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Thread: Trying to get it back on the road....

  1. #1
    On the Road
    Join Date
    Apr 2011

    Trying to get it back on the road....


    The prologue to this post is the "Died on the side of the road!" thread I started last week, while I was... dead on the side of the road and then having to limp across the state, I'm trying to get the puma going again and needed some input.

    Here's the issues-

    Bike is a '77 R100S and will start, but the left cylinder sputters and backfires constantly, right cylinder runs fine.

    After a few minutes the left header starts to glow from the heat- most have diagnosed this as a lean burn condition. But dammit! what is causing it!!

    Prior to it dying (that was caused by a loose battery terminal) it was driving fine, but at higher speeds/rpms I could feel a little hesitation and maybe some small backfires. It felt like I had something in the bowls- The sensation started to get worse after about 30-40 miles. I stopped to check, they were fine, but then it just went to hell... it was constantly backfiring when I started it back up....

    So far I have:

    - Checked the throttle cable and choke cable, both seem fine.
    - Adjusted the pilot screw- seemed fine
    - Cleaned the bowls multiple times, removed the jets and cleaned them out- all seemed fine
    - Checked fuel levels in the bowls, level is fine, gas is available from the lines.
    - Checked and adjusted the valves, (the exhaust on the left side was a bit tight, but not much)
    - Checked the plugs- right is (obviously fine) left looked fine, but was wet with gas.... is that normal for what seems like it should be a lean burn? I'm guessing it's just from the cylinder running so crappy that not all the gas is getting combusted..?
    - Swapped the leads, spark plugs and coils.. no change.
    - Swapped the hoses between the carb and head from side to side to check for airleaks, no change.
    - Looked in the front cover to see if water/sludge etc had gotten in there- (I did drive thru a rainstorm the day before everything went to hell), but it's been a week of dry weather since then and everything looked fine inside there. I wd40'd all the connections. But it seems to me that if there was a problem with the points/condenser/timing then BOTH cylinders would be outta whack. But the right one is running strong...

    Next Steps:

    - I'm going to replace my inline fuel filters today, to see if that does anything.. I doubt it because it's getting fuel to at least the right cylinder in appropriate quantities... and the crossover is fine...

    - I suppose dissemble the carb... I've already looked at the needles etc.. but could a torn diaphragm cause a lean burn? Not quite sure of what to look for when I'm in there... but maybe something will jump out.. OH- I noticed when I took apart the elbows that gas was getting blown up the elbow towards the airbox.. I guessed that's from the backfires blowing fuel the wrong way- but might that point to anything?

    So any help would be greatly, and I mean GREATLY appreciated.

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  2. #2
    Grammarian no, Rider yes ISAMEMON's Avatar
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    Jun 2006
    Kings Valley Oregon
    compression check time. at least that would be my next step. burned intake valve ? just starting to brain storm

  3. #3

    Absolutly a compression check

    You may have a valve issue in there, back firing and fuel getting pushed back into the intake tells me you may have either a stuck intake valve or the seat might have come loose and the valve is bent and open. One thing rain or excessive moisture can do is knock carbon loose or the rapid cooling of the head (when big water hits it) can cause the valve seat to fall out. Hopefully if that's happened it only did a little bad stuff.

  4. #4
    On the Road
    Join Date
    Apr 2011
    Okay-- thanks for the beta so far-

    I guess I'll go get a compression tester during lunch... any tips on using it would be appreciated.

    Regarding checking for a stuck or bent intake valve- does that necessitate removing the head, or is that something that can be done by only removing the head cover? When I adjusted the valves last night- it didn't seem like it was stuck- it would activate back and forth in place... Am I missing something?

    And Brewmeister- I was going to go back through the jets and needles tomorrow morning when I was planning on going through that carb. But regarding the timing/advance/cond. etc- wouldn't the right cylinder running good negate those as a possibility?

  5. #5

    A quick check under the valve covers

    Quick pull your valve covers off again, take your spark plugs out and turn the motor over slowly and look at the tops of the valves and rockers. You should be able to tell by close observations if something is wrong and a valve is hanging up or sticking. Now a bent valve or loose seat may still move in and out so you really have to look close and maybe even take some sort of measurement of the valve stem. That might give you a hint right away if something bad happened but I would also plan on a compression check as well.
    When you do it: fully charged battery, both spark plugs out, properly and safely ground plug wires, throttle will be fully open, do one side at a time and let it turn over about ten revolutions or until the needle on the gauge stops. repeat the test on each side about three times just to be sure about your readings

  6. #6

    Oh yeh

    If you do have an obviously bent valve or damaged seat and its confirmed by the compression test (one side will be 115-150lbs and the other a lot lower like in the 20s) you will be pulling the head anyway so you might as well do both sides and get it over with all at once.

  7. #7
    Liaison 20774's Avatar
    Join Date
    Sep 2005
    San Antonio, TX
    For the compression test, the engine should be warm and the carbs should be removed. At the very least, you will need to prop open the CV carb slides...they won't open just by twisting the throttle.
    Kurt -- Forum Liaison ---> 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!

  8. #8
    TNSTAAFL Troutluck's Avatar
    Join Date
    May 2006
    West Fork, Arkansas
    It's my understanding that the gasket around the choke mechanism can get sucked into the housing, causing an air leak and a lean condition. I would check that and the choke return springs. Is the choke completely off on the left side in standard running condition?
    Jack Pate | '09G650GS | '95 R100RT | Previously: '00R1100R, '87K75T
    NW Arkansawyer |
    BMWMOA #125430 | Fraternal Order of the Mudshark (FOM #0001)

  9. #9
    Registered User
    Join Date
    Jun 2003
    Northern Front Range, CO
    Quote Originally Posted by bobinthemtns View Post
    And Brewmeister- I was going to go back through the jets and needles tomorrow morning when I was planning on going through that carb. But regarding the timing/advance/cond. etc- wouldn't the right cylinder running good negate those as a possibility?
    you are correct. If one side runs fine, and the other does not, then you can pretty much rule out any "combined" parts of the fuel/ignition system- points, timing, condensor, fuel quality, tank sludge, air filter, etc.
    Ride Safe, Ride Lots

  10. #10
    Registered User 88bmwjeff's Avatar
    Join Date
    Aug 2004
    Walnut Creek, CA
    You might want to try switching carbs (e.g. left to right). I know they're really not designed to run that way, but for short run I think it could be helpful. If the problem switches sides, then you know it's the carb. And, if it doesn't, then you'll know it's not the carb.
    Jeff in W.C.
    1988 R100 RT (the other woman)
    "I got my motorcycle jacket but I'm walking all the time." Joe Strummer

  11. #11
    Sounds like my R90. I've done the same as you, and more. Still backfiring. I hope it's not a messed up valve. I am going to swap plugs/coils with my other airhead to rule those out, too.

    Doesn't sound like diaphragms. When those tear, the bike runs fine up to 60 miles an hour. Then, no more power. It doesn't backfire, it just bogs down.

    When you inevitably take your carb(s) off, check your intake spigot. Sometimes those loosen. Check the rubber hose there, too.

  12. #12
    Registered User lmo1131's Avatar
    Join Date
    Jan 2010
    Pismo Beach, CA

    Bob.... you up'n running yet?
    "It is what you discover, after you know it all, that counts." _ John Wooden
    Lew Morris
    1973 R75/5 - original owner
    1963 Dnepr

  13. #13
    On the Road
    Join Date
    Apr 2011
    No! it's still running like hell!

    But, put in a LOT of time working on this weekend-

    - Checked compression-- it's @ 147 psi in both cylinders (yay for that)

    - Bribed a buddy who's an engineer to help yesterday, and we traced every single wire across the whole bike... (sidenote: now I know why I haven't been able to get my brake lights to work!)

    But it's still backfiring!

    But at this point, it's got to be the carb. We've checked everything else. I'm going to swap the carbs per Jeff's suggestion to make sure- but we've checked everything else...

    I partially disassembled the bad carb in question and couldn't find anything that looked out of order... diaphragm looked fine- the one thing I saw was that on the pilot screw and the pilot jet, both o-rings were dry-rotted.. but I would assume that would result in a rich mixture, not lean... but they've been replaced at this point.

    Couple of questions regarding the carbs:

    - I was running it without the airbox elbows and the carb on the bad side was spitting fuel around in a spray, with a fair amount of it splashing back towards the airbox... But the good side carb didn't seem to spitting fuel around like that.... might this cause any issues?

    - When reinstalling the pilot jet, do you screw it all the way in until it seats? Or do you need to back it out a touch like the mixture screw? Ditto for the main jet?

    The Bike only has 33,000 miles on it- so I'm guessing that it probably doesn't need the needle replaced..?

    Any other thoughts that could be causing issues???

  14. #14
    I recently solved a problem similar to what you describe by ditching a Dyna ignition and reverting to points. A backfire into the intake side suggests a badly mis-placed, or should I say, mis-timed spark. The Dyna that I tossed was the kind with 2 pick-ups to time each cylinder seperately.

    A bent valve will kill compression on that cylinder. I enjoyed an episode of that a few years back. A bent valve will not close off the cyinder and the entire 4 stroke process will be defeated. The cylinder will not fire, it will be dead cold. It won't even draw the air/fuel mixture thru the carb to dump unburned fuel in the exhaust.

    A lean mixture will cause the cylinder to super heat, as you've described. Your spark plug would be whitish like new. A lean mixture can be caused by (1) an air leak on the intake side of the head, (2) an obstructed jet or passage in the carburetor, or(3) an inadequate fuel supply to the carburetor, which would make the bike run like you are about to need to switch to reserve.

    Great idea about switching sides with the carbs, by the way. All the jets should be seated in their bores, only idle air mixture screw is backed out. It varies from some carbs to others but generally the initial setting is 3/4 to 1 full turn out from bottom.

  15. #15
    Registered User krpntr's Avatar
    Join Date
    Jan 2010
    Mt Vernon WA
    i had a problem like that once. Took the bike apart trying to find something wrong. Turned out to be coils. I recently read that about 90% of carb problems are electrical.

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