TIL: my K75 won't idle with the oil filler cap off

[DavidATL]

I am sure many know this.

If you've just changed your oil, mostly refilled the crank case, yet left the filler cap off intending to start the bike and top off the level, be warned it won't idle worth a crap. Extra air will throw off the air/fuel sensors. Put the cap on and life is good.

If you want to test it, try loosening the oil cap while the bike's running. It will very likely stumble and probably die.

May my lesson save someone some time some day some how.

[The Mighty Gryphon]

That is one of the ways to check for a breather air leak that is messing up the mixture. 

The bike idles like crap, and when you loosen the oil filler cap it doesn't get any worse.

[Chaos]

also, if you take it for a ride with the filler cap off it makes a chuffing noise similar to the oil sight glass blowing out.  And it oils your right leg. 

[AtLarge]

Duly noted.

[K1300S]

opening the fill cap is how you simulate the infamous crankcase vent hose crack!

[Laitch]

If you've just changed your oil, mostly refilled the crank case, yet left the filler cap off intending to start the bike and top off the level, . . .

In all the vehicles I have operated, engine oil level has always been checked or topped up with the engine cold, or cooled down for a few minutes after it has been shut off. The Brick's engine is no exception.