[3rdMarDiv]

A friend of mine just bought an engine for his G14XL and said that it ran fine when the seller demo'ed it...Later, after he installed it, he took off the oil cap while it was running and said oil misted onto his hand.

I tried it on my engine and also had oil mist out of the filler tube top while running...Shows good compression 100lb+ on both cylinders...:fing32:

I'm assuming this is normal and nothing to worry about...I hope...raying:

Hoping someone who knows more about small engines than me tells me I got no problems...:00000060:

I have a twin vert B&S 16hp...:thanku:

 

[cosmo74]

any engine with full pressure lubrcation will do that if you pull the dip stick out even a regular car should do that that's why you are supose to check the oil with the engine off on a level surfice so don't wory every thing is fine if it dose not do that and you have a engine with full pressure lubracation you know you have a problem

 

[Walt 2002]

"Hoping someone who knows more about small engines than me tells me I got no problems"

No problem and full pressure lubrication has nothing to do with it. It is caused by the natural "breathing" of the engine as the piston goes up and down displacing air in the crankcase.

Walt Conner

 

[3rdMarDiv]

That's music to my ears...:thanku:

I had just never given it a second thought as I would not run my engine without the oil fill cap on...:thThumbsU

Now you have me thinking about this as when I took the fill cap off I noticed a pitch change in the engine. Sooo, that pressure in the crankcase must be causing some effect on they way the engine preforms...Right?...:fing20:

Is the effect positive where it adds to the overall performance, or, is it neutral and is simply a result of the engine normal functions...:fing32:

Where I'm headed here is if say a sump gasket is blown, is replacing it sooner better for the engine engine or does it matter...

I replace worn gaskets, seals, because I just don't like oil leaking out and mixing with grass, dirt, etc., and making my machines filthy...:thSick:

Sounds like there may be a mechanical reason to keep the casing tight and dry...???...:bannana:

Thanks for helping me expand my small engine knowledge...:rauch10:

 

[tomw0]

An opposed piston engine, such as the flat-four Subaru and Volkswagen engines, will have two pistons moving 'in' to the sump, and two pistons moving 'out'. In theory, there should be no volume change, and thus no pressure inside the crankcase.
A single piston engine will raise the crankcase pressure as the piston comes down on the firing and intake strokes, and lower the pressure when the piston is on the compression or exhaust stroke.
A vee engine will have higher and lower peaks of pressure, I think, due to the geometry.

In the past, you could take the oil filler cap off an idling car engine, and use a piece of cardboard or paper to check the functioning of the PCV system. Or use your hand. It was supposed to generate a slight vacuum at idle speed, I think.
tom

 

[G-1]

3rdMarDiv, I'm not an expert on this, but here is what I know:

The reason for ventilating the crankcase is to get gas and water vapors out while maintaining a pressure/vacuum that does not stress the seals. Keep in mind pressure can come from blow-by past the rings as well as mechanical movement.

Probably more ventilation is better than less, but nobody wants a messy engine. Also, if the ventilation is not controlled, then carb adjustments would likely be affected. For one thing, most engines these days send the vent gases directly to the air intake. Venting somewhere else would affect the air mixture.

Now this part I'm not sure of, but it seems like even if the gases did not normally go to the air intake, the idle speed adjustment might be affected if the crankcase pressure was not controlled.

 

[3rdMarDiv]

3rdMarDiv, I'm not an expert on this, but here is what I know:

The reason for ventilating the crankcase is to get gas and water vapors out while maintaining a pressure/vacuum that does not stress the seals. Keep in mind pressure can come from blow-by past the rings as well as mechanical movement.

Probably more ventilation is better than less, but nobody wants a messy engine. Also, if the ventilation is not controlled, then carb adjustments would likely be affected. For one thing, most engines these days send the vent gases directly to the air intake. Venting somewhere else would affect the air mixture.

Now this part I'm not sure of, but it seems like even if the gases did not normally go to the air intake, the idle speed adjustment might be affected if the crankcase pressure was not controlled.

Someone else mentioned that, at least in his opinion, the rings do have a gap in them and that should cause some pressure from "Blow-By, by Design". Even though the rings are off-set there is still a gap...mg:

I'll let you know what B&S has to say about this. This is just a matter of curiosity for me now. Mainly, I'm curious as whether this is an engineered feature and if there are levels of pressure that are expected and others that are "Too High". No idea how one would measure these pressure levels. I would think that if you can measure the level that could help in a diagnosis...:thThumbsU

Actually, this brings up a couple more questions to me...

If there is expected pressure in the case, then, what are the pressure limits of say the lower crank seal, sump gasket, etc...

If you do a compression test and are getting say 85lbs, what does that equate to in increased crankcase pressure. How much does that blow-by effect the case pressure...(reaching for the aspirin bottle):lalala:

 

[Lead Head]

Automotive engines should have little to no "blow-by" if you take the oil cap off while running. The pistons moving in different directions tend to negate each other.

Small engines normally operate on a very slightly crankcase vacuum. The breather is usually a reed valve. It lets air out, but not back in, creating a vacuum. Taking the oil fill cap off while a small engine is running basically completely bypasses the reed valve breather, making it blow air, combustion gasses and oil mister everywhere.

 

[3rdMarDiv]

Automotive engines should have little to no "blow-by" if you take the oil cap off while running. The pistons moving in different directions tend to negate each other.

Small engines normally operate on a very slightly crankcase vacuum. The breather is usually a reed valve. It lets air out, but not back in, creating a vacuum. Taking the oil fill cap off while a small engine is running basically completely bypasses the reed valve breather, making it blow air, combustion gasses and oil mister everywhere.

So, if the breather is not working properly (bad) would or could that cause increased or decreased pressure within the crankcase...:Stop:

Is that the sole purpose of the breather valve...:banghead3

 

[38racing]

So, if the breather is not working properly (bad) would or could that cause increased or decreased pressure within the crankcase...:Stop:

Is that the sole purpose of the breather valve...:banghead3

A bad breather can certainly cause excess pressure. My L twin was blowing oil into the air cleaner because the breather was gone

 

[H-D]

I don't recall the source, but I read an opinion that a negative pressure (vacuum) in the crankcase reduced the effect of vacuum in the cylinder (on the intake stroke) causing oil migration past the oil-control rings & intake valve guide to stem gaps. It makes sense to me.

 

[Lead Head]

With a non functioning breather reed valve (or just a plugged breather), there will be excessive crankcase pressure and it will tend to blow oil past the rings. As mentioned, the breather plays an important roll in oil-control. An engine will often smoke heavily with a plugged breather.

A damaged breather that doesn't seal well will often coat the intake (or side of the engine in older engines) with oil.

 

[3rdMarDiv]

A bad breather can certainly cause excess pressure. My L twin was blowing oil into the air cleaner because the breather was gone

Now, that's good information...:thanku:

I do not have any oil coming back up my carb and the filter is clean...:fing32:

Had I found oil on the inside of my filter, I would not have known that a bad breather might cause that to happen...:fing20:

 

[3rdMarDiv]

With a non functioning breather reed valve (or just a plugged breather), there will be excessive crankcase pressure and it will tend to blow oil past the rings. As mentioned, the breather plays an important roll in oil-control. An engine will often smoke heavily with a plugged breather.

A damaged breather that doesn't seal well will often coat the intake (or side of the engine in older engines) with oil.

Hence the oil in 38racing's air filter...:crybaby:

I have a 1972 Ariens Tiller with a 4hp B&S Horz "that is" getting oil back up the carb and does smoke when running...:Stop:

I only use it twice a year and have just been putting up with it...sounds like I will be putting in a new breater valve...:00000060:

I'll bet that is also, one cause, of it only producing 70lbs of compression...:fing20:

Interresting...:rauch10:

 

[Fcubman]

3rdMarDiv,
I would say that your crankcase pressure and low compression are due to worn rings. A leakdown test would show whether your rings are good or bad. A compression test dry, and the with oil added to the cylinder, would also be a indicator. The breather may be clogged or bad, but I suspect that would be an effect, not a cause.

 

[H-D]

A little Mystery Oil (or equivalent) in the cylinder in the off season would help with possible ring gum/sticking issues as well. Just a thought.

 

[3rdMarDiv]

3rdMarDiv,
I would say that your crankcase pressure and low compression are due to worn rings. A leakdown test would show whether your rings are good or bad. A compression test dry, and the with oil added to the cylinder, would also be a indicator. The breather may be clogged or bad, but I suspect that would be an effect, not a cause.

In the tiller I would agree that the rings stand a good chance of being worn...:crybaby:

In the B&S 656cc they were good in the spring at over 100lbs...:fing32:

I think I'm good for now on that engine with some oil misting out of the fill cap while running...:Stop:

 

[3rdMarDiv]

A little Mystery Oil (or equivalent) in the cylinder in the off season would help with possible ring gum/sticking issues as well. Just a thought.

You know I have a can of mystery Oil, but have not used it as I did not want to use something that might cause more harm than good...:Stop:

How do you use it and in what ratio...:trink39:

All ways looking to improve performance...:fing32: