Topic: 1976 CB750K Mystery oil leak on #4 Exhaust?

[Rhymin]

Hello all

I had the recent pleasure of buying my first rideable bike and it ended up being a 1976 CB750K in very nice original condition with about 10,000 miles on it. I did a few things to the bike to get it ready for the season and proceeded to ride the hell out of it for about 2 months. I love the bike and have had almost no problems until now. I rebuilt the carbs a week or two ago and when i got everything back together i was intending to borrow a tool to properly sync the carbs, but was still riding it to work in the meantime. I checked the oil on thursday and it was only slightly low. Everything was fine. Then im warming it up the next morning and i notice a new drip underneath the left side of the bike (It always had a small leaking coming from somewhere near the alternator housing and a little from the head gasket on each side of the bike). I check my oil after it had been running for a few minutes and suddenly there is barely anything on the dip stick. So apparently i lost about 1/2-1/3 of a quart in less than 50 miles on my commute.

I couldn't quite tell where the leak was coming from but i could see that it was collecting on the exhaust flange of the number 4 cylinder. So i decided to run some UV Dye through the engine to try and pinpoint where it was coming from. So i top the oil back up to the proper level (which took almost a quart) and ride it around town for about 30 minutes. Then i get back and i dont see that any oil has collected on the exhaust flange. I check my oil again and suddenly im overfilled by quite a lot. So i figure that maybe it was just a fluke and i got a false oil reading, as ive heard that can happen sometimes with these bikes and with it being a dry sump system. So i decided ill ride it to work again and see what happens.

And sure enough when i got to warm it up the next morning its leaking quite badly and collecting on the bottom of the exhaust flange of the number 4 cylinder. I check it out with the uv light and glasses and i can see where its collecting, but i just cant see where its coming from. It is definitely not coming from the valve cover as i would definitely be able to see that, so i figure the only possible place it could be coming from is the head gasket, right? But it seems strange to me that the head gasket would just suddenly go like that, and it seems like the head gasket would not leak that badly even if it were blown. Not to mention that the headgasket is technically below the exhaust flange so it shouldnt be able to drip UP to it. But then again this is my first one and im not terribly familiar with these engines, so i could be completely wrong.

If anyone has an idea of what im dealing with i would definitely like to know before i start tearing it apart and resealing the engine. 

[ekpent]

Oil level does fluctuate in the tank on these 750'S depending on the condition of the oil stopper in the pump. Always check oil after its been warmed and ran for awhile.
   I will let the other guys chime in on some tips for finding it like spreading some powder etc..

[Airborne 82nd]

When you checked it the second and it was over filled did you ever drain that extra oil back out?

[Rhymin]

Thanks everyone for the replies. No i havent drained the extra oil out yet. But i got some tracing powder like what calj737 and ekpent recommended and when i look for leaks with that i will make sure its at the proper level.

Another question i had was about how exactly to check the oil on this bike being that it is a dry sump system, as im not familiar with them and it doesnt specify in the owners manual. What i was doing is checking it with it cold and not running, but now im getting the impression that thats not the proper way to check these engines.

[MauiK3]

When my K3 sits for a few days the oil drains into the engine and the tank appears very low. I crank with the kill switch off until I see oil pressure (I have a gauge, it jumps to about 60 psi after a bit of cranking). Then I start it and watch the tank. The oil is slowly pumped out of the engine back where it belongs. I bet the first few minutes of running it's a real oil slinging mess inside the crank case. I check the oil on the center stand after the oil level settles out in the tank. If I didn't do this it would be overfilled every time. I like having the gauge, takes away some of the guesswork.

[Rhymin]

Ok, so i have some news. I cleaned the engine off again, drained the excess oil and sprayed some of the tracing powder, then let it dry and started it up. Immediately i saw liquid spilling from the TOP of the exhaust flange on the number 4 cylinder. I realized this made no sense so i checked and sure enough it was gasoline coming from this small hole on top of the exhaust pipe. Considering that i thought i was low on oil the same time this leak formed i never even considered that the leak could have been anything other than oil. I suppose this was a lesson in assuming a problem for me.

So now my question is; What typically cause almost raw gas to come from this tiny hole on top of the exhaust? Is the cylinder running too rich? Does it just need a new seal? Seems to go away mostly when its warmed up, though the exhaust pipe does smoke along its length for a while after it has stopped. Could this have been caused by my carb rebuild? I wouldn't think it would do that just because they're not synchronized yet.

[Rhymin]

Pulled the plug and yes, very black. Not a whole lot of buildup so im not sure its fouled enough to weaken the spark, but its definitely very black. Is it possible the float is too high or should i simply replace the plug and lean out the mixture on the air screw?