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Author Topic: Air Blowing In My Crankcase  (Read 2712 times)

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Offline campbmic

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Air Blowing In My Crankcase
« on: February 22, 2010, 07:26:54 PM »
I thought I had seen a thread on here awhile ago stating that if you take off your oil dipstick and you feel air blowing in there then its blowby from the combustion chamber which means worn rings. Is this right? I would hate to tear into my engine again  :o

I searched and couldnt find anything

Its a 75 CB550 by the way
« Last Edit: February 22, 2010, 07:32:27 PM by campbmic »
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Online scottly

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Re: Air Blowing In My Crankcase
« Reply #1 on: February 22, 2010, 07:44:05 PM »
It's normal for the pressure inside the crankcase to fluctuate, due to the displacement from the bottom side of the pistons going up and down in the cylinders. If you had excessive blow-by, you would have more symptoms, such as puking oil and fumes out of the crankcase vents. Relax.... 
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Re: Air Blowing In My Crankcase
« Reply #2 on: February 22, 2010, 09:01:34 PM »
Here's a trick to figure out if you have blow-by going into your crankcase. Take a plastic bag and put it over where the dipstick goes in with the dipstick out. Put a rubber band around it. Or something to"seal" it to the surface where the dipstick goes. Turn on the bike. If the bag blows up and stays blown up you have blow-by. If its just vibrates alot you're fine. I saw a video of this on youtube a few days ago but I cant find it now. Ill look some more.
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Offline Kframe

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Re: Air Blowing In My Crankcase
« Reply #3 on: February 23, 2010, 04:20:07 PM »
Quote from: coheed on February 22, 2010, 09:01:34 PM
Here's a trick to figure out if you have blow-by going into your crankcase. Take a plastic bag and put it over where the dipstick goes in with the dipstick out. Put a rubber band around it. Or something to"seal" it to the surface where the dipstick goes. Turn on the bike. If the bag blows up and stays blown up you have blow-by. If its just vibrates alot you're fine. I saw a video of this on youtube a few days ago but I cant find it now. Ill look some more.

That's a cool test, I'd like to see the video if you find it.
:)
-K
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coheed

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Re: Air Blowing In My Crankcase
« Reply #4 on: February 23, 2010, 10:04:41 PM »
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Offline Steve F

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Re: Air Blowing In My Crankcase
« Reply #5 on: February 24, 2010, 07:46:09 AM »
Cool test, but what if you have a dry sump engine?  Is there a similar test?  What about the breather tube...what do you do with that?  Can it (the breather) be used to do the same test?  What about the tube at the rear of the engine by the trans (750), should that be blocked off to do this kind of test?  I'm just trying to visualize all the places where the engine could vent through, and if they could have an affect on the test.
Steve F
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coheed

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Re: Air Blowing In My Crankcase
« Reply #6 on: February 24, 2010, 07:56:00 AM »
Honestly Im not that familiar with dry sump engines.  But in that case I probably would make sure all ports/ holes are sealed properly on the bottom end and just choose one of them to do the test. But you would need to find a hole or port or something that wont dump oil out, and it has to be on the bottom end.


But Im not sure the same principle will work. As I stated Im not really familiar with dry sumps.
« Last Edit: February 24, 2010, 07:59:37 AM by coheed »
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Offline Gordon

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Re: Air Blowing In My Crankcase
« Reply #7 on: February 24, 2010, 08:20:25 AM »
It doesn't have to be from the bottom end.  The housing on top of the valve cover vents the crankcase, too.  Ultimately, though, a compression test would be a better way to go.
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coheed

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Re: Air Blowing In My Crankcase
« Reply #8 on: February 24, 2010, 09:40:48 AM »
Yea a compression test is the ideal way to go. But if your cheap or a broke college student and dont know anyone with a tester this is a cheap alternative.
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Offline Kframe

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Re: Air Blowing In My Crankcase
« Reply #9 on: February 24, 2010, 11:02:03 AM »
Cool videos, thanks!

This is a good rough test to see if there's a problem, and if so, then a compression test could isolate which cylinder and how bad.
Very cool.
-K
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Offline lone*X

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Re: Air Blowing In My Crankcase
« Reply #10 on: February 24, 2010, 02:10:18 PM »
?  The two videos appear to be of the same bike.  Note the stickers on the clutch housing.  In one video the bike has no blow by, in the next it does.  Makes one wonder about the validity of the demo videos.  How do you turn blow by on and off?  Wondering minds want to know.

I don't have any comment on the test method, just the validity of the video.
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Offline kos

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Re: Air Blowing In My Crankcase
« Reply #11 on: February 24, 2010, 02:17:16 PM »
You could hook bag to oil tank on CB750 dry sump.... as the tank is vented to the engine with that big hose behind engine. And/or the breather tube that runs from oil tank and is pointed to ground.

KOS

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coheed

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Re: Air Blowing In My Crankcase
« Reply #12 on: February 24, 2010, 07:17:57 PM »
Quote from: lone*X on February 24, 2010, 02:10:18 PM
?  The two videos appear to be of the same bike.  Note the stickers on the clutch housing.  In one video the bike has no blow by, in the next it does.  Makes one wonder about the validity of the demo videos.  How do you turn blow by on and off?  Wondering minds want to know.

I don't have any comment on the test method, just the validity of the video.
Isnt it possible they rebuilt the engine?
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Offline Deltarider

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Re: Air Blowing In My Crankcase
« Reply #13 on: February 24, 2010, 11:37:05 PM »
Great idea. With worn rings and an attached bag big enough, one could start hot air ballooning. With these old Honda's, the sky really is the limit. :D
« Last Edit: February 24, 2010, 11:38:45 PM by Deltarider »
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Offline Steve F

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Re: Air Blowing In My Crankcase
« Reply #14 on: February 25, 2010, 02:46:28 AM »
Quote from: coheed on February 24, 2010, 07:17:57 PM
Quote from: lone*X on February 24, 2010, 02:10:18 PM
?  The two videos appear to be of the same bike.  Note the stickers on the clutch housing.  In one video the bike has no blow by, in the next it does.  Makes one wonder about the validity of the demo videos.  How do you turn blow by on and off?  Wondering minds want to know.

I don't have any comment on the test method, just the validity of the video.
Isnt it possible they rebuilt the engine?
They just plugged the vent holes in the second video.
Steve F
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