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Thread: Crancase vent

  1. #1
    Atomic City Boxer 154048's Avatar
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    Crancase vent

    Greetings,
    On my 1980 100RT is it the suction in the airbox that draws oil vapor out of the crankcase (then into the 'horns' to be run through the carbs and be burned)?

    I am getting oil building up in the bottom of the box.

    I may end up looking at the reed valve and drain hole, but wonder if clamping the vent hose before it enters the airbox (under the starter cover) would have any good or bad effects...?


    Thanks
    Steve in Santa Fe
    1980 R100RT
    2005 DR 650

  2. #2
    Administrator 20774's Avatar
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    AFAIK, the airbox has not effect on pulling anything out of the breather. The breather opens when the pistons are moving from TDC to BDC...the pistons are creating an overpressure in the crankcase, so the breather opens to relieve the pressure. As the pistons move back towards TDC, the breather valve closes.

    If you have oil in the bottom of your airbox, I would think the problem is most likely a crack in the lines that take the oil mist into the carbs...maybe a tee in the connection has come loose.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #3
    Registered User dwyandell's Avatar
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    Kurt has it right. . .the crankcase vent hose and junction, esp. where it comes through the engine cover and into the airbox, will crack over time, leaking oil into the airbox.
    The next time you have the airbox apart, pop the top cover off and inspect this vent hose. You may find that the hose needs replacing. There are also two little gaskets--one at the outlet cover where the hose starts, and the other at the reed cover .. . these can leak and cause a mess too.
    Dave in Vermont
    '84 R80ST
    '81 R100 hack

  4. #4
    Atomic City Boxer 154048's Avatar
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    Gents,
    Looking closer shows that one of the 'T' hoses had popped loose from the intake horn. Simple enough to snap it back in...
    Thanks for your replies

    Steve
    Steve in Santa Fe
    1980 R100RT
    2005 DR 650

  5. #5
    Nickname: Droid
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    Two things can cause too much oil in the airbox from the crankcase breather:

    1. Oil level too high in the crankcase. Oil level too high means less air volume in the crankcase to compensate for the pumping action of the pistons. The excess oil will get forced up the breather to the airbox, if it does not force its way out the main seal at the back of the crankcase.

    2. Gummed up/stuck compression rings on the pistons. Stuck compression rings cannot adequately expand as the piston nears the bottom of the stroke and pressure from combustion gets by and increases crancase pressure enough to force oil out the breather. This can happen even if the oil level is right. If your bike smokes on relaxed throttle/decel it also indicates possible stuck compression rings.

    In both my 76 R100RS and 94 R1100RS I have never had excess oil in the airbox on either bike.

  6. #6
    Administrator 20774's Avatar
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    Quote Originally Posted by ANDYVH View Post
    Two things can cause too much oil in the airbox from the crankcase breather:
    Apparently there is a third cause...the connectors to the lines that carry the oil mist to the carbs comes loose!
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  7. #7
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    Quote Originally Posted by 20774 View Post
    Apparently there is a third cause...the connectors to the lines that carry the oil mist to the carbs comes loose!
    I was curious to know, Kurt. Since your '78 R100/7 only had one tube going back to the right air horn, and my '78 R100/7 actually has two (look stock - single tube out breather then "T's" into two - 1 going to each air horn, do you have any idea when the factory made the change? Was the two-tube method earlier or later in time than the 1-tube design?

    Ironically, I don't have any oil in my airbox as this thread first got started, but I definitely do have MORE oil going to the left airtube. I know this because, when I replaced the Needle Jet and Jet Needles on my carbs, the left carb clearly had an oil residue all over everything. Right carb did not.

    I can't see where there could be any other source for the oil residue than through the breather tube(s).

    Hmmm?

  8. #8
    Administrator 20774's Avatar
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    I don't have any specific time period...I always thought it was later, like after the 1980 models. My bike is an early model built September 1977. I wonder if maybe after January 1, 1978, then made changes. I've heard that was the cutoff for bikes to meet stricter EPA standards in the US. Does that track with your build date?

    Update: I entered my VIN into RealOEM and went to this picture under Housing Ventilation:

    http://www.realoem.com/bmw/showparts...55&hg=11&fg=15

    I'm not sure I really understand all the bits and pieces...some of which weren't offered as the table doesn't identify all pieces. I do see "Up to 9/1978" so maybe that's an indication when some kind of change was made.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  9. #9
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    Quote Originally Posted by 20774 View Post
    I don't have any specific time period...I always thought it was later, like after the 1980 models. My bike is an early model built September 1977. I wonder if maybe after January 1, 1978, then made changes. I've heard that was the cutoff for bikes to meet stricter EPA standards in the US. Does that track with your build date?
    Where do I find the build date?

    Somewhere on the frame, or by serial number and then look up somewhere?

    Also, I have a question for you through PM

  10. #10
    Administrator 20774's Avatar
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    Quote Originally Posted by jimmylee View Post
    Where do I find the build date?

    Somewhere on the frame, or by serial number and then look up somewhere?

    Also, I have a question for you through PM
    There are several ways to determine build date. See the first section of the Resources and Links thread:

    http://www.bmwmoa.org/forum/showthre...rces-and-Links
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  11. #11
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    Jim: here is a list of '78 R100/7 U.S. bikes built during each month

    Month From To
    " 07/77 6145001 6145082
    " 08/77 6145083 6145843
    " 09/77 6145844 6146303
    " 10/77 6146304 6146715
    " 11/77 6146716 6147120
    " 12/77 6147121 6147573
    " 01/78 6147574 6147680
    " 02/78 6147681 6147759
    " 03/78 6147760 6148109
    " 04/78 6148110 6148177
    " 05/78 6148178 6148184
    " 06/78 6148185 6148190
    " 07/78 6148191 6148196

  12. #12
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    Quote Originally Posted by EMSimon View Post
    Jim: here is a list of '78 R100/7 U.S. bikes built during each month

    Month From To
    " 07/77 6145001 6145082
    " 08/77 6145083 6145843
    " 09/77 6145844 6146303
    " 10/77 6146304 6146715
    " 11/77 6146716 6147120
    " 12/77 6147121 6147573
    " 01/78 6147574 6147680
    " 02/78 6147681 6147759
    " 03/78 6147760 6148109
    " 04/78 6148110 6148177
    " 05/78 6148178 6148184
    " 06/78 6148185 6148190
    " 07/78 6148191 6148196
    According to this, my build date was 04/78.

    However, you need to really need to be careful of anything coming from Mike. I am sure he has tampered with the numbers somehow. He seems to have a penchant for messing things up!!

    Sorry, Mike, I couldn't resist!

  13. #13
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    Quote Originally Posted by jimmylee View Post
    According to this, my build date was 04/78.

    I am sure he has tampered with the numbers somehow.
    Darn! You smelled the rat! I moved your VIN into that part, so that you knew yours was an April Fool's joke.....

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