Topic: Oil Pressure Light

[k2 sohc]

Due to covid I haven't been able to ride the honda 4 as often, one trip 6 months ago i rode 200ks, bike ran sweet, recently on a ride, only had travelled 70ks the oil pressure light came on, had a good look over bike, nothing unusual, oil level good, rested bike for 30 mins rode on, at another 20ks, it happened again, stopped, oil level good, decided to trailer bike home, In the shed had another look, nothing unusual again, started bike, oil light goes out, let bike warm up choke off idling at 1200 rpm, all good, did a complete oil and filter change, cleaned all the wire connections, went for a ride, 20ks again light comes on, now if the revs gets below 3000, light comes on, around town if i don't select low gearing and revs at 3000 light comes on, idling around town light always on, when bike gets up to a real operation temperature the oil light comes on, there is no oil leaks around motor, The only thing I haven't removed or replaced is the oil pressure switch, cos it a prick to get at, Why is this happening and why

thanks Alex

[newday777]

Howdy Alex
I guess we have to ASSUME......(Not a good thing.....) What bike this is on?? Your signature says cb750 but always as if going into a shop tell what bike it is and miles on it, saves having to ASSUME....

What is the ambient temperature when this happens?
Specifically, what oil and filter is this happening on?
Did you remove the 1&4 exhaust side tappet covers and run the bike to see if the fender gets squirted with oil on revs?
Yes a bad pressure switch can be the problem(I think I had to replace a bad switch once back in the 70s.) It's not that hard to get to with a deep socket to inspect and replace.
Did you inspect the wire back to the harness tape for cracks or chew marks? And the harness tape to the headlight bucket for chews/chafing?(varmints) A touch to the frame will cause the light to come on.

This thread has a result of a replacement oil hose being too long and kinked being the final cause.

http://forums.sohc4.net/index.php?topic=107915.0

Reply back with the answers and findings.

[newday777]

Stock motor and this just started happening after a long history with the bike? Or modified?

[newday777]

More threads on 750 low oil pressure

http://forums.sohc4.net/index.php?topic=132735.0

http://forums.sohc4.net/index.php?topic=19006.0

[k2 sohc]

Hello Newday777

My bike is CB750 K2 1974, only modification is it has a 836 kit in about 3 years ago, have always used honda oil filters, and GTX 20/50 oil on changes, all oil hoses and breathers were replaced when 836 kit went in, the ambient temp when riding is 20 to 25 Celsius, this has just started,  all wiring on bike is in mint condition, I have inspected and traced wiring from pressure switch to tell tale light no shorts, will inspect oil squirt on 1 & 4 exhaust rockers,

thanks alex

[Terry in Australia]

G’day Alex, I’m in Melbourne mate, it could be your switch, or it could be a failing pump, so the easiest way to find out is to buy an accessory oil pressure gauge that threads into the oil gallery behind the points cover. Your bike should be pumping out 60-80 psi when cold, and 50-60 psi at 60 MPH (100 Kph) in top gear, regardless of whether it’s hot or cold. At idle when warm it can drop below 20 psi, but your oil light will not come on even then, if it’s working correctly.

[k2 sohc]

I purchased an oil pressure gauge, all readings are perfect, on start up, idle and highway speed, then after 20 kms again oil pressure light came on again, oil pressure gauge showing zero, I have removed oil pump, all parts associated with rebuild have been ordered, will update soon to give result
alex

[Terry in Australia]

Good idea mate, I hope you didn't ride too far with a zero reading on your oil pressure gauge, as soon as you lose oil pressure you have metal on metal, with no oily stuff in between.

[newday777]

Alex
Did you inspect the squirt on 1 & 4 rockers like you said you would?

[k2 sohc]

yes oil squirt is strong on 1 & 4 rocker when u give it a few revs, havent checked if its the same when oil pressure light comes on,

[newday777]

yes oil squirt is strong on 1 & 4 rocker when u give it a few revs, havent checked if its the same when oil pressure light comes on,

That is the important time now.......

[Terry in Australia]

I wouldn't run it at all with the oil pressure light on, I restored a K2 in 2019 and didn't clean all the sludge out of the bottom of the engine (I dropped the sump pan and pump, but there must have been more crap lurking down low) and because I fit an oil pressure gauge on every CB750 apart from my K0, I was riding along the freeway @ 60 MPH when my oil pressure light came on, so I looked at my oil pressure switch and it was down to nothing, and I could hear the top end of the engine starting to clatter, so I switched the engine off, pulled in the clutch, and coasted to a stop.

I'd already sold that bike to a co-worker, so I had to quickly rip that engine out and install the engine out of my bike for him. I haven't pulled the other engine apart, and I'm not sure that I want to.......

[Don R]

 Be sure to clean the bottom of the oil tank too, they build up sludge below the drain hole.

[k2 sohc]

Hi All,
still waiting on a few "O" rings from Honda to arrive for a complete overhaul of oil pump, they are on back order. purchased a used oil pump for a spare. still waiting on Aussie post to deliver, when all arrive, I will post of outcome, the oil tank was cleaned out, very little sediment in bottom
Alex

[k2 sohc]

Hi All,
original oil pump and spare oil pump were both recondition, have oil pressure gauge installed, installed original pump, oil pressure guage shows 70psi as I travel, bike is running sweetly at this stage,  as soon as 20 kms have been travelled, oil pressure light comes on, oil pressure gauge drops from 70psi to zero, Installed spare oil pump. the same happens all over again, Surely two oil pumps cant be crook, All previous recommendations have been re checked, Leave bike sit for 30 mins, then test ride again, oil pressure back to normal, then all happens again, Oil tank is recirculating, The only observation i have notice is the oil tank seems to have a higher level when pressure light comes on then at start of test ride, Whats my next shot,
thanks Alex

[Terry in Australia]

Wow that’s a head scratcher mate, what a pain in the bum! Do you have an oil cooler? Probably not, but some oil coolers have thermostat operated valves that open when your engine gets to a certain temp and will drop your oil pressure by around 10 psi depending on the ID of your lines, but not drop it down to zero. Usually low oil pressure is caused by a bad pump or worn out main bearings, or crap inside your engine blocking the pump. Was there any crap in your pump?

[k2 sohc]

Hi Terry,
bike hasn't got oil cooler, internals of both oil pump were immaculate, no wear, all within specs, motor only has the normal clutch cage rattle, no knocking, only 4 years ago I put a 836 kit in, no play in main bearings, bike only done 4thou kms since 836 kit,  replaced primary chain about 10 years ago,

alex

[Terry in Australia]

That is really weird mate, and seeing that the engine hasn't been a aprt for 10 years makes me think that nothing is amiss, particularly as when you let it cool down everything is fine again. I'll keep thinking about it, but I'm out of ideas right now.

[newday777]

Alex
Did someone use any RTV or liquid gasket in the motor?

[BenelliSEI]

Just reading this post for the first time. Very strange indeed..... you describe the oil pressure loss as “sudden”. To be clear, you get 20 miles and it “drops to zero”? So what can change so dramatically?

1. When you reworked the oil pumps, you had the piston out of the high pressure relief port? Replaced it? It moves freely in the bore and replaced the spring?

2. Have you closely inspected your oil hoses (from the tank)? Is the supply line (pump suction side) clear and open. Any chance it collapses when it gets really hot? No supply = No pressure.

3. Dumb, but have to ask. Right after you go to zero oil pressure and shut off (in that moment) how much oil is in oil tank? Any chance the scavenge portion of the pump is loosing ground and the tank is ending up empty (engine full)??

4. I probably missed it. What oil are you using?

[k2 sohc]

Hi Newday777

when cases were split 10 years ago to replace primary chain I used Yamaha bond on mating surface, installation of 836 kit four years ago, i used Hylomer aerograde sealant on base gasket, rubber caps under cam blocks, and head gasket,

alex

[k2 sohc]

to BenelliSEI
all working parts within the 2 oil pump bodies were in excellent condition, all moving parts were removed cleaned and refitted with engine oil, then submerged in oil and wheel rotated moving oil thru out pump, 6 "O" rings were renewed, next step is to check both oil lines from tank to see if they are collapsing or collapsed during operation, when oil pressure is lost the oil in tank is at normal level, I have only used Castrol GTX 20/50 since new. on cold start oil pressure is 70, it drops to 60 at normal running temp highway use, around town pressure 40-50, One thing to note when oil pressure is lost if I keep revs above 3000 rpm oil pressure with be 40-50 and the oil pressure light will be slighty dim, once I drop revs below 3000 rpm total lose of oil pressure

alex

[newday777]

Hi Newday777

when cases were split 10 years ago to replace primary chain I used Yamaha bond on mating surface, installation of 836 kit four years ago, i used Hylomer aerograde sealant on base gasket, rubber caps under cam blocks, and head gasket,

alex

Very sparingly I hope.

[Terry in Australia]

I'd reckon if it was all done years ago, that shouldn't be the issue, especially since Alex has had the oil pump out, and cleaned the oil tank.(?) I'm interested in John's collapsed oil lines between the oil tank and the engine theory, it was the only thing I hadn't thought of, but should have.

Next time you take it for a lap Alex, when the oil pressure drops to zero, keep it running, but jump off and take a look at the oil lines and see if they're deforming as they try to suck themselves inside out? One of the guys here (in the US) makes really nice stainless braided steel oil lines, if you need new ones, or some of the automotive hose places can put new hose on your OEM hose ends.

[BenelliSEI]

Alex….. it’s also very easy to generate a “twist” in the oil lines when tightening the ends at the oil tank. That can pinch the line half closed to start with.

When you remove the hoses, make sure the OUT tube/fitting on the tank is not clogged with some mystery “lump”…… Good Luck!

P.S. Post your questions in the “regular” topic areas. You will get a lot more answers and suggestions.

[k2 sohc]

Hi All,
I can say my problem with oil light has been rectified, after a lot of suggestions, it has worked out, fitting an oil pressure guage was a big problem solver, it showed that I had reasonable pressure and that I didn't have to lip test riding,  replacing an oil pump didn't solve the problem, original re-installed, washing out the oil tank didn't solve the problem, it all came down to a twist in the oil line, only noticed this when removing the oil tank to wash out, once carefully reconnecting oil lines and keeping them straight, the oil pressure light didn't glow whilst test riding, I have down 400kms now oil pressure holding,
thanks to all who offered suggestions

alex

[newday777]

Good find on that Alex. We're the lines the original rubber lines?

[Terry in Australia]

Well done Alex, and John wins the “Head scratcher of the month” award!

[BenelliSEI]

Well done Alex, and John wins the “Head scratcher of the month” award!

Apparently if you make enough mistakes (“been there, done that”), it can help others out! Great.

[Terry in Australia]

Well done Alex, and John wins the “Head scratcher of the month” award!

Apparently if you make enough mistakes (“been there, done that”), it can help others out! Great.

Ha ha, yes mate, one of the young “executives” I work with couldn’t understand why I get paid twice as much as he does, so I explained that after doing basically the same job for the last 45 years I’m being paid to do the same job he’s learning, without making all the mistakes he’s making as he learns the biz.

[k2 sohc]

newday777
Good find on that Alex. We're the lines the original rubber lines?

Hi Newday777
they are the original rubber oil lines, no signs of delaminating on the insides, The good old GTX oil is working to its name, lol

alex