Topic: Oily back tire.

[Old Moe Toe]

In the past I have been fooled regarding the amount of oil in my engine (750) because of faulty stopper valve in the oil pump.
You know, it looks like the oil tank is empty so you add lots of oil and when you go for a ride it spews up all over the back tire/wheel.
After an engine re-build (836cc) and new stopper valve in the oil pump I still have a problem with oil ending up on the back tire. Pretty certain it is coming from the oil tank breather tube.
I have been keeping the oil tank filled to the top mark on the dip stick. Would have thought this was good practice?.
Obviously not.
Interested to hear from those who know more about this matter than I.
Cheers

[PeWe]

If from oil tank, too much oil.
Many CB750's oil level decrease while parked. Oil flows into engine.
Do not fill more until engine is really warm, 20 minute ride will fill tank.

[Nurse Julie]

Have you got the correct oil tank fitted for your model and have you got the hoses fitted from where they should be to where they should end up 🤔

[Old Moe Toe]

Yes Julie, I think I have got the right hoses in the right spots and the right oil tank.
Surely most folk fill the oil tank to the upper mark on the dipstick?

[DickL]

Check the overflow hose. It goes from back of tank to rear back of engine. Pull the battery and you can the hose.

[Nurse Julie]

Yes Julie, I think I have got the right hoses in the right spots and the right oil tank.
Surely most folk fill the oil tank to the upper mark on the dipstick?

It depends on the age / model of the engine but some are breather pipes, ie, go to atmosphere, others are oil pipes that plug back into the back of the engine. No, we never fill up to the top mark on the dipstick on a CB750/4

[ekpent]

 I'm a little more careful about oil level now but back in the olden days when I did a filter and oil change I would just dump all 4 qts in. Never seemed to have any problems and also nowadays run my oil tank at or near the top line. I had a bike project where the rubber breather hose from the valve cover area was totally blocked and was losing oil from the tank all over the back of the bike.Luckily I wasn't test riding it, member Sean B was 

[seanbarney41]

Only time I had an oily back tire (other than test riding Ekpent's super duper junky bikes, lol), it was leaking from a cracked case by the front sprocket.  I would hope you don't have that problem but if you have never checked?...

[Henning]

Assuming you haven't simply overfilled with oil, and the pipe from the oil tank to the back of the crankcase is good, the reason it's blowing oil from the oil tank breather is too much crankcase pressure. Check that the breather from the valve cover is clear and that it's not being pinched between the swingarm and the rear of the engine casings. It could be something else like a bad head gasket or broken rings. In my case I found broken top compression rings on cylinders 1 and 4. Get too throttle happy and it blew oil out of the tank. A steady 60 mph into a strong headwind and it blew oil. Bike ran well enough in spite of the broken rings. I ran the bike with oil low on the dipstick but it didn't make any difference.

[scottly]

I always fill my oil tank to the top mark on the dipstick, checked after the motor has been run a bit.
The tank breather tube should be routed to the right side of the tire. If the drain tube from the oil/air separator in the tank is split, (which has been reported before) it can dump oil right in front of the tire.

[PeWe]

My CB750 K2, Oiltank has a ventilation hose to free air. Must be routed to the side to avoid tire if it decide to puke oil.

My hose is not that long, just so the end is beside oul lines with valve cover ventilation.

My K6 had longer valve cover ventilation hose hanged in a special holder in the rear foot peg bolt angled outwards and down.

I always fill-up oil to max on dipstick AFTER a ride. A harder ride can fill tank a little bit more.
My K2 has 5mm over max after last oil change.
No oil puke.
I thought I had an oil leak yesterday. I checked the oilfilter bolt before digging deeper.
Lucky me, it was not tightened enough. I tightened it by very light hand at last change.

I have had thoughts about too hard tightened oil bolt that squeeze o-ring beyond its flex and start leaks making the front part of engine really dirty.

This seems to need a retighten after a real warm up.

[markreimer]

I had the same issue on my old 750 after an engine and oil pump rebuild. After the pump was freshened up I noticed the oil level in the tank dropped a lot at startup and took a while to equalize. It was strange! I’d do an oil change so I knew beyond a doubt it had the right amount. It wouldn’t leak down when parked. But after a few minutes riding it would read as though it was nearly empty at the tank. Shortly after that it would come back up and equalize.

It took me a bit to figure this out. Before i did, I was adding more oil and getting a wet tire. I also checked all the hoses as described, didn’t find any cracks, pinched or other issues.

Reminds me of my XR650L. It can read bone dry on the dip stick if you check after a long layup, and the oil level doesn’t come up for several minutes of riding, but then eventually returns to the normal oil level. Easy to overfill as a result.

One way to check if you’re doing this without realizing it is measure the quantity of oil that drains out during a change


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