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Thread: 83 R100 Compression Test

  1. #1
    Arctic Art
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    83 R100 Compression Test

    What is the best way? Seems like I have to warm up engine,take off front cover,disconnect bean can,pull or prop open carbs pull both plugs and put tester on cyl. That unplugging of the bean is a PIA. Is this it or is there a better/easier way. Thanks

  2. #2
    Administrator 20774's Avatar
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    I don't think you need to pull the bean can and the front cover. You do need to ensure that the engine won't fire. You can do that by dropping the float bowls...no gas, no fire. Also, put the plugs back into the high tension caps and secure the threads of each plug to the engine case...this allows the spark generated to find a path to ground, protecting the coils. Pulling the carbs is the best, but if you can prop the slide open on the carb, twist the throttle to open the butterfly, that will be OK for government work.
    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
    Day Dreaming ... happy wanderer's Avatar
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    I may be going to check out an 82 RS next week. I was wondering about the compression test procedure. My RS has Bing 42mm CV carbs with the rubber diaphragms on top. Do these carbs have slides in them or just a long needle that comes down into the jet?

    I'd like to check compression with disturbing the carbs if possible. I read somewhere to just open the throttle wide open, ground the spark plug to the head and crank away. Cold or warm should not matter.

    Not so?
    MJM - BeeCeeBeemers Motorcycle Club Vancouver B.C.
    '81 R80G/S, '82 R100RS, '00 R1100RT

  4. #4
    Administrator 20774's Avatar
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    Quote Originally Posted by Happy Wanderer View Post
    I may be going to check out an 82 RS next week. I was wondering about the compression test procedure. My RS has Bing 42mm CV carbs with the rubber diaphragms on top. Do these carbs have slides in them or just a long needle that comes down into the jet?

    I'd like to check compression with disturbing the carbs if possible. I read somewhere to just open the throttle wide open, ground the spark plug to the head and crank away. Cold or warm should not matter.

    Not so?
    CV carbs that you have have slides with a needle. But twisting the throttle only operates the downstream butterfly...it doesn't directly move the slide. So, you will need to remove the carbs, or as a bare minimum, remove the air tubes and prop open the slide and twist the throttle to open the butterfly. Actually, it would be easier to just loosen the clamps on the head-side of the carb and rotate the whole thing out of the way...you have to remove the air tubes no matter what. This won't disturb your carb synch.

    Warm engine would be better and give you a truer reading. Cold gives you a general idea and maybe what you want to do if you're looking at a potential bike.
    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!

  5. #5
    Day Dreaming ... happy wanderer's Avatar
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    Thanks Kurt.
    It figures that cheatin and the easy way is usually not the right way.
    MJM - BeeCeeBeemers Motorcycle Club Vancouver B.C.
    '81 R80G/S, '82 R100RS, '00 R1100RT

  6. #6
    Registered User lkchris's Avatar
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    It's super easy to simply pull the carbs off the manifolds and get them out of the way somehow. Without fuel dripping. No need to do anything to the cables. Without the carb attached to the manifold, things are by definition wide open throttle.

    Then just push the removed plugs back into the the wire connectors and lay them on top of the cylinders so they ground. The entire metal portion of the plug (the hex, the threads) is ground and it can easily contact a cylinder fin or two or the block. Just keep gasoline away from the spark.

    I recognize this is a little more difficult with the pre-1980 carb tubes, but still not that bad.

    You do know that the 1980-on carb tubes are removed by first sliding the rubber ring down on to the the plastic elbow. Then the assembly simply falls off.
    Kent Christensen
    21482
    '12 R1200RT, '02 R1100S, '84 R80G/S

  7. #7
    papafoxtrot
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    Compression Interpretation

    I'm just starting into rebuilding an R90S engine that has 40K on it. I pulled a compression test and both cylinders came in at 115PSI (8Kg/cm2). I've got the factory repair manual which seems to show a chart with lines for each cylinder with readings running anywhere from 4Kg/cm to 11Kg/cm. can someone help me interpret what I'm looking at or provide a PSI figure for an in-spec engine?

    Thanks,
    Paul_F

  8. #8
    Administrator 20774's Avatar
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    Paul -

    Not sure what lines you're talking about. A rule of thumb that I use is to determine the nominal compression by using the compression ratio of the engine. For the R90S, it's 9.5 to 1. Add 1 to that (to account for the starting atmosphere) to get 10.5 and multiply that by 14.7. You get around 155 psi. That's what you should be seeing. Under what conditions did you measure the 115?
    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
    papafoxtrot
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    Kurt,

    The engine is out of the bike, so the measurement was made cold on a 50 degree day. But the carbs are off as suggested. Sounds like she's a bit low but both sides are equal. I'll probably go to the first overbore that's what I was trying to decide. I have to strip the engine down to bare bones as the PO decided to paint everything orange.... Block, trans, fork sliders & rear drive housing!!!!!!!!

  10. #10
    Administrator 20774's Avatar
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    Paul -

    You should probably do a leak-down test to confirm where the trouble is. Then, once you strip it, you can do some final measurements to see exactly where it stands relative to the specs, especially the cylinder diameter and ovality. You can also check your ring end gap. Once you do that, you'll know what needs to be done.
    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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    I check the compression on a certain entrance ramp, coming around the curve in third, hold it wide open until it zips past 6K, click into gear four, whack it wide open again, hold it a while until I feel a little guilty, and then back off. If it takes a short while to git back to 65, the compression is OK. What can I say, it was minus 2 this morning and I'm dreaming.

  12. #12
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    Haynes says 145 (psi) is good, 123-145 normal. On some engines with aggressive cam grinds the late closing of the intake valves allows a bit of backwash on the intake/compression change resulting in lower cranking compression but at operating rpm cylinder filling is better. I'd go with the spec book or even better, the leakdown test.

    Re: grounding the spark plugs- Years ago I soldered a couple of old (but usable) spark plugs to alligator clips and use them for spark testing. I clip them to a good ground on the engine (usually a cooling fin). That way I don't have to worry about the plug falling away while the engine is cranking.
    Last edited by 40401; 01-02-2013 at 03:06 PM.

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