[allen]

 

[Lomcevak]

On certain older bikes, there was a positive correlation between the plug being left in and the crankcase seals failing. Old habits die hard.

Not sure if you're suggesting that the plug should be removed during normal operating, but if so, I'm afraid you're completely wrong.

The Owner's Manual (found under your seat) specifically states that the plug is only to be removed to drain the tube and to remain plugging the tube while riding.

 

[Maxidax]

Pure genius.

 

[spdklls]

manual says to check and clean periodically...mine is still as clean as the first mile i put on it 6000 miles later...works for me..im leavn the plug in too..if it wasnt supposed to be in there..they wouldnt have put it there

 

[AlexCBR250]

you made me cry (of laugh)

 

[Parakalien]

If that Audi thing is really true...... wow....

 

[Deanohh]

None of you guys know what that hose with the plug really is, do you?

hint: all cars have crankcase breathers, but they don't have one of these. (maybe a few do)

 

[MotoMike]

None of you guys know what that hose with the plug really is, do you?

hint: all cars have crankcase breathers, but they don't have one of these. (maybe a few do)

Apparently some haven't bothered to look where the other end of the pink see-through drain hose is connected to, or where the black rubber hose connected to the fitting at the rear of the crankcase leads to...

 

[Deanohh]

"I normally remove that plug, even on the dirt bike.."

Don't leave it out. The "real" crankcase breather connects to the airbox so that crankcase gasses are fed to the intake and burned. Condensation can collect in the airbox and there is a separate tube that the condensation, if any, will run out of the airbox and this is the tube with the plug in it. Without the plug, it is a hole in the airbox after the filter and sucks unfiltered air right into tha intake manifold.

some older or modified bikes may have a different set up.

 

[MotoMike]

On most Off Road bikes, the air box drain hose connects to the air box before the air filter, not between the filter and engine intake as it is on the CBR250R. As Deanohh said, on the CBR250R the plug has to be in place to prevent crap from being sucked up into the filtered air of the intake tract.

 

[jsonder]

Additionally, what has to happen to get negative pressures in the air box? If this is the result of negative pressure from the crankcase, what is the condition that causes this? Will oil frothing (from overfilling) create that condition?

And yes, the "older bikes" I referred to were designed in the early 80's (Honda Rebel). Bone simple design and easy to work on. We no longer have any, so I can't check the location of the tube and the filter.

 

[Deanohh]

Additionally, what has to happen to get negative pressures in the air box? If this is the result of negative pressure from the crankcase, what is the condition that causes this? Will oil frothing (from overfilling) create that condition?

And yes, the "older bikes" I referred to were designed in the early 80's (Honda Rebel). Bone simple design and easy to work on. We no longer have any, so I can't check the location of the tube and the filter.

John!!! Come on, man.... from the engine running...... Pistons pumping air in through the airbox.

 

[jsonder]

John!!! Come on, man.... from the engine running...... Pistons pumping air in through the airbox.

Those were twins with 180-degree ignition offset.

A single will have changing "air volume" on the crankcase when the engine is running so you get a sinusoidal pressure curve with ups and downs displacing about a quarter liter of air.

So to test this, I went to the garage and started the NX250. The amount of push/pull felt at the open end of the tube (engine speed 1400 rpm) was hardly noticeable. I am not worried about my 250 single sucking water up into the tube.

for anyone curious about this 26-hp, 300-pound bike, try this url: http://www.bikez.com/motorcycles/honda_nx_250_1989.php

 

[Deanohh]

I didn't realize you meant breather tube instead of air box.

of course it makes no sense to block off a true crankcase breather.

 

[jsonder]

I didn't realize you meant breather tube instead of air box.

of course it makes no sense to block off a true crankcase breather.

No, the puke tube takes the condensate. Admittedly, my old enduro has a small (200 cc) flask-like unit that lets gases from the crankcase cool and has the drain/puke tube at the bottom and a side port to the air box.

 

[GPZ550]

several things to consider here.
First the reason for a breather. In a single cylinder engine crankcase volume decreases when the piston moves down and increases on the up stroke, though volume and pressure fluctuate the net effect would be zero. However there would be a pumping loss in a closed system. Even if we were willing to accept that loss a breather would still be needed. All engine develop pressure in the crankcase. Minor cause is the expansion of heated oil and air, major cause is blow by. No matter how good the engine a small amount of pressure above the piston ( both compression and combustion ) leaks past the rings, though this is very small per event it quickly adds up due to the number of events at operating RPM. Old engines ran open breathers but due to emissions regulations the systems are now closed. The vapors come out of the engine and are routed back to the air box after the filter ( so as not to foul the filter with oil) to be added to the intake air. Some of the oil will fall out of suspension and accumulate at the bottom of the air box. This is the reason for the drain, it is not a direct breather, only a drain. Now to the reason for the plug. As soon as air moves its pressure drops ( the reason carburetors work ) so any time the engine is running air box pressure will be negative and as stated above the drain is after the filter so if run with no plug unfiltered air would be drawn in. Many motorcycles have no plug in the drain, if you pull the filter out of one of these and use a mirror and light you will find a very small foam or fiber filter over the drain hole. For those interested in a clearer view of the system look at any old air cooled 4 cylinder Suzuki. Look at the center of the cam cover and you can see the breather tube exit the plenum and go directly to the top front of the air box well forward of the filter element.