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Thread: Advance Mechanism

  1. #1
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    Advance Mechanism

    I have read of the sticking advance mechanism causing a hanging idle. How does one determine that the mechanism is free and not sticking? I had a hanging idle on my 78 RS and cleaned the advance mechanism with carb cleaner and then lubed it with some graphite "Lock-Ease". Things seemed OK for a week or so, then the hanging idle came back.

  2. #2
    Administrator 20774's Avatar
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    A high idle can either be a hanging advance or it can be improperly synched carbs. Have you performed any carb work recently?
    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
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    I rebuilt the carbs two years ago under the watchful eye of a certified BMW wrench. I did not run the bike until about a month ago and put in non alcholhol gas. I did the intial shop syncs, verfying that the butterflies were opening at the same instant. Mixture screws were set at 1-1/2 turns out. When I did the start up, it did not want to idle and I ended up moving the idle speed stop screws about 4 turns to get the idle to around 1000rpm. Then I took a test ride and the bike ran fine. Got back home and checked the idle speed on each cylinder by alternately pulling the plug wires off (yes, I grounded them to dump the spark). Got th where I had equal idle around 600-700 rpm on each cylinder and 1000 rpm with both. Took another ride, maybe 4 miles and when I got home, the idle was up around 2000. Loaded the engine by easing the clutch out and the idle settled back to 1000 rpm. So, now l you know.

  4. #4
    Administrator 20774's Avatar
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    Hopefully that first test ride was 15-20 minutes in order to fully warm up the engine before synching the carbs. Have you checked for vacuum leaks at the carb to head sleeves?
    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
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    Yes, it was about a 15 minute ride on county roads. I have not checked for vacuum leaks.

  6. #6
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    If you have an induction timing light, or want to get one, the advance mechanism can be checked. Both Haynes and Clymer manuals describe checking the advance mechanism operation using a timing light. A Haynes or Clymer manual (and preferably both) have detailed step-by-step instructions. Some details are explained more or differently in either manual. Good luck!


    p.s. These airhead engines are not difficult to diagnose and get running well. However, diagnosing any engine problem requires attention to detail and being meticulous. Skipping steps and shortcuts - especially without the guidance of experience - is a recipe for frustration and spending extra time and money. Similarly, trying to work on these motorcycles without a decent manual is a bad idea.
    Stan

    AH# 13238

  7. #7
    Monza Blue 1974 R90/6
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    Sounds like you are on the right track (carb sync and cleaning the advance).

    Whenever I have idle problems, I always check the valves first.

    How did the advance unit look? If it was really dirty or rusty, you might need to take it apart to clean around the pivots. Also, do you know if the springs have been in the last 10,000 miles (or so)? The stretch out over time.

    One more thing -- slack in the throttle cables? Sure one of the isn't pinched between the tank and one of the relays or the wire bundle underneath? A pinched cable, or one with not enough slack can hold a butterfly open, making you think it is a high idle.
    Barron

  8. #8
    beemerfield
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    I just learned this weekend that I had channelled the cables improperly by not running them through the little triangle opening at the steering head gussett. I doubt if this is your issue, but I have learned that there could be a combination of small issues "lining up", so It pays to check and re-check until it is sorted out. I have spent most of this summer trying to sort out some issues and with LOTS of help, finally have it going. Just keep trying until you get it!
    beemerfield

  9. #9
    R100GS, '89 Guenther's Avatar
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    How does one determine that the mechanism is free and not sticking?
    Definitely with a stroboscopic timing light with an inductive sensor. It's a simple procedure and you don't need an "upscale" light for this. Either buy one (they come as low as $40) or, borrow one.

    /Guenther

  10. #10
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    I believe I have found the problem. On the LH carb, the enrichener bell crank was not going all the way down to the run position when the choke lever was pulled up to off. Took a ride today, warmed up the bike, then stopped and looked at the idle--was up around 1500-2000rpm and the LH bellcrank was not fully off. Pushed it down and continued the ride. The idle was still a tad high, like maybe 1200rpm. Got back home and re-set the idle speed balance on the carbs, took another test ride and the idle was at 1000rpm where it should be. The problem with the bellcrank appears to be due to a kink in the inner choke cable right where it passes through the hole in the set screw. I will work on that later.

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