SOHC.co.uk Forums > CB750

Oil pressure dropped

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yozzer74:
Think it was 10 / 40 semi.

Bryanj:
Forget the semi it isn't worth the cost, and if it is hot used 20W50

r1_pete:
Its probably the oil pressure relief valve which is not seated properly, if the engine is left standing dry crap builds up as the oil coating deteriorates, when you run it again the valve lifts off it seat as it should to prevent over pressurizing, the valve then doesn't seat properly and allows oil to bypass all the time, instead of shutting off at low revs.

westfieldandy:
Hi,

Just spent a fortune on tooling and stuff redesigning oil pump for the cb750, I've been testing the prototype now for 10 months and all seems good, wish i'd had one when we were racing.

anyway,  what you are describing does not sound uncommon to me,  most used pumps / engines with a semi synthetic 10/40 motorcycle oil, when the engine gets really hot, ( ie you cant hold your hand on the fins, ) its probable you will see 50lbs pressure when revving above 2500, below this it can drop to almost nothing at a slow tickover.  the oil light is designed to trigger at 8psi.

I have found 10/60 oils designed for air cooled harleys work very well in improving on this, but I now use much cheaper 10/40 semi. and it works well, but i do change regularily.

Its the sitting about in traffic or riding too slow that is the worse as the engine loves air flowing over it.

shimming the relief valve spring we always did to our engines, but, this only changes the upper cut off point to the maximumum pressure at high revs, ie it max pressure.

in old customers bikes displaying this problem, I have now sourced a reasonably priced 15/50 semi oil which helps this problem, or just change your oil more, and semi is less likely to break down with the heat, we had a lot of problems with oil when racing, if the bike got far too hot just once on the line or an extreemely hot meeting it never recovered viscosity again.

finally the full cure would be split the engine get the bearing clearances all good, get a good pump with good tolerances, check cam etc etc.
then you will find you will have 15/20 psi at hot tickover and 60psi above 2500 (relief valve wont let it higher).

ps, make sure you oil supply line is clear/ and not partially blocked or kinked.

www.good-bits.co.uk
https://youtu.be/cOcQZ2oVeFE

andy

Trigger:
Changing the relief valve spring and a good clean makes a shed load of difference. I sell the new spring to OEM standard and the stopper kit.

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