s and it's been sitting since November of 2001, so I'm not surprised.

[BenelliSEI]

Good idea, look at that pump. Either the pickup screen is varnished over with gunk or time for some pump work…..

[rotortiller]

So it's lower than the filled-from-dry scenario Hondaman suggested?

Basically if you put 3 liters in the oil tank during the oil change the oil level in the tank should be higher than in the picture.

[pickleknuckles]

Good idea, look at that pump. Either the pickup screen is varnished over with gunk or time for some pump work…..

👍👍

So it's lower than the filled-from-dry scenario Hondaman suggested?

Basically if you put 3 liters in the oil tank during the oil change the oil level in the tank should be higher than in the picture.

Thanks! That's the input I was looking for.

[Don R]

 Let us know what you find, if anything. You aren't the only guy that runs into this issue.

[pickleknuckles]

Let us know what you find, if anything. You aren't the only guy that runs into this issue.

Will do! I've got a couple good pumps here and I ordered a rebuild kit w cometic gaskets, a new screen and rubber parts, and new springs from CycleX. Also shot a coat of paint on a pan I had vapor blasted a while back.
Just dropped the bike off to have the frame checked and straightened though so it might be a few weeks till I get it swapped out.

[HondaMan]

Your forum name reminds me of a fella I met once in Missouri at one of their public shooting ranges. He'd ordered a Rowland conversion kit for his old Army .45 and had a 'smith build it up, took it shooting at the same range where I was, next lane over. For some reason he decided to remove the slide and on reassembly, the VERY strong recoil spring was fighting back, big time. He asked he to help him out by holding the frame while he installed the spring, saying his 'pickleknuckles of fingers' had trouble with small parts like the bushing. His fingers were BIG (like him, a good 300 lbs, 6'6" at least) and thick, bigger than my thumbs!