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Semi-professional MoRider
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122 Posts
Discussion Starter · #1 ·
Short version:
Root cause: clogged air filter due to high engine oil level makes engine run rich
Solution: wash or replace air filter and adjust oil level at next oil change

Longer version:
The stink or '919 stank' has puzzled generations of 919 riders. Different theories and remedies have been contemplated, from higher octane fuel, hotter thermostat to windscreens and pipes but no conclusive solution has been reached to my knowledge,

I am in the camp of 'I don't care unless I wait at a stop light with a tail wind'. However, recently I started riding with a friend; he is a 'returning rider' and rides behind me. He mentioned the stink repeatedly. Still, I didn't really care but it seemed to get worse recently.

Earlier this year, spring cleaning and maintenance on the 919, I pulled the air filter. It was black and oily. I washed it out with soapy water and dried it (it takes about two days to dry even in my 90F + garage).

The stank disappeared, and the engine was much more responsive. But the stank came back after a few rides.

Pulled the air cleaner again, and again it was oily and dirty. Rinse and repeat.

I started thinking where the oil gunk comes from. The crank case breather hose goes into the intake duct somewhere seems to be the most likely culprit.

My oil level was at the top line of the glass.

When I then changed my oil, I was very careful to adjust the oil level so that it is exactly in the middle of the glass, not the top.

Now the air filter stays clean and the stank is gone.

What do you guys think, did I find it?
 

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McTavish
Joined
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6,570 Posts
yes I got a center stand
While the centre stand provides for the bike being level on the yaw axis, it introduces error on the pitch axis.
Which is why the factory manual calls, admittedly in rather inferred way, that bike be on its wheels and balanced on its yaw axis and fill to the high mark.
Depending on your front end, and worst case is a stock front end with the ride height (preload) adjusters fully backed out, being on the centre stand significantly changes the pitch of bike thus screwing up the accuracy of the sight glass.
In other words, oil level at max on the sight glass when the bike is on the centre stand = overfilling.
 

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McTavish
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6,570 Posts
By the way, low mount single out exhaust conversions do help re the stink. Because the stock exhaust exits are no longer dumping into the very strong low pressure turbulent zone behind the rider's back (or the passenger if there is one).

I'd also expect a Cat' type Y Pipe would help, as long as the PAIR system is intact and operating correctly. But exhaust is still exhaust and the stock outlets are in a really bad place for good diffusion behind an upright rider.
 

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Semi-professional MoRider
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122 Posts
Discussion Starter · #6 ·
yes you are right about the pitch angle. I verify by sitting on the bike and taking a picture with my cell phone.

But my point is, the stink is likely related to oil level too high and crankkase breather gunking up the air filter.
 

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McTavish
Joined
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6,570 Posts
yes you are right about the pitch angle. I verify by sitting on the bike and taking a picture with my cell phone.

But my point is, the stink is likely related to oil level too high and crankkase breather gunking up the air filter.
My opinion is that the infamous stink is due to unburned hydrocarbons, and is also a function of the fuel formulation, and I don't mean ethanol content. Gasoline has not smelled like gasoline for decades, because of the other stuff that makes up recipes going back to the '80s. True, if the air filter is gummed up, that's going to screw up the fueling. But squeaky clean air filters are also associated with the infamous stink.
 

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McTavish
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6,570 Posts
A bit more on the "stink"............................
So we have fuel formulation as a factor.
We have unburned hydrocarbons as a factor.
There is also partially combusted hydrocarbons as a factor, that result in intermediate species.
There's very complex chemistry involved.

More....
When there is incomplete combustion, CO (Carbon Monoxide) is formed.
The EPA is after CO and seeing as CO is combustible, the PAIR system is used to supply air at the exhaust port to allow the CO to combust and form CO2.
Depending on the reactivity of the unburned hydrocarbons, and the uncombusted intermediate species hydrocarbons (a.k.a. partially combusted hydrocarbons) vis a vis the conditions at the end of the exhaust port just before transition into the header tube, some of the aforementioned hydrocarbons will burn.
Not lost on me is how many have reported exhaust popping ceasing upon the disabling or removal of the PAIR system.
I doubt that CO to CO2 conversion by PAIR causes popping. But a combustion of hydrocarbons would.
 
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