Topic: oil light comes on tickover

[D DIAMOND]

hello i have just rebuilt my cb750k2 new mains new big end shells have replaced all the seals etc did 30mls today came to a junction oil light came on at tickover 1000rpm.the engine showed 50lbs when i purged the oil pump with the batterie on the bench there are no rattles from the engine is there any faults with oil switch or oil pump or as i was reading on some old topics about oil temp thanks DD

[bryanj]

If the switch is original change that first, its cheapest and easyest

[D DIAMOND]

thanks thats the cheapest to start with when it starts from cold and it stalls it takes the oil light a long time to to come on it led me to think  the oil pressure was good thanks DD

[Bodi]

Cold oil is considerably thicker than hot oil. Using a pressure gauge, with cold oil the pressure is often popping the pump bypass around 60psi even at idle and goes higher with revs as the bypass can't dump all the overpressure.
Hot oil reads lower, maybe never hit the bypass pressure.
The pressure switch trips at quite low pressure, 5-10 psi.
The switches do wear out, sometimes with a leak.
Try a gauge to see what your pressure really is. A worn pump or worn bearings will cause low pressure, poorer bearing lubrication, and more bearing wear. I don't know what's really needed, my 400 will be around 40 psi at riding rpm once fully heated up and drop to 10 or so at idle. It hasn't blown up yet - and the pressure hasn't changed for decades.
If the light comes on above 10psi, I would replace the switch. Automotive switches may trip at higher pressure? And beware, the case bore is not the 1/8 NPT thread that almost all American and Japanese car oil pressure switches use - they will thread in but that damages the case threads. You can get BSPT (case thread) to NPT adapters if you want to use an American switch the right way.
The Honda OEM switch is not hard to find.

[D DIAMOND]

thanks bodi i have a 75 cb750 and a spare engine which has just docked in the uk today the 75 bike has a oil pressure gauge on it the engine has a oil pressure switch i will try those i also will have a spare oil pump i was wondering if the oil pressure valve has been sticking open its just a thought does anyone know if they can stick open and would it make the oil pressure drop thanks DD

[Bodi]

Technically possible for it to stick open I suppose. It is a metal piston that slides in a bore, with the pump output pressure pushing it against a spring. When it slides up from high oil pressure a port back to the sump opens. Anything can happen but I don't think this piston, constantly lubricated with engine oil, would stick. Something in the oil could jam it I guess, hopefully there isn't much in the way of that stuff in the oil. The pump works on unfiltered oil so it's possible.
It would have to be a small leak to keep much pressure at all with hot oil though. Stuck wide open, the system pressure would be very low.

You can inspect the pressure pump rotor and measure some clearances. The rotors will have obvious damage, that's normal and not a problem unless really bad. Usually the scavenge pump looks worse (this has the thicker rotors). Not sure why, anyway the pressure pump rotors normally look better than the scavenge pump rotors. Doubtful you can find a new pump, it will be $$$$$ if you do. Every used pump I've looked inside of is scarred up about the same amount. Only one was definitely NFG, looked like it had been pumping gravel for a long time - probably a bearing disintegrated and chunks went through the pump. It came from a salvage shop, never saw the engine it was from.

[markreimer]

I’ve had similar issues. Oil light when I come to a stop, hot engine. It would flicker and go out if I raised the rpm just a hair.

I replaced the switch and was a complete and utter fool, over tightened the pipe thread and cracked my cases. Don’t do that!!!

Fast forward, new bottom end, had a similar issue. Rebuilt the oil pump and it was much better. I also installed a pressure gauge and could verify I had good cold and hot oil pressure.

Start with the sender unit. Then the pump


Sent from my iPhone using Tapatalk

[D DIAMOND]

ready to throw in the towel have swapped the oil pump fitted 2 oil gauges one from the r/h side of crankcase one off the the oil switch changed oil filter casing and filter when i start from cold i get 45psi  let it tickover for 5 mins @1000rpm i get less than 10psi i thought it was the crank so fitted a very good one and new shells replaced the head and barrels still get the same oil pressure it looks like the crank or oil leaking internally compression tests 150psi on all cylinders DD

[Maltboy]

Just curious, what viscosity are you running?

[ekpent]

 Probably not your problem but maybe help someone out some time in the future. My weird oil light problem on a K4 had me scratching my head for awhile once. The red light was bright but when the engine was running in neutral id dimmed but still glowed. Turned out that I had a small hole burnt through the plastic that holds the idiot lights into the dash and light was bleeding through. Small blob of black rtv and the oil light problem was fixed.

[PeWe]

Oil pressure gauge is a good thing. I have seen it as a not needed gadget for decades, difficult to read when riding.
I was so wrong!
Easy to have a quick look now and then. A hot summer (often around 30-32C) made me to verify pressure when light could flicker at idle when hot.
A pressure gauge will also show when oil has decaded and have lower viscocity causing  lower pressure.

Both my CB750s have pressure gauges and new pump rotors from MessnerMoto. Plus the gaskets, seals.
Around 70PSI when idling direct after cold start.
40-60PSI @ 4000 rpm cruising when warm.

Idle at 1200 rpm is a well working rpm when riding in the city (easier take offs and not that abrupt throttle off) and avoid flickering pressure warning when hot.

CB750 must have 20W-50 oil as its owners guide Honda wrote specifies.
10W-40 for lower temperatures, below +15C.

Honda might not had thoughts about riding when weather allows +35C.
I should use 15/20W-60 oil that period.

Too low pressure with 2 pumps is strange. I have heard that too high can be reached by mounting the pressure relief valve upside down.
Maybe if spring is not there.
Possible to put a shim washer on spring and increase pressure.

The other valve must also be able to slide free inside. I had to clean pump with drive chain cleaner spray and use fine grit paper on the valve (steel pipe). This is to keep oil in tank when parked.

If spring and washer for oil filter is mounted wrong, pressure will be affected when filter can not seal against case? Or maybe missing not pressing filter against case.

[D DIAMOND]

i have been using semi synthetic 10/40 as i thought honda spec was 10/40 i have used the same oil on my other cb750s for about 6 years this the first i have heard about using 20/50 oil DD

[PeWe]

i have been using semi synthetic 10/40 as i thought honda spec was 10/40 i have used the same oil on my other cb750s for about 6 years this the first i have heard about using 20/50 oil DD

Search forum and find.
Viscocity is the most important detail of an oil (next to no anti wear additives that can cause clutch slip). It specify the pressure the oil film between crank vs bearings withstand, gearbox etc. (Separate  gearbox oil is usually much thicker 80-90)

Engine will get higher wear with wrong viscocity.
The oil label on side cover specify 20W-50, 10W-40 too without more information.

Modern engines use lower viscocity oils due to tighter clearances.
The construction of crank - rods with their lubrication another thing. Our bikes needs higher pressure. Crank is floating on a film of oil with hopefully no metal to metal contact.

I read about viscocity related issues on a Porche forum where some users ruined their engines when using too low viscocity high quality oils. More sensitive when camchain tensioner was hydralic.  Those engines also made for 20W-50, 5/10W-40 not good.
A test of a not much used quality oil showed that viscocity degraded rather quick and oil changes to be done in time.

This is up to all users to decide. Good to know what to use for longer life before shorten the life of the engine.

My users guide I got with bike was very clear.

[jonda500]

"(Separate  gearbox oil is usually much thicker 80-90)"
Since buying a two stroke KTM380 with separate gear box oil (but still with wet clutch) I have been schooled big time about engine/gear oils over in the KTM forum. Now you wont believe this and will have to google it to verify, but gear and engine oils use completely different viscosity rating systems - I can't remember any specific conversions but gear oil uses a higher number than engine oil to mean the same thing! - so an 80-90 gear oil is not much if at all thicker than a 20-50 engine oil...
Sorry to go off topic with this can of worms, please disregard and carry on (and if, like me when I first read it, you don't believe this - go look it up!)
John

[PeWe]

I did not know the viscocity difference gear oil vs motor oil.
Good info about oils is important.
I have never understood the problem with oil threads. Its like a bad religion to complain about oil questions.
The world have changed since the 70's when the Castrol GTX 20W50 was the oil.

Even the very popular Shell Rotella has got its receipe changed with less zinc and phosporous our old designed engines needs which is discussed in some threads were people had contacted Shell for specification.

It should be an own oil classification for the old CBs, not only for wet clutch, enough of metal against metal lubricating phosporous, zinc as well. API class that was the latest when these bikes were produced.

If searching, you can find oil for classic engines.  "Classic Motor Oil",  API SG or SH in oils I have found when searching.
And explaining articles as this one:
https://www.transdiesel.com/blog/passenger-car-motor-oil-blog
"So while API SJ, API SL, API SM and API SN PCMOs are perfect for modern petrol engines, they may not be ideal for older engines, especially those fitted with flat tappet cam followers."

This can be the future help if not knowing an oil with specified amounts.

Separate additives can contain anti friction stuff that is not good for clutch causing slip.

This make me to feel right to order a batch of more Spectro HD!

EDIT:
The 2 CB750 heads I have recently bought on eBay, 392 head from UK ( probably US import) and K6 head from USA had both really bad exhaust valves while IN were fine. K6 head looked like a really low mile not tampered with before. I guess the old owner used bad oil. Too thin or not for the old type of engine.

[spanners]

I don't suppose your problem is an oil inlet flexy pipe restriction rather than a problem of dumped pressure on the outlet pressure side?

Only mention it as by chance this article by Hondaman made me think:

http://forums.sohc4.net/index.php/topic,187289.msg2172527.html#msg2172527

Reference collapsed internal part of the line only when hot.