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Thread: Stuttering at low rpm

  1. #1

    Stuttering at low rpm

    Hey all,

    Been working on my R100T 81' and notice some kind of stuttering when at low RPMs or my throttle is at about the 1/8-1/4 turn.
    I messed around with the jets and what not when I had done the carb rebuild and updated the exhausts and it runs great except when im in first and second gear. I can't tell if it's maybe not getting enough fuel or maybe too much? I know the idle jet is only in effect at the 1/8-1/4 turn on the throttle or something like that which makes me believe its the idle jet. It idles great though. I Haven't checked the spark plugs to see how they are running but I don't think the idle jet would affect the coloring too much on those. I could be wrong.
    Anyways, is there a way to figure out if I need to go bigger or smaller with my idle jet or if it might be something else?

    Much appreciated

  2. #2
    Liaison 20774's Avatar
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    How's the valve clearances? The '81 was the first year of the poor metallurgy and the clearances could be tight. Idling issues is a symptom of tight valves. I'd be sure of that before looking at the carbs.
    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!

  3. #3
    Quote Originally Posted by 20774 View Post
    How's the valve clearances? The '81 was the first year of the poor metallurgy and the clearances could be tight. Idling issues is a symptom of tight valves. I'd be sure of that before looking at the carbs.
    I believe good. I did the .15 on the intake and the .20 on the exhaust. They sound nice and tappy. I know the airheads tighten overtime and I just did the valve adjustment so maybe I should crack them open again and check it out?

  4. #4
    Liaison 20774's Avatar
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    If the valve clearances are OK, then probably wise to look at carbs. Do you know what size jets you ended up with? How would they compare to what the Bing specs are? How did the carb synching process go? Maybe something didn't get adjusted right causing an issue in the low RPM area.
    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!

  5. #5
    B Reams brook.reams's Avatar
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    Quote Originally Posted by salt View Post
    Hey all,

    Been working on my R100T 81' and notice some kind of stuttering when at low RPMs or my throttle is at about the 1/8-1/4 turn.
    I messed around with the jets and what not when I had done the carb rebuild and updated the exhausts and it runs great except when im in first and second gear. I can't tell if it's maybe not getting enough fuel or maybe too much? I know the idle jet is only in effect at the 1/8-1/4 turn on the throttle or something like that which makes me believe its the idle jet. It idles great though. I Haven't checked the spark plugs to see how they are running but I don't think the idle jet would affect the coloring too much on those. I could be wrong.
    Anyways, is there a way to figure out if I need to go bigger or smaller with my idle jet or if it might be something else?

    Much appreciated
    Hi Salt,

    I'd start at the beginning and work my way through things in a methodical way.

    I'd first ensure I have the correct carburetor model for your year/bike:
    64/32/324, 64/32/323

    Then, I would put all carburetor jets and settings back to stock:
    Main Jet: 150
    Needle Jet: 241
    Jet Needle: 2.66
    Idle Jet: 50
    Jet Needle Position: #3

    There is no reason to deviate from the stock setting unless you have significantly changed the inlet (removed stock filter and added K&N filters for example), heads changed (i.e., dual plugs), exhaust system changed, or the altitude of the bike is 5,000 Ft or more.
    --> You mention you "updated the exhausts", but you don't tell us what changed. Was this a "performance" exhaust, or replacement of standard exhaust?

    Check your choke cables
    Be sure the choke on both is completely off when the choke handle is off. The affects low end strongly.

    When did you do the carburetor rebuild?
    If it's over 7-8 years ago, your rubber o-rings, and/or diaphragm are getting ready for replacement. You can check this by removing the idle jet and/or the main jet stack. If the rubber is deformed, tears upon removal or seems flacid, it's time for all new o-rings and diaphragm.

    How many miles on the bike?

    When did you last change the jet needle and needle jet?
    These wear from vibration and enlarge the passage causing unstable running.

    Did the bike sit for a year with fuel in the float bowls?
    If so, there can be gunk in the passages from the ethanol/water sitting. A common location for water is in the main jet/needle jet stack. If there is water inside, it will cause problems, particularly at low RPM. Pull out the jet stack. If you see a bubble of liquid, that's water. Clean it out. Blow out the passages with air, or carb cleaner, and put it all back together again.

    The choke housing gasket can fail.
    This will cause rough running. A common cause is the screws holding the cover on can back out and the gasket gets sucked into the choke cavity. Check this.

    How Use Plugs To Check Rich/Lean Condition
    You can check rich/lean using new plugs. You can check this at idle, mid-range and full open if you wish. Start the bike and get it up to operating temperature. Then install new, gapped plugs. Start the bike (without the choke) and let it idle for a couple mins. (you might use a fan in front to keep the engine cool). Shut if off pull the plugs. Both should show faint signs of light brown/tan on the porcelain.

    Reinstall the plugs. Go for a ride and keep the RPM down, 1000 - 2500 or so. Ride it for 5 mins, then pull the clutch and hit the kill switch at the same time. When stopped, pull the plugs again. You want to see a bit more brown/tan if the mixture is correct.

    And, you guessed it, you can do a high speed run (5000+ RPM) and do the same shut down drill to check the main jet mixture.

    I hope this helps.

    Best.
    Brook Reams.
    Brook Reams - Arvada, CO
    Endeavor to Persevere
    Various Two-wheeled Vices, All BMW || Website: Airhead Rebuild Projects
    2004 R1150-RS||2002 F650-GS||1983 R100RS||1977 R100RS||1973 R75/5

  6. #6
    Registered User 6322's Avatar
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    All that being said, carbs and valves being dialed in, you might want to make sure that the timing is spot on. From experience, if the timing is slightly retarded, the result can be a stumble or stutter coming up off of idle. Good therapy working on a bike. Take care and have a good winter. gp
    Gary Phillips - #6322
    Wildland Firefighter, Retired, Riggins, ID
    Heartland Moto Locos BMW Riders
    '77 R100/7 Dirt Hack, '83 R80ST, '85 K100RS w/EML, '00 R1100RS

  7. #7

    Stumble

    Quote Originally Posted by salt View Post
    Hey all,

    Been working on my R100T 81' and notice some kind of stuttering when at low RPMs or my throttle is at about the 1/8-1/4 turn.
    I messed around with the jets and what not when I had done the carb rebuild and updated the exhausts and it runs great except when im in first and second gear. I can't tell if it's maybe not getting enough fuel or maybe too much? I know the idle jet is only in effect at the 1/8-1/4 turn on the throttle or something like that which makes me believe its the idle jet. It idles great though. I Haven't checked the spark plugs to see how they are running but I don't think the idle jet would affect the coloring too much on those. I could be wrong.
    Anyways, is there a way to figure out if I need to go bigger or smaller with my idle jet or if it might be something else?

    Much appreciated
    I can't recall if '81 was electronic ignition or not. If not, check points gap. My R90 idles poorly soemtimes and more often than not, it's the points gap getting tighter. IF it's electronic ignition, then check timing.

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