One rich carb on my airhead

Discussion in 'Airheads' started by Craig in Alabama, Nov 18, 2010.

  1. Craig in Alabama

    Craig in Alabama Adventurer

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    Hi Folks,
    I have an '84 R100RS that I rescued about a year ago. I've been remedying the neglect, abuse and lack of maintenance ever since. I'm really close to getting everything sorted out, but a new problem has cropped up. I promise I've searched diligently, but I didn't find an answer.

    The bike has been running fine, although not as smooth as I think it should. When I checked the valve clearance recently (it had been 2,000 miles) I was surprised to see the right side plug was very black and sooty. The valves were right in spec so i didn't mess with them. This plug was previously just fine, and I don't know when it began to run rich. Gas mileage has always been poor (about 35 mpg).

    I have replaced all the o-rings in the carbs and cleaned them up. Carb diaphragms are good. Plugs and coils are good. It's just one side so I think it's the carb and not timing.

    The most interesting thing is that in order to get the right side to run, I have to turn the mixture screw all the way in or it bogs down. That's full lean, right?

    Could it be that my float level is too high? I checked it and it looked to me that if anything it would be too low, but I could be wrong. I thought perhaps I had assembled the choke incorrectly, but the little kidney shaped hole is at the top where it should be. Besides that, I did the carbs about 1,000 miles ago. Is there anything else in the choke assembly that I should look at? It really seems like a choke problem to me.

    So any idea what could affect it so bad at idle? I've been messing around with it for about a week and not gotten anywhere. I thought it best to ask the collective masses.
    Any ideas?
    Thanks!
    Craig
    #1
  2. bikecat

    bikecat Long timer

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    Timing and valves clearances must be alright before diving into carbs .... that's what I learnt.

    Cheers
    #2
  3. Gravytrain

    Gravytrain house husband

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    I would think mixture screw all the way in = full rich :huh
    #3
  4. durtwurm

    durtwurm Talented Amateur

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    I would check the tension adjustment where the choke cable attaches to the carb. Each side must be the same. Also the nut that maintains tension at carb can loosen and cause all sorts of problems.
    #4
  5. rufusswan

    rufusswan Been here awhile

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    Sometimes plugs, wires, and coils, etc. fail dude. I wouldn't hesitate to either swap plugs or replace the fouled one, or swap plug leads. Then you can start your diagnosis of the real issue by starting with the symptom.
    #5
  6. Craig in Alabama

    Craig in Alabama Adventurer

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    Hi Folks,
    Thanks for the replies. The valves and timing are fine. I've checked the coils and they measure fine. The spark plug wires are new NGKs and I put in new plugs when I checked the valve adjustment a couple weeks ago. They were also black and fuzzy after a ~50 mile ride. Bike seemed to run pretty good, though.

    I swapped the coils over as well, but that didn't change anything.

    All the cables are new and I checked to be sure the choke wasn't hanging up. I still think it must be choke related though.

    As for the mixture screw, I thought screwed all the way in was full lean as it's not allowing any fuel to pass. Is that not correct? This is why I think it must be either the choke or the float bowl level.

    Thanks,
    Craig
    #6
  7. fishkens

    fishkens Long timer

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    You are correct:

    "Turning in a clockwise direction results in a leaner mixture and turning in an anti-clockwise direction in a richer mixture. (Note: this is just the opposite of the direction required in Bing slide carb mixture settings)."


    page 8 Bing carb manual
    #7
  8. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    The idle and low-speed mixture is set by the idle fuel jet and idle air bleed supplying fuel to the idle and bypass openings in the throttle bore. THe idle mixture screw provides fine-tuning of the mixture and should provide a good mixture in the 1 to 1-1/2 turns-out specified. If it has to be turned in all the way something is amiss.

    Check the o-ring on idle jet-- if it is deteriorated, more fuel will flow through/around the jet. Check if the jet opening has been enlarged. Look at the throttle plate-- is it aligned with the throttle bore and installed correctly (ie, not upside-down)? Notice the idle and bypass drillings in the throttle bore-- there should be an idle opening (the mixture screw will be in that opening) and the throttle plate should be centered on it at idle. Above it there is another opening, the bypass drilling, which provides fuel for the low-speed (part throttle) mixture; the throttle plate should not pass by it at idle.

    Check the float adjustment and double-check it against the fuel level in the float bowl. The fuel level should be the same in each float bowl (I disremember what it is in the later R100); if not, suspect that a float has gotten "waterlogged" with gasoline, is too heavy, and needs to be replaced. Of course, double-check that the enricheners are correctly assembled and adjusted.
    #8
  9. Craig in Alabama

    Craig in Alabama Adventurer

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    Thanks, fishkens. I should have specified that it's a CV carb.

    And thanks Bill. I will check the o-ring on the idle jet and the jet itself tonight. I did not disturb the throttle plate and it doesn't look like anyone else has either. It all seems to line up correctly, but I'll check it's position with the idle and bypass openings as well.

    The floats "float" and appear to be new versions. In fact I found a pair of old bing floats bobbing around in the fuel tank, but that's another story...

    If all this checks out I may block off the choke area as a quick test. It still seems like the culprit even though it appears to be fine.

    If these carbs were clear it would be a lot easier to diagnose this stuff!

    Thanks for all the help! I'll report back.
    Craig
    #9
  10. Airhead Wrangler

    Airhead Wrangler Long timer

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    You mention neglect. I also spent a good long time reviving an airhead that sat for 20+ years and found that the needles and seats in the carbs had corroded considerably and were nowhere near there former size and shape. My bike ran, but only got about 25 mpg. I replaced the needles and seats and boom. Everything was right again. Worth a look if your bike sat for a long time.
    #10
  11. fishkens

    fishkens Long timer

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    :huh

    PO: "Hmmm, what do I do with these old carb floats? I know, I'll toss them in the gas tank. They'll happily live out their days floating in gas."

    :rofl:rofl:rofl
    #11
  12. mykill

    mykill odd

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    I'm on the idle jet oring path as well. It also may not hurt to remove the mixture screw to check for deformity or corrosion at the tip. Blow some carb cleaner through the passage while you are at it.
    #12
  13. jwegman

    jwegman Been here awhile

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    I may have missed mention of it, but has the main needle setting been checked to verify that they are in the proper clip position and the same on both carbs?
    #13
  14. Craig in Alabama

    Craig in Alabama Adventurer

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    I didn't mention it, but the needles are at the stock setting. They weren't when I checked them though! The two mixture screws look identical. I swapped them and the problem stayed in the right carb.

    This was one of those cases where I'd find something mis-assembled and think "That's it!" and put everything back together. Like the diaphragms (which were new) had been put in with complete disregard for the correct origin. PO: What are these tabs for? Who cares!
    It's been an adventure...

    Anyway, I didn't replace the needles and seats and other hard parts when I first went through the carbs, but I see now that that was clearly a mistake. Time to place another order.
    I just didn't think this over-rich mixture at idle was needle, jet or seat related. But new stuff will take the uncertainty out of the equation and it can't hurt.

    I'll probably still mess about with it this weekend, because I'm stubborn and have music and beer in the garage.

    Cheers!
    Craig
    #14
  15. durtwurm

    durtwurm Talented Amateur

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    All the above! I recently had similar problems. It turned out that I had a bad China condenser. And defective China points, and no the rubbing block was not too high. Replaced condenser and the points. Set timing. Set valves. Cleaned the carbs and replaced all o rings. My floats and settings were fine. Also checked the wire between the coils and cleaned. (Did you do this?) Cleaned the fuel filters. Replaced plugs. Runs well, very well now. Don't forget your many settings for choke and fuel as mentioned. Also the chokes cables where they attach and the gaskets for same.
    #15
  16. Craig in Alabama

    Craig in Alabama Adventurer

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    Hi Folks,
    Well I did some experimenting today. I rechecked the choke assembly and I can't see anything wrong. The float level is about the same in both bowls (15mm or so). I compared the idle jets and they are the same (4.5) and I swapped them, hoping the problem would switch to the left cylinder. But it did not! Dang!
    It's still idles highest with the mixture screw all the way in (right cylinder only).
    So I went for a ride anyway. After about 40 miles of nice moderate riding, I checked the right plug. It looked pretty good. And the bike was running fine at cruising speed. At low rpm though, it still isn't right and when I got back to the house the plug was getting black and fuzzy again from just a little bit of riding in traffic. Gas mileage was an all-time low at about 32mpg.
    I would suspect that there is a problem in the carb body where the jet screws in except that it was fine until recently. This is perplexing. I guess I just need to order new jets, needles, etc. and start from scratch.
    Oh, when I put new plug wires on, I cleaned up all the coil connectors and swapped the coils side to side. Problem stayed with the right cylinder. I need a spare R bike to swap parts on!
    Thanks for all the help.
    Cheers!
    Craig
    #16
  17. Overdog

    Overdog 'Burghherfer.....

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    In all of the disassembling/assembling of the carbs, have you checked to see that you haven't bent the needle?
    #17
  18. fishkens

    fishkens Long timer

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    From Bing:
    "If a too rich jet is selected, OR if the carb passages are partially blocked, these conditions will be indicated by the requirement of excessive screw setting in the CW direction."


    15mm of fuel in the bowl?

    Snowbum suggests 24 mm. Although that doesn't seem like it that would require turning the idle mix screw all the way in.

    From Snowbum:
    http://bmwmotorcycletech.info/bingcv-2.htm

    "Actual bowl fuel level: You can turn the gas off again, replace the bowl, turn gas on, allow bowl to fill, turn gas off, remove bowl quickly. Measure the fuel depth from the center bottom to the fuel top, using something small...such as the tip end of a vernier caliper, or a thin, narrow, machinists steel rule.
    Do your best to have the bowl sitting flat and level on its base, and estimate the best you can with #1 eyeball, as there are meniscus errors, the exact height of the fuel from the bottom.
    For the 32 mm carburetors, use 24 mm fuel height.
    For the 40 mm carburetors, use 28 mm fuel height.
    "

    A clogged passageway?
    #18
  19. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    Good. You've eliminated a lot of what the problem is not.

    From your check of things and your test ride, it is clear that the problem is in the idle/low speed circuit. You are getting too much fuel metered to the air at that speed, which suggests too large an idle jet, a bad o-ring (which allows too much fuel to leak around the jet or a blocked idle air bleed (jet/drilling or passageway). An air bleed that is blocked will not allow enough air to "bleed" into the idle/low speed circuit. The effect is that the mixture becomes richer and richer at part-throttle as the speed increases up to the transition to the main (high-speed) circuit.

    I used to have some good books on carbs from Solex and Weber which explained "how they does it" pretty well. I've not been able to find (yet) a great online explanation. Yet.

    Here is a pretty good explanation, though:

    http://www.chevyhiperformance.com/techarticles/83118_carburetor_basics/index.html

    And Wikipedia has a good general overview:

    http://en.wikipedia.org/wiki/Carburetor

    I'll keep looking.

    Meanwhile, remove the idle jet and blow out the idle air passage with compressed air.... :)
    #19
  20. Craig in Alabama

    Craig in Alabama Adventurer

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    More great info!
    I don't think the needle is bent. But it's about to be replaced anyway.

    "If a too rich jet is selected, OR if the carb passages are partially blocked, these conditions will be indicated by the requirement of excessive screw setting in the CW direction."
    This sounds exactly like the condition I have. Where exactly do I look to clean out the idle air passageway?

    I reckon I'll go take that carb off again and see if I can find any blocked passages. I thought I cleaned them up pretty good, but there are a lot of nooks and crannys in there.

    Thanks for the links, Bill! Good reading.

    Back to the garage! If I can't find anything today, I'm ordering new jets and needles and float nedles tomorrow.
    Cheers!
    Craig
    #20