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Thread: R75/5 CV Carbs: Base Idle Stop Settings?

  1. #1
    Registered User
    Join Date
    Apr 2012
    Los Angeles, CA

    R75/5 CV Carbs: Base Idle Stop Settings?

    Hey all,

    So was doing a carb sync tonight using the ol' 'ear method' and a few questions came to mind.

    1. What is a good base setting to start with, for the idle stop / speed screw?

    I want to be sure I start with the same or nearly same adjustments on each side

    2. I noticed that at some point, one carburetor would seem to be "in charge" or "taking over". Only way I can describe this is that at one point i noticed one cylinder would seem to be trumping the other. I would adjust the idle speed / stop screw in and out for a ways and very little change would happen. Also when adjusting the idle mix screw on that same carburetor, it would not seem to change the engine rpm at all. I noticed this happen specifically on both sides, on different occasions. Once I adjusted the "in charge" carb down slightly, the other carb would seem to once again be able to be adjusted.

    What causes this phenomenon and is it normal?

    Thanks all!

  2. #2
    Administrator 20774's Avatar
    Join Date
    Sep 2005
    San Antonio, TX
    jbf -

    As for #1, the start position should be turn the idle stop screw in until it makes contact with the throttle flange/arm and then turn one more revolution CW.

    I'm not sure I have anything for #2, but I think you should be using the shorting method to make the settings...personally, I think it's too difficult to "hear" the changes because they can be so subtle. Snowbum discusses this method. The idea is that with your initial one turn in setting, you temporarily short the spark to ground on one cylinder and observed the running RPM of the running cylinder. Release the short and then short the opposite side...note the RPM. If the RPMs are not the same, then adjust the slower side up so that you get the same RPM. If after all of the synching process (including the idle mixture), the idle RPM is too high, then you can turn each idle stop screw down the exact amount...probably wise to go back and recheck the shorted RPMs to be sure they're still the same.
    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
    Join Date
    Mar 2010
    Lethbridge Alberta
    My method.

    Make sure you have free play on both cables and the idle mechanism is at its closed stop. I use a narrow strip of paper and put it between the stop and the screw. First back out the idle screw so it isn't touching the stop. Slide the strip of paper between the stop and the upper part of the screw. Turn the idle screw in slowly until it has contact with the paper. Gently pull the paper back and forth until you feel some resistance on the paper, remove the paper. Then turn the idle screw one full turn in on both carbs. This sets your base line.

    If you have the Bing book you can look up your carb numbers and find the base setting for you idle mixture screws located at the very front of the carb directly in front just before the flange that goes into the rubber fitting that attaches to the cylinder head. You could start with 1/2 turn out. Turn the screw in carefully until it stops then back out 1/2 turn.

    The above is your carb basic setting. Providing the rest of the engine is in a good state of tune, valves & timing this will get you really close. If you have vacuum gauges or can borrow some then you can get the carbs sync'd. Nothing wrong with the shorting method, but it only works with point ignitions, and it will likely ruin any electronic ignition.

    Don't confuse the mixture screw at the front of the carb with the idle screw ( located on the piece where the cable hooks into ). They do different things. For the most part the idle screw just sets the idle speed and in turn the vacuum you will see on the gauges. You normally have to paly both of these screw off against each other to get a good solid idle. The engine has to also be warmed up to operating temp. So make sure you've taken the bike for a ride and get it well warmed up.

    Each cylinder is going to pull a different vacuum due to condition, compression etc and what you are doing by balancing the carbs is to have both carbs pulling the same amount of vacuum and therefore generating the same amount of power. Within reason that will accommodate the different state of condition of the two cylinders.
    1971 R50/5 SWB with R75/6 drivetrain
    2013 DL650

  4. #4
    #4869 DennisDarrow's Avatar
    Join Date
    Jan 2006
    Sugar Valley, Ga
    This topic has been covered MANY times in the past and a search for carb adjust or sync will show you many threads about this.

    The latest that Kurt, others and myself spent time with that answered your questions with techniques and thoughts are covered in depth...........

    Good luck and God bless...........Dennis
    Last edited by DennisDarrow; 09-04-2014 at 01:08 AM.

  5. #5
    P Monk
    Join Date
    May 2005
    Port Neches, Texas

    shorting out to set idle, but what about at highway speed?

    after finally getting my timing right (another thread), I decided to set the carb needles in the #1 position and see how it ran. It has always been in the #2 position, but the bike was running rich. Obviously I had to re synch the carbs after setting valves anyway, I used the one eye on the right throttle spring and one finger on the left throttle spring and adjusted the cables as close as I could for an initial setting. Took it on the road for a good warm up and the bike ran fine, however it really had an unusual vibration at 65mph.

    Hooked up the carb tune to the carbs and even though they were close at idle, at 3-4k one carb was obviously out pulling the other. Takes a little fiddling but I finally got them to where they both open at the same time and both pull very close to the same vacuum at 3-4k rpm.

    Honestly don't see how it is possible to get the carbs right without vacuum gauges, whether it is mercury sticks, homemade manometer, or twin max etc.
    P. Monk
    74 R90/6 (also know by bride as the Black Hole). 09 R1200 GS. My wife, 1953 model who has survived aplastic anemia and a bone marrow transplant.

  6. #6
    Join Date
    Sep 2014

    unstable idle cv carbs

    Carb inlet hoses a little hardened really can affect vacuum at idle. Plugged idle atomizer air bleeds at carb inlet creates a very rich unstable idle. Check w the filter tubes off,at idle seal the small air inlet w a finger and notice a change in idle. It should be the same on both sides. Check vacuum pistons for movement on throttle blip w a finger tip or a small extension mirrow to protect the eyes. Shorting plugs seldom bothers electronic ignitions, it's the open circuit voltage that does. Hope thi helps

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