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Thread: poor compression '76 R90/6

  1. #1
    Registered User ezwicky's Avatar
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    poor compression '76 R90/6

    howdy all,

    i sent this same message to the airheads email list, but i wanted to post it also here.

    i recently bought a '76 R90/6 and among other issues, it has poor compression, at 100psi in the right cylinder and 75psi in the left. i did multiple readings and they were all consistent.

    i am comfortable with removing the heads and cylinders and pistons (i
    have the clymer's and haynes manuals and am a reasonably competent
    instruction-follower) but i do not have the precision tools nor the expertise to
    evaluate the condition of the top end components.

    is there any way to determine whether it is the heads or the piston rings that are
    the cause of the low compression? is there something i could do at the home
    garage and be confident?

    or would it be more adviseable to send the heads and cylinders and pistons off
    to a competent BMW mechanic and leave it to them to determine?

    i am in the south-western suburbs of philadelphia, closer to wilmington
    DE than PHL. so i am about an hour away from rubber chicken garage,
    which i have heard a lot of good things about.

    i also head back home to richmond a couple times a year and pass right
    by bob's bmw between baltimore and DC. they seem also to have a good reputation.

    thanks in advance for your time and advice,
    BMWMOA #182796

    '76 R90/6

  2. #2
    Administrator 20774's Avatar
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    Ensure that your vavles are properly set...step zero.

    First, tell us how you did the compression check. It should be done after a warm-up ride, maybe 15 miles long. Ideally, the carbs should be pulled from the intakes stubs...at a minimum, the slides have to be physically held open since you have the CV carbs...just holding the throttle wide open doesn't do it.

    As far as determining where the problem is, you need to do a leakdown test. You need a special piece of equipment...I bought a test at Harbor Freight for $50. There are places on-line talk about making one, but after all that, you could just buy one. Or find an Airhead friend in the area.

    Same thing, you need to do this with a warm engine. The difficulty is that the cylinder under test must be at TDC on the compression stroke and you have to find a way to hold the engine there...the in-rushing air will just force the piston to BDC. But, the amount of air leaking will tell the story, along with where the air is escaping. Through the carb - intake valves; through the muffler - exhaust valves; through the oil file hole - rings.

    Cutter or Bob's would be great choices. Also Hermy's in Port Clinton...not sure how far away that is.
    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 ezwicky's Avatar
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    Quote Originally Posted by 20774 View Post
    Ensure that your vavles are properly set...step zero.
    i did adjust the valves. they were not really off, but i did set them with the gaps according to the specs in clymer's.

    Quote Originally Posted by 20774 View Post
    First, tell us how you did the compression check. It should be done after a warm-up ride, maybe 15 miles long. Ideally, the carbs should be pulled from the intakes stubs...at a minimum, the slides have to be physically held open since you have the CV carbs...just holding the throttle wide open doesn't do it.
    ahh.... well, i did not do all that. i just screwed he compression tester into the sparkplug hole and cranked the starter over until the dial did not increment anymore. i had both plugs out but nothing more. and the engine was cold because i couldn't even get it to start (new plug wires took care of that yesterday. i will try again on a warm motor).

    Quote Originally Posted by 20774 View Post
    As far as determining where the problem is, you need to do a leakdown test.
    a guy on the airhead list sent me a message that i should do a leakdown test. i googled it and i will look for a tester.

    Quote Originally Posted by 20774 View Post
    Cutter or Bob's would be great choices. Also Hermy's in Port Clinton...not sure how far away that is.
    never heard of port clinton before, but google maps says it's an hour and a half, thanks for the tip. but cutter and bob's for various reasons would be more convenient.

    thanks for your reply!

    -eric
    BMWMOA #182796

    '76 R90/6

  4. #4
    Registered User ezwicky's Avatar
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    Quote Originally Posted by 20774 View Post
    As far as determining where the problem is, you need to do a leakdown test. the cylinder under test must be at TDC on the compression stroke and you have to find a way to hold the engine there...the in-rushing air will just force the piston to BDC.
    would it work to leave the other sparkplug in, and also put the bike in gear?
    BMWMOA #182796

    '76 R90/6

  5. #5
    Administrator 20774's Avatar
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    I don't think the other cylinder will help...it's at TDC on the exhaust stroke, so there's nothing going on there. The piston can either go the proper direction or it could go backwards...path of least resistance. I suppose the bike in gear might work, but it's important to keep the piston very near TDC...much movement either side and the valve begins to open and you've lost integrity.

    I devised a way to use a wrench to hold the alternator bolt in place and taping the wrench to the frame. There might be easier ways.
    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!

  6. #6
    Registered User ezwicky's Avatar
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    Quote Originally Posted by 20774 View Post
    I devised a way to use a wrench to hold the alternator bolt in place and taping the wrench to the frame.
    that was going to be my next question... whether that could be damaging in any way. i'll assume "no"

    thanks!

    -eric
    BMWMOA #182796

    '76 R90/6

  7. #7
    Benchwrenching PGlaves's Avatar
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    I would think in gear, holding rear brake would be sufficient.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://www.bigbend.net/users/glaves

  8. #8
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    walls

    If you think oil has glazed your cylinder walls, you need to break the glaze on the walls and replace the rings.

    Might want to check your compression gauge.

    Marvel mystery oil will help to raise the compression.

    Jon

  9. #9
    Registered User ezwicky's Avatar
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    update : good news

    ok this time i warmed up the engine and re-did the test while holding the throttle wide open. now i have 135psi on both cylinders.

    i sure am glad i posted here. i thought all i had to do for a compression test was crank the starter a few times. live and learn.

    thanks again everybody.

    -eric
    BMWMOA #182796

    '76 R90/6

  10. #10
    Benchwrenching PGlaves's Avatar
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    Quote Originally Posted by ezwicky View Post
    ok this time i warmed up the engine and re-did the test while holding the throttle wide open. now i have 135psi on both cylinders.

    i sure am glad i posted here. i thought all i had to do for a compression test was crank the starter a few times. live and learn.

    thanks again everybody.

    -eric
    If air can't get in, it can't get compressed.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://www.bigbend.net/users/glaves

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