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Thread: R100 compression

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  1. #1
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    R100 compression

    Question: I just did a compression check (with an automotive comp tester) and found 125 on the left and @ 95 on the right on my 1992 R100 - the bike burns no oil (although there is slight oil showing from the right push rod exhaust seal), doesn't smoke, good acceleration for a 20 year-old BMW - It rides well (did Michigan to California trip Summer 2011 with no engine problems - needed a new drive shaft, but that's another story) - Are rings/valve-seat grinding/guides needed? The bike has 64K miles and has been maintained with fluids, adjustments, etc. Not sure where to take this airhead, but it's been a great ride in my 8 years with it -

  2. #2
    Benchwrenching PGlaves's Avatar
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    How did you do the test. With CV carbs vacuum at cranking speed, with the throttle plate held wide open, won't fully withdraw the slides. So the slides block airflow into the cylinder. If air can't get in, it can't be compressed. So, the reading is low.

    To get a reliable compression test, do it with the carbs removed from the head spigot and swung back out of the way. The airtubes need to come off first, of course.
    Paul Glaves - "Big Bend", Texas U.S.A
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    I would first check and then recheck valve clearances - especially exhaust valves. The problem may be deeper than a mis-adjustment, but nothing is less expensive than starting with this. Also, I would ensure that the spark plugs are tightened, but not over-tightened.

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    Bluenoser
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    A lot of time using an auto type compression gauge will result in lower readings, but they would be consistent on both sides.

    You don't mention plug colour. Is there any difference between the two? One side running rich could also mess up your readings.
    1971 R50/5 SWB with R75/6 drivetrain
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    Quote Originally Posted by cycleman2 View Post
    A lot of time using an auto type compression gauge will result in lower readings,
    Why would that be? Compression is compression; an engine is an engine.
    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!

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    Clarification, Please. . .

    Sorry to hi-jack, but I've had the experience of "using an auto (?) compression tester" on both an old LOW mileage, pampered Honda Goldwing, which displayed cylinder-level psi figures that were about HALF what they should have been, but the bike ran (still does) like a scalded-ass ape, a fact which told me "not to worry" about the psi readings. (Yes, I did the test properly, I think. . .) Recently, I just had the SAME experience on a '78 R100RS -- that is, readings of 85 on one side and 90 on the other, but the bike runs great - good starting, strong pull in all gears, and tops out at 115-120mph indicated.

    Paul -- are you saying that ALL airheads need to have the carbs REMOVED to get an accurate reading? Some source I read said that "only the 60's" need to have the carbs REMOVED, whereas for the larger-displacement bikes, all that is required is that the choke be "off" (plate open) and the throttle plates opened at the throttle to do the test. Maybe not enough air coming in with carbs in place to get accurate reading for ALL displacements?

    Also, I understand that a "leak-down test" is MUCH more valuable vs. compression testing, as it really pinpoints the potential trouble-spot. I've never done this, but am tracking down a gauge and compressor right now to see how it comes out.

    As always, THANKS to all the knowledgeable folks posting here. A guy can learn a lot, every day.

    Walking Eagle

  7. #7
    Nickname: Droid
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    Don't buy a Leak-Down tester as the device is very easy to make and a lot less expensive if you do it yourself. Just search the web for home built leak down tester.

    I've been using my home built one for over eight year now.

  8. #8
    Benchwrenching PGlaves's Avatar
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    Quote Originally Posted by Walking Eagle View Post
    Sorry to hi-jack, but I've had the experience of "using an auto (?) compression tester" on both an old LOW mileage, pampered Honda Goldwing, which displayed cylinder-level psi figures that were about HALF what they should have been, but the bike ran (still does) like a scalded-ass ape, a fact which told me "not to worry" about the psi readings. (Yes, I did the test properly, I think. . .) Recently, I just had the SAME experience on a '78 R100RS -- that is, readings of 85 on one side and 90 on the other, but the bike runs great - good starting, strong pull in all gears, and tops out at 115-120mph indicated.

    Paul -- are you saying that ALL airheads need to have the carbs REMOVED to get an accurate reading? Some source I read said that "only the 60's" need to have the carbs REMOVED, whereas for the larger-displacement bikes, all that is required is that the choke be "off" (plate open) and the throttle plates opened at the throttle to do the test. Maybe not enough air coming in with carbs in place to get accurate reading for ALL displacements?
    Carbs should be removed for all of them. But, you can get a fairly decent reading with pure slide type carbs as on some /5 bikes, and I think the early R60/6.

    Any CV type carb - Bing for sure - has a throttle plate that is opened by the cable, and a slide that is pulled upward by vacuum. At cranking speed and wide open throttle plate the condition is very low vacuum and the slide will stay closed or almost closed, blocking inlet air. Since the engine is sucking in very little air the compression reading will be abnormally low.

    I'm not sure about the Dellorto units. They may be strictly throttle plate controlled. If so, there is still some restriction but the reading would be only a little low instead of way low.

    Best practice is to remove and swing away the carbs so that the entire inlet throat is open and unobstructed.

    For those interested, injection throttle bodies on BMWs are purely throttle plate controlled. There are no slides to worry about. I would be OK with a compression check on these just holding the throttle full open. This is certainly true for classic K bikes where the throttle bodies are a major job to remove. If yopu do an Oilhead with TBs in place, and then with them off, the reading is a little bit higher but not much.

    Also, by the way, unless the carbs or throttle bodies are removed, a partially clogged air filter can also cause a low reading.
    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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