Topic: Oil light slow to turn on after stopping cranking

[KayOne]

Installed the engine this weekend and primed the oil system today. I heard of all the problems of people priming their CB750's so preheated the oil before I filled the tank. Kicked it over approximately 30 times (without plugs) and then gave short bursts on the starter. The oil light went out very quickly once on the starter. Confirmed oil to 1/2 and 3/4 rocker boxes via the forward tappet covers. Tank level went down quickly and I filled it to the specified quantity.

Any way, I was happy with the way it went but noticed the oil light took 30 to 40 seconds to turn on after cranking was stopped. Has anyone seen this before? Light goes out very quickly when first cranked. This is not a wiring problem as the main harness is new and gauge harnesses have been cleaned and checked, all new indicator bulbs. Is it possible the pressure sensor is sticky?

[Don R]

Mine takes a few seconds. As long as it comes on I wouldn't be too concerned. I'd think it could indicate a nice tight engine.

[AJK]

Yeah, your engine is nice & likes to 'hold' the oil. This is due to the oil pump stopping in whatever position is stops at but continuing to 'seal' in the oil pressure along with every other place that is under that pressure like bearing journals & rocker assy orifices, etc.

I'm assuming this was done when the engine was fairly cold, not after a 30 mins ride. Go for a ride, get the engine hot then take note how long it takes for your oil light to come on. Probably just a few secs at most because the oil is a lot thinner when hot so quicker to run out of all the gaps in the engine & lose oil pressure quicker turning you oil light back on.

As DonR says, you have a good engine, enjoy.

[Bodi]

This could mean you have fairly thick oil and a lot of air trapped in the galleries and filter. The compressed air (by oil pressure) could upply oil to the various outlets - bearings and valvegear - for a while.
After some actual running of the engine most of the air will be blown out and the delay eliminated. An air-free oil system will go to zero pressure instantly on engine stop, but the filter will always trap some air because of its orientation.

With warmed oil and the low rpm of starter cranking... I suspect a sticky switch - staying pressurized for 30 seconds is not very likely IMO. Original switches can still work perfectly but failures are pretty common: ~40 years is a long time for the thing to keep working, when you think about it.
The switch actuation pressure is quite low - 5-10psi. Car switches will work but usually they activate at higher pressure so an OEM one is best if you need a replacement.

[KayOne]

Thanks for the replies guys. The engine has not been fired yet so the engine was cold with preheated oil. Sounds like good news all around! Should be firing engine within the next week if I don't encounter problems.

BTW, the oil was 15/40.