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low oil pressure at idle

scully39

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Mineral Ridge, Ohio
I have a 1974 cb750k which I rebuilt into a café style bike. I have a oil pressure gauge mounted to the oil gallery which shares the oil pressure light. I have found when at idle (900-1k rpms) the oil light comes on and pressure doesn't register until I increase the revs. I replaced the filter and oil (10w-40) and put in a rebuilt and tested oil pump thinking perhaps pump cams were worn. Same problem. I have an aftermarket oil tank and lines with fittings that are 1/4" I.D. i'm thinking they may be too small for the viscosity of the oil and the inflow can't keep up with the pump at idle. If the return and inlet lines were reversed by mistake there would be no pressure at any rpm correct? I'm sure they are on right but I want to cover all bases. Only other thing I can think of is a blocked oil passage but the engine has relatively low miles and was clean inside with no debris in filter after first oil change which was done after only a few hours on the engine.
Any suggestions on what to check will be greatly appreciated. I have a lot of hours into this and would like to finish (if they are ever finished).
 
A 1/4" oil feed IS a blocked hose to me there. However blocked hose would show worse at more rpm not better.

The rear hose off oil tank is the feed to pump and attaches to the rear engine port as well.
 
What kind of oil tank? If it is a harley tank then it wouldn't matter because both holes are just that....holes, the 750 really needs to have the return in the tank have a bung that extends inside the tank just like the oem, it will slowly leak oil back into the motor if you dont because there is no check valve on the scavenge side. I figured that one out the hard way, but only once! If you look inside an oem tank you will see that there is a tube that sticks up in the tank on the return. I also agree the 1/4" is too small for my liking but people have been getting by with it. But this wont cause no pressure at just idle. Does it have no pressure when cold also or just warm? How high do you have to get the rpms before you get pressure?Honda does state that if oil hot it is normal for the light to come on at 800 to 900 rpm...but honda spec idle is 900 to 1000. Light comes on at little over 7 lbs. But non of my motors ever drop below 30psi. I would guess that it is a combination of larger bearing clearances and oil orings inside going lax and letting some pressure bleed off. The orings under the cam towers and the one on the left side transmission bearing support will bleed a fair amount of pressure when the orings get old and hard.
 
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