Topic: 73 CB750 Oil Tank breather spews oil when throttling

[Vincent P]

Hello all,

My issue first off is that at hi-throttle what I think is the breather hose top rear of the tank will spew lots of oil. The cylinder head had a valve job over the winter, re/re done by myself; while this happend I stripped the bike and had the frame powder coated.

It runs great except that I have some puffing out the breather hose off the rocker cover gasket, but my main concern is the oil spewing from the breather hose on the oil tank.

The connections are as follows. I have the front compression oil hose to the  to the front mount at the side of the engine, and the rear compression hose at the rear of the tank to the rear connector on the engine. there is what I believe to be a breather return hose from the rear of the motor connected to the  botton connector at the rear of the tank, and finally a breather hose at the top to atmosphere (this is the one that leaks when the bike is fully throttled).

1. What could be causing this?

2. What is the correct connections for the hoses on the tank. Does anyone have a schematic I could follow.

Thanks for any help all,

Vincent

[Old Scrambler]

Going from memory.................The bottom connector is the vent from the tank (internal from top).............the rear case vent tube should go to the higher tube on the back of the tank. Please do not ride until this condition is corrected.

[Vincent P]

Thanks Scrambler will give this a try.

[Vincent P]

Ok from what I can gather Scrambler via some internet research, the hoses were connected correctly.
The smaller hose from the back of the engine was connected to the smaller connecter at the back of the tank, and the larger ID hose at the top rear near the oil cap is simply a breather.

So my real question then is why is this spitting oil?

Got me scratching my head?

[scottly]

Sounds like you have over-filled it with oil. The oil stopper tends to leak, allowing the oil to drain into the sump when a 750 sits for a length of time. It's called "wet-sumping". If you have done like I have done, you checked the oil level in the tank, found it to be low, and dumped in a quart or more of oil into it. After starting the bike, the oil scavenge pump pumps the oil hiding in the sump back into the tank, causing it to flow out of the tank breather hose.

[754]

As Scott said, let me guess..one day you looked and it was low on oil..

[scottly]

A few months ago, I checked my oil tank and found it to be very low, as expected. I poured in about 1/2 quart of Marvel Mystery oil, thinking maybe it would help flush the stopper, and the wintergreen oil might soften the rubber a bit. After I changed the oil a few weeks ago, I've seen no evidence of the oil level in the tank dropping, while before the MMO soak, the level might drop a noticeable amount in a few days.?

[Vincent P]

After I had re-assembled the bike I had an oil issue where no oil leaking from the bike but the tank would run dry and the oil light came on. I ended up putting about 6 liters in her to get the level up. I then dropped the oil pan, which gave me back all the oil I had put in. I then dropped the oil pump and cleaned this as the screen was filthy. Put all back together andi it runs fine.

During the whole carb syncing process, and putting on new points and adjusting, it was running fine. I have started to ride, short ones at first just around the block and on some local side streets she is fine.
Yesterday was my first ride on a major highway/freeway, rode from home to work and it spit out about a liter of oil from the breather hose at the top of the oil tank.

Note: if I crack the throttle i.e. wap wap nothing comes out; but if I hold a steady rev lets say 4ooo RPM then the breather tube will start to leak oil.

Note: I also have white smoke coming from the breather hose on the rocker cover, this will come out in a nice steady stream when the bike is throttled, and will lightly puff out when it is idling. This is not a blue smoke but a pure WHITE smoke.

This really has me scratching my noodle!!!

[Vincent P]

It almos seems as though I'm getting to much crankcase pressure at high RMP under load.
I have re-adjusted the carbs so that they are not running so rich; does not seem to have changed anything.

[crazypj]

If the tank wasn't over filled because of oil pump leakage, you may have stuck or weak rings with excessive blow-by
Unfortunately, it's a pretty major job on CB750 to pull motor  for top end removal
I'm pretty sure there is no measurement for free ring gap on older Honda's, the fitted gap can be perfect but still leak during combustion if ring tension is low
I just did a CB360 with Suzuki pistons, (378cc conversion) that pushed about a pint of oil through breather in 20 miles
 Ring gap should be approx 9mm but was only 4.5mm, the fitted gap was 0.016" (Suzuki gives free end gap)

[Vincent P]

If I had blow by on the rings, would I not see this coming from the exhaust, or at least see signs of it on the plugs i.e. on the spark plugs either gummed up, or wet??? I see none of this?
On a compression test I had 140psi plus on all cyclinders.

I'm starting to thing that I have a crankcase vent plugged as the pressure seems to be coming from behind the pistons back into the oil tank which I think is causing it to also come up throuth the oil drain holes in the top end of the motor.

I will look at the crankcase breather at the rear of the motor, and drop and re-inspect the oil pump valve, along with the tank itself. Hopefully this will uncover something.

Thanks for all the suggestions all.
Vince

[fire113]

...my K1 did the same, @WOT oil comes out
I found a broken piston ring @#1,put a 836-Wiseco-kit and no more oil was leaking from the oil tank 

Good luck...Georg

[Henning]

I had this problem on my K1 for many years; this past winter I did some work on the engine and it looks like I finally cracked it. So far in 1200 km I have not had a problem. The downside is that I can't tell you what it was that made the difference.

Sooo, make sure the basics are right:

1) Put in the correct amount of oil for your model. Drain it and measure it if necessary. It's 3.5 liters for a K1.

2) Replace the small hose from the back of the engine to the oil tank so you are sure it is OK.

3) Make 100% sure that the oil tank breather and the valve cover breather are clean and are not being pinched in any way. Drag a rag through them using a wire and route them out through the frame under the oil tank.

If this doesn't help you can try:

1) Replace the three big oil pump o-rings with genuine Honda parts

2) Clean the oil pump screen (you already did this)

3) Dismantle, clean and inspect the oil pump pressure release valve and oil stopper valve

4) Replace the small o-ring behind the gear cover with a genuine Honda one

5) Crank down the head bolts some more

I did lots more than this, but did NOT have the head off. My compression is 135, 135 135 125.

Good luck!

[crazypj]

If I had blow by on the rings, would I not see this coming from the exhaust, or at least see signs of it on the plugs i.e. on the spark plugs either gummed up, or wet??? I see none of this?

Nope, combustion pressure is over 1000psi, you can loose some down side of pistons but oil in crankcase doesn't get past oil rings.
 Compression test can also be good (or even slightly high)
All sorts of weird and wonderful things happen inside engines at high rpm
 When breather is blocked or pinched the oil seals leak because of excess pressure build up

[Vincent P]

I've narrowed it down to number 2 cylinder; if I pull the spark plug wire one at a time while running 1,2, and 4 will cause the engine to buck and sputter, but if I pull 2 there is no change.

Question: I had the valves redone on this enginem could it be something as simple as valve adjustment???

[bollingball]

I've narrowed it down to number 2 cylinder; if I pull the spark plug wire one at a time while running 1,2, and 4 will cause the engine to buck and sputter, but if I pull 2 there is no change.

Question: I had the valves redone on this enginem could it be something as simple as valve adjustment???

Do you mean #3??
Ken

[Vincent P]

Sorry meant to say 2 the others will stumble and run rough if the plug wire is pulled.

[Duke McDukiedook]

Have you run a dry and wet compression test to confirm any of this?

[Vincent P]

Will borrow a tester from a buddy this week and confirm.

[crazypj]

Valve adjustment won't cause crankcase to pressurise

[Vincent P]

Well compression test has revealed the following:

1 and 4 Cylinders have 90psi. 

2 has 30psi. 

3 has 125psi.

Will pull her apart soon and keep you updated on what I find, but am leaning heavily towards rings...

Thanks for all the input brothers....

Vincent

[scottly]

Squirt some motor oil into the # 2 cylinder and check compression again; if it goes up, suspect the rings, if not, suspect the valves.

[Vincent P]

Scotty, the cylinder head was rebuilt professionally over the winter new valves seals springs lapped and planed

[scottly]

I don't care what was done to the head; a tight valve adjustment may be your compression problem. The compression test with added oil will help determine if the problem is the rings or the valves. 30 PSI is seriously low for bad rings...

[Vincent P]

I though about this too, but had a buddy tell me to tight valve i.e. I said could not be closing correctly would cause this; he said no.

I will use the oil trick (how much should I shoot in the cylinder?)...

I'll adjust the valves and give it another compression test.