Topic: 1973 CB750 Oil Line Leak?

[stnick]

My 73 CB750 appears to be leaking oil, a lot of oil, out of the oil line at the engine connection. It only leaks when I run at sustained speeds of 70-80mph for a few miles. It does not leak when I ease around town and the back roads under 60mph. Why would it only leak when Im running at higher speed and not leak  drop otherwise?
Are the lines for the 73 the same as all the other years?
I have a 76 I bought new and I can swap the lines out for trouble shooting if necessary.

Thanks for your thought!

Nick

[kghost]

Pretty much the same until the F and K 1977 on up.......
Where is it leaking? Engine to line? Around the crimped fittings? From the actual flex part of the line? Oil pickup or scavage return?

I've seen some lines that were completely shot that don't leak.

If you have a decent hydraulic hose place near you...they can probably put a hose and new swag fitting on the stock hardware.

That's what I do when I want a Stainless braided oil line.

[stnick]

I cleaned everything up and headed for the Interstate... 75mph for about 2 miles pulled over and right side of rear wheel covered in clean oil.... it looks like the hose is leaking at the crimp. It s the hose that's lowest on the case.
Why doesnt it leak in stop and go traffic and cruising 60 and under?

Thanks for your time!

Nick

[Black 750K8]

Higher speed = more vibration and more oil pressure. I take it you don't have a oil pressure gauge on the bike.

edit: About the vibration My bike has what I call a sweet spot and a sour spot. That sweet spot is when it just sounds sooo smooth  and it pulls strong and nice around 6000-7000 on mine and that sour spot hell there ain't one.

[scottly]

My 73 CB750 appears to be leaking oil, a lot of oil, out of the oil line at the engine connection. It only leaks when I run at sustained speeds of 70-80mph for a few miles. It does not leak when I ease around town and the back roads under 60mph. Why would it only leak when Im running at higher speed and not leak  drop otherwise?

Thanks for your thought!

Nick

Hey Nick, welcome! The most common cause of oil leakage after sustained high revs is an improperly installed drain tube from the back side of the oil tank to the nipple on the rear of the engine case. At higher revs, the oil pumped from the sump to the tank tends to get foamed up. Inside the oil tank there is an oil/air separator; the oil gets drained back into the case, and the air is vented through a larger hose nearer the top of the tank. If the small nipple on the oil tank is not connected to the nipple on the back of the case, the oil collected by the separator will dump oil all over the back of the bike.
Here is a pic of the drain hose on my K1 engine. Since my battery and other electrics have been relocated, it's easier to see than on a stock bike.
 

[Jerry Rxman Griffin aka MuthaF'er]

Be sure both bolts are sufficiently tightened (without stripping them!).

Replace the o-ring. 

Swap the oil lines. K to K as the F has one hose that is slightly longer.

+1 higher speed = more vibes and more pressure.

[stnick]

Great advice guys! I'll verify the small hose on the rear of the oi tank is connected and then swap out the suspected braided hose if necessary.

Nothing like a good group forum!

Nick

[stnick]

The hose from the anti-foam unit to the rear of the engine case is in place... now I'm looking for decent oil lines.

Nick

[Henning]

Sounds to me as if you have a problem with too much crankcase pressure, which causes oil to be blown out of the vent pipe at the top of the oil tank, and dumps it on the right hand side of your rear wheel. This can be due to a bad head gasket, broken ring or bad valve guides. It's load dependent, so it won't show up if you just rev the bike in neutral, and typically shows up at highway speeds over four grand. Put a catch bottle on the vent pipe to confirm.
I doubt very much that the oil lines are faulty; then it would leak all the time rather than just at highway speeds.
Is this a new-to-you unknown bike?

[stnick]

It is a new to me bike... It starts cold and hot very easily. Idles well and pulls well. Is very quiet. No oil leak at head gasket. I say all this because broken rings and leaking head gaskets, valve guides all indicate a loss of power and overall performance from start up, idle and on up the revs.
Does the 73 oil tank have a vent with no hose on it... that doesnt sound like Honda and I dont see a hose from beneath the bike.
Can the vent you mention be seen easily?
Thanks for your thoughts!
Nick

[Henning]

I think your K3 is the same as my K1, in that there are two breathers that vent to atmosphere. The one from the top of the engine exits between the back of the engine and the front of the swingarm, left hand side, the one from the top of the oil tank exits same place, right hand side. If you take off the tool tray (easy), you will see the big oil tank vent pipe at the top of the oil tank. I suggest you make sure both are clear by pulling a piece rag through using some wire, and then attach a catch bottle to confirm the oil is coming from the oil tank breather.
Take a look at this thread from another member battling the same problem http://forums.sohc4.net/index.php?topic=121715.0. There's a picture on page two of how I rigged up a catch bottle.

And the seller, did he "forget" to mention this little issue when he was selling you the bike?

[Duke McDukiedook]

My 73 CB750 appears to be leaking oil, a lot of oil, out of the oil line at the engine connection. It only leaks when I run at sustained speeds of 70-80mph for a few miles. It does not leak when I ease around town and the back roads under 60mph. Why would it only leak when Im running at higher speed and not leak  drop otherwise?

Thanks for your thought!

Nick

Hey Nick, welcome! The most common cause of oil leakage after sustained high revs is an improperly installed drain tube from the back side of the oil tank to the nipple on the rear of the engine case. At higher revs, the oil pumped from the sump to the tank tends to get foamed up. Inside the oil tank there is an oil/air separator; the oil gets drained back into the case, and the air is vented through a larger hose nearer the top of the tank. If the small nipple on the oil tank is not connected to the nipple on the back of the case, the oil collected by the separator will dump oil all over the back of the bike.
Here is a pic of the drain hose on my K1 engine. Since my battery and other electrics have been relocated, it's easier to see than on a stock bike.

I got a crack in my oil crankcase return line like Scottly describes. It would leak out on my swingarm and rear wheel when I was pushing highway speeds.
Check out my old thread for some info, replacing that breather A hose solved the problem for me.

http://forums.sohc4.net/index.php?topic=118995.0

[stnick]

Ok, I took the black rubber hose( .5 inch pprox diameter) connected to the top of the oil tank and rigged the other end, down near the swing arm, into an empty water bottle. Ny tied that dude to the pass foot peg and headed out to the interstate. Running 70 for a couple of miles produced nothing but vapor drops in the clear bottle. Then at 80-85 for about a mile I could see oil in the bottle... another mile and the bottle nearly filled up ( 10oz). WTF! There must be something else I can do, this engine runs so smooth, quiet and strong I just cant accept the broken ring theory. Could the device inside the tank be defective and allow the oil to be drawn out once it reaches operating speed and temp?
Here's a pic of the rig...

[Johnie]

Just to rule it out...are you 100% sure the vent hose which comes from the top of engine at the back of the valve cover and drains to the atmosphere is open. Take it off and blow through it. I had a K1 do that and that vent tube was blocked.

[stnick]

Thanks Johnie, I'll ck that next. I noticed that vent line in the online manual and wondered how the vent from the oi tank can leak sooo much oil and yet the one from the valve cover not leak a drop? My experience with all overhead cam engines is most of the oil is in the cam area during higher rpm's.
I'll pull the gas tank and ck that valve cover hose and also the compression while I'm there. BTW, the smaller (.250 diameter ) hose going from themiddle of the oil tank straight to the back of the block looks good. Clamps on each end and the hose is nice and dusty outside... indicating no cracks. lol

Appreciate your thoughts, again

Nick

[HondaMan]

This might seem a question from 'left field', but is very important today: what oil are you using?

If you have a high-detergent oil, like Castrol, Havoline, or Valvoline automotive oil, you have FAR too much detergent in the oil. This is a perennial problem since 2002 when the EPA edicted that zinc be removed and replaced with higher detergents, in automotive oils. The cure: use low-detergent, so-labelled, motorcycle oils. One good example (not the best oil though) is the Castrol 4T oil in 20w50 weight. At bare minimum, use 15w40 weight in this engine, never anything lighter.

Another point: if you are using 10w(anything) oil, it WILL foam at hiway speeds in the 750 SOHC4 engine. This is a given, and has been the source of endless "oil fights' in various forums - but, that's the simple truth...

[ekpent]

How much oil are you using ?  Always check the dipstick after the engine has ran for a bit. Because the oil can drain back into the engine from the tank past the check valve some new owners may think they are way low on oil when cold.

[Duke McDukiedook]

^^ This, it is easy to overfill if you check it cold or hot, you want to wait 10-15 minutes after a decent-length ride.

[stnick]

Ok, I pulled the gas tank and verified the valve cover vent line is clear... there goes another cheap solution!
I bought the bike at Barber back in Sept and the owner said he'd just changed the oil and it looked clean on the stick. It was a little over full and I hoped that was the problem. However, after a couple of interstate runs the level dropped to about half way on the hash marks of the stick so I knew that was not the casue.
I will change the oil and filter so I will know I have 20W something as Hondaman suggest. I'll post the results.
Thanks all for your idea's!
Nick

[Henning]

This is all bad news - the only positive is that you now know where the oil is coming from.
I am not an engine guy, so I can't help you from here on. I know of two guys who found broken rings, two cases of bad head gasket, a post by Hondaman mentioning worn valve guides on middle to later models, plus one guy who claimed to have fixed the problem by swapping out the oil tank. To me that sounded like dealing with the symptoms rather than the cause. In my case, I fixed it by cranking down the head bolts. Good luck, mate.

[stnick]

Great news! I was just about to roll the bike off the lift and as I was slipping the valve cover vent hose back on the nipple, I thought " I never verified the nipple was clear". Anyway, I blew in the hose, which I knew was clear, and then slipped it on the vent nipple and immediately felt restriction. I pulled the 3 screws holding the vent cover on and saw the problem. Dirt daubers had completely stopped up the nipple!!! Couldnt wait to get her back together and road test. Not one drop from the oil tank vent!!! Ran 80mph steady for 2 miles...
Thanks everyone for your suggestions and especially "Johnie" who recommended verifying the valve cover vent hose.

All the Best!
Nick