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Oil dripping out of airbox breather tube

12K views 32 replies 9 participants last post by  WSC 
#1 ·
I recently cleaned my carbs and adjusted mixture screws and the bike is really roaring. Recently, after 5-10 minute rides and longer I get a little puddle of oil draining from the airbox tube. Never happened before. Last oil change was about a month ago and it is perfectly in the middle of the sight glass. When cleaning the carbs I put a new stock airbox on where I was riding with a K&N in stock box before. Any thoughts? Anyone else experiencing this? Cheers!
 
#3 ·
Good call, I have read that before. But, why is it dripping now and never has before? That tube hasn't had a plug in it for as long as I've owned the bike...going on 2 years now. I have read people removing the tube altogether when they go to pods or other mods. Curious as to what conditions other than overfill that it would start to drip oil now.
 
#12 ·
When you replace the airbox with pods, the crankcase still needs to breath, so a standard replacement is a small K&N or equivalent pod filter. I believe the plug that others are referring to is the short vent tube at the bottom of the airbox, forward of the filter element. As the engine "breathes", the "blow by" collects in the bottom of the airbox and needs to be drained periodically. The short vent mentioned above should be capped to keep bad air from entering the intake.

My apologies if this info was already known to you.

I'm going for a short spin tonight and will place a towel under the pod filter to see if that is indeed the area of my leak. If so, I'm betting on an overfill, causing an excess of pressure being built up and forcing itself out through the path of least resistance.
 
#5 ·
When you dropped the old oil, did you open the filler plug first or just the sump plug?

It has been reported on here that draining the oil without loosening the filler plug can put a great strain on the oil slinger seal and damage it - resulting in oil being spat back into the air filter box.

It could also just be a coincidence and the slinger seal was already on it's way out.

To test it - remove the pipe from the airbox and run the engine. If you get oil coming out then I'd say it's knackered!
 
#8 ·
So glad this topic was posted, and shall follow it with interest.

Like swaymix, I've discovered the same symptoms with my 99 TBS.
This has just started to occur within the past few days, and like swaymix, I had R/R the carbs, changed then oil/filter and cleaned the breather filter. The bike is set-up with pods, and a small KN filter for the breather.

After short 4-5 minute test rides, I park the bike in the garage, only to find a nice sized pool (about 5") of fresh oil on the floor underneath. After tracing the oil back to it's apparent origin, I've found that the breather filter has been/was leaking.

Upon reflection, it was discovered that I re-filled the oil case with 4 full quarts of Mobile1 T4.

I'm hoping that it was only overfill and the symptom will cease.

Arfer, I'm trying to follow the comment of the oil slinger. How can the oil slinger be compromised by failing to "vent" the crankcase when removing the sump plug? In my case, I probably did removed the fill plug before draining oil, since that is the method I have used for years, to assist in good gravity flow. The filter was R/R after the oil was drained.
 
#9 ·
How can the oil slinger be compromised by failing to "vent" the crankcase when removing the sump plug?
I can't say it's a dead cert, but many here have said that the vacuum and subsequent "plunging" effect created when the oil is drained (and filler plug not opened) puts a huge strain on that floppy lipped slinger seal and damages it.

Logically it could happen, although I have no direct experience of that happening, rather just offering up collective "wisdom" as a possible cause and/or solution.
 
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#13 ·
Draining Oil

Of course it makes sense, especially when the oil is hot. The sudden out flow of oil from the drain plug can (and has done in My experience) invert the lip of the slinger seal. When the oil is 'glugging' out it creates a partial vacuum in the crankcase that in some cases is not overcome by the breather tube and thus the lip of the seal takes the load. With the oil filler cap removed there is no 'glugging' out, just a steady stream of oil that is not being held back by vacuum.
 
#14 · (Edited)
How much oil flows out, and how fast, compared to the change in crankcase volume when the engine is running? The math for a 3 cylinder is pretty complicated, but for a parallel twin it's simple; the crankcase volume changes by the displacement (e.g. 650cc for a 650cc engine) once per revolution. No glugging oil can compete with that. The 3 is a lot less violent, I just don't know how much less. An in-line 4 has no change in volume; the only problem there is blow-by.
 
#16 ·
Draining Oil

How much oil flows out, and how fast, compared to the change in crankcase volume when the engine is running? The math for a 3 cylinder is pretty complicated, but for a parallel twin it's simple; the crankcase volume changes by the displacement (e.g. 650cc for a 650cc engine) once per revolution. No glugging oil can compete with that. The 3 is a lot less violent, I just don't know how much less. A parallel 4 has no change in volume; the only problem there is blow-by.
I am simply not interested in 'math' but simple physics. The volume of oil exiting the drain plug (when the oil is hot/cold) has to be replaced by an equal amount of air. This air can come from either-the 'glugging' effect of the oil back thru the drain plug, via the breather hose if it is clear and unobstructed or via the filler cap if it is removed. If the filler cap is not removed and the breather hose is obstructed (crap in the filter box end) then the breather seal can invert, and has done in previous experience. All I am simply pointing out is- that it is a good idea to remove the filler cap prior to removing the sump plug-it is not an order or some great mathematical problem, just common sense from past experience.
 
#15 ·
While your comment is (I think) directed to the OP swaymix, I'll answer your comment since I have/had the same problem with the breath tube.

What began after an oil change, and short runs on the bike, was about a 5-6" shallow puddle of fresh oil under the bike (two times). Tracing it to the airbox pod filter, it was indeed flowing out of that. Since then, the volume has dropped substantially, with only a half-dollar size underneath. The fill level was above the center of the sight glass, so in my case, the leak was due to overfill.

While in the previous mentioned times the puddle formed, that had occurred sometime overnight. I had watched for oil to flow while the motor was running, and after switch off of the engine....nothing. So, it must have been a slow release of internal pressure due to overfill that caused my leak.

Yesterday, I had the TBS out for a good long run, and checked periodically for seepage from the filter, as well as evidence of a leak underneath. While I did see/feel the seepage after a good run, there wasn't enough to accumulate on the pavement while parked for a good half hour at the lake. that being said, a half dollar sized drip was discovered this morning.
 
#17 ·
I do enjoy the Technical banter guys...I'm learning something everyday! Thanks for your post Ovni, the symptoms are essentially what you experienced although I was dead on in the middle of the sight glass with another person holding the bike a straight as possible. Either way I put a plug in the tube as it was missing, now no more puddles. I will just have to monitor the oil level and check that tube periodically. Cheers
 
#18 ·
Same issue and the puddles get smaller and smaller until they are just a small drip at the kickstand caused by a slight leak at the clutch pushrod seal. (nearly all of my trips are short and furious runs to the grocery store)

At exactly halfway up the sight glass I still get blow by. Just below that and everything is cool. My experience has shown that anything below that magic "just below half" and I get shifting issues. I have one of those long mirrors on a stick that you can get cheaply at auto parts stores. I check my levels once a week and it never seems to move much. A very small amount of oil makes a very big stain.

Been through this cycle many times after each oil change, I always put a little too much in. New problem is leaking airbox that allows oil to drip out. Waiting for a few more parts and then I am changing it out with a fresh one. It is long overdue.

I was once chastised on this forum for worrying too much about little stuff. Forgot who it was but they were right. My bike marks its spot a little bit now but has never run better. Slowly addressing little issues but it is mostly because I like to pretend I am a mechanic.

BTW, my tube is plugged with a cork I got at Lowes. I used a real cork plug for a vintage/retro look. :)
 
#22 · (Edited)
Draining Oil



It makes not a jot of difference what the pistons do or don't do. As the pistons are spaced at 120 deg intervals on the crank as one goes up the other comes down, and so on, crankcase vacuum/pressure is virtually equalised unlike a Brit parallel twin where the whole mass (pistons) is moving at once. The oil seal is designed to accommodate that small amount of differential. What it is not designed to do is support the sudden outflow of the sump oil (especially when hot) hence the advice to remove the filler plug first.
 
#23 ·
John,

'cuba is incapable of thinking thru' anything once he has an idea in his head. All you get is 100% cognitive bias, rinse repeat!

As you've already pointed out John, a 120 deg crank triple doesn't balance crankcase pressure over its rotation. But rather than take note of what you've already written and apply some of Kahneman's 'mode2' thinking*, or gosh, 1min of google research, 'cuba prefers to ignore all that. (Note also the straw man diversion into whether you'd remove the oil filler anyway... )

(* mode2 thinking is what tend to mean when we use the word 'thinking', but it's not our default mode as it requires time - at least minutes, often longer, of undisturbed cogitation. Rather, unless we make actual conscious effort, we remain in 'rapid' mode 1 ... which just uses automatic heuristics to juggle with whatever cognitive biases we've picked up thru' life, that may or may not have accurate insight into the question at hand... it may look like a duck, but it may be a platypus... mode 1 will never re-examine the data. All this was verified by decades of research by Psychologist Daniel Kahneman, and later verified working with brain scanners..... explains sooo much about human affairs... )

We all do this stuff to a greater or lesser extent, but 'cuba takes the biscuit, seemingly virtually anti-engineer and/or anti-science and proud of ignorance..
 
#30 ·
Intelligence/Stupidity on show for All



Either you have been at the bottle, or maybe something stronger? The above rant is just so absolutely pathetic that one has to wonder what Planet you are on. Sure, debate can be robust and we all have the right to agree/disagree with each other but to lower oneself to your level Mike is, well, really quite sad! It may come as a surprise to you but you are no oracle, no better than the rest of us and with rants as above perhaps you are our lesser!
 
#24 ·
Right!

With 120 degree crank spacing you have two pistons moving in one direction and one in the opposite direction at any point in time. There are instants when they equalize (any time a piston is at TDC or BDC), but most of the time they don't. Immediately after TDC the piston moving up is moving faster than the one moving down (or vice versa for BDC). (Piston speed is at its maximum when the piston is 90 degrees before or after TDC and zero at TDC or BDC.)

Only if the crank throws are 180 degrees apart do they equalize all the time.
 
#25 ·
On a different note, the seal is not in the path the crankcase gasses take to the breather tube. It is only there (AFAIK) to prevent oil from migrating down the shaft from the oily side to the dry side of the slinger. Can someone explain why excess crankcase pressure or vacuum would put any stress on the seal anyhow?
 
#26 ·
Draining Oil

If you had ever replaced a seal, you would understand. The seal is so designed to give extra 'vent' when an event occurs within the crankcase causing excess pressure. The lips of the seal are very soft and flimsy and will release their grip around the oil slinger shaft when extra pressure is detected. Otherwise an ordinary oil seal would suffice That's why! And insofar as the pathetic and childish response from the other, would one expect otherwise?
 
#28 ·
The lips of the seal are very soft and flimsy and will release their grip around the oil slinger shaft when extra pressure is detected.
Then all you should need to do is pressurize the crankcase and blow it back in place.

You're right, I've never been up close and personal with it. How much extra area (compared to normal) do you get to relieve the pressure when the lips relax? (Is it significant?)
 
#27 ·
Yeeehaaaa. Now we're having fun.

FYI, I have a plug in the overflow tube and rode about 100 miles and checked it after cool down and only a couple small drops. At the end of the day I may have overfilled the oil by a couple of ounces as it still seems smack in the middle of the sight glass but the dripping has slowed considerably. Regardless, I will continue to remove the filler plug when draining used oil as it has always been my practice, whether or not that damages the seal, I leave to the guys on this thread to work out. Cheers
 
#29 ·
Draining Oil

The seal is designed to undertake two tasks-one is to prevent excess engine oil entering the left side engine cover (with the engine running), the other is to vent excess blow by. The seal cannot be 'blown' back into place once it has allowed excess crankcase pressure to vent, the lip of the seal, by virtue of it's design, will simply clamp back around the oil slinger shaft and continue doing it's job. As mentioned before, this is a very special and expensive seal, it looks nothing like a normal oil seal. When fitted to the shaft the convex lip of the seal face's outwards, there is a process to go through when fitting the seal to ensure that the lip maintains a reasonably firm contact with the shaft, it is this particular convex fitting which allows the lip of the seal to temporarily open if crankcase over pressure is present and that over pressure cannot escape through the slinger and hollow shaft for any number of reasons.
It's a pity that you have not removed the left side engine cover or actually seen the seal in question WSC because then all would become apparent. As stated earlier, I have removed a cover on a Triple due to the airbox spewing out oil from all it's joints, the engine cover had a large amount of oil sitting in the bottom and we found the seal partially inverted and slightly mangled. The only possible way this could have occurred was from suction on the concave side of the seal and the only real way that could occur was by dropping the sump oil without removing the filler cap.
It's not important if Folk do not believe this, not to me anyway, but the simple expedient of removing the cap prior to draining the oil may save someone the grief I experienced. It's a public forum, take it or leave it.
 
#31 ·
I meant blow it back when it becomes inverted.

How did it get mangled?

Do you know how much vacuum 1/4" hg is? (0.12 psi) I doubt it could move anything. I'd bet just cranking over the engine with the starter would create more vacuum than that.

Still doesn't make any sense to me. Air can rush in through the vent much faster and easier than oil can flow out through the drain. It's all about viscosity. Even if the vent were plugged (how did it get plugged if the problem is yet to occur...) the pressure on both sides of the seal will still be the same. If no air flows through the vent there can be no pressure differential across the seal.

I believe there has to be another cause. No harm opening the filler before draining the oil, although I don't think I've ever done that and (knock on wood) do not have oil spewing out anywhere.

I've said all I have to say on this topic. A mind is like a parachute. It works best when open.
 
#33 · (Edited)
I think there was a photo of the seal posted a while back in a different thread. No photo in it's natural habitat, though.

It may have been here but the links are broken. Note the last post, though, apparently stating the Bonneville, a parallel twin, uses the same arrangement.

Here is a picture of the oil slinger.
 
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