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Thread: R75/7 Carb adjustment

  1. #1
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    R75/7 Carb adjustment

    After cleaning both carbs and installation of o rings and diaphragms I have been trying to adjust them.
    The bike starts and runs reasonably well with a bit of a vibration. Once warmed up there is a bit of a backfire from the exhaust on deceleration. The plugs (dual plugged)on the right show it to be running a bit rich on that side.
    I do not have any type of vacuum balancing equipment. How do I get these things adjusted better?

    Info:
    1977 R75/7
    24000 orig miles
    Dual plugged
    Dyna 3 ignition
    recent valve adjustment

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    Have you checked the timing on both cylinders? If I remember, the Dyna 3 uses two different pickups, one for each cylinder. So you can't just rely on timing the left and assuming the right is correct.
    You can get really close without even an el cheapo vacuum gauge.
    First run it one cylinder by grounding out the plugs on one side. Adjust the running carbs idle to something where it will idle by itself. Then do the same thing to the other side. Now when you start it on both cylnders, it will be running really fast. Just back the idle screws out the same amount on each side until you get a proper idle. Then you can fine tune the idle mix screws. Then with the engine off, stand on the right side of the bike, reach down until you can feel the left throttle arm where the cable attaches. Put your head where you can see the right throttle arm. Slowly move the throttle. You should see the right arm move the same time you can feel the left arm move.
    This synchronizes the carbs to their cylinder as well as making sure they open together. Worked for years on my 75/7.

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    Grounding plugs is a little difficult on a dual plugged machine. I believe Snowbum discusses ways of doing that.
    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!

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    I just went through this with my /6 and the problem turned out to be the float level was set too high.

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    Thanks guys
    I will re-read the snowbum thread, I have head so many horror stories about screwing up coils trying to ground out plugs.

    I will also read about -then try checking float levels

    As for the timing, that will have to wait, I did not know about checking both sides???

    I may need to find an airhead mechanic.

    Pete

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    MOA #24991 Pauls1150's Avatar
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    That just sounds insulting...

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    Grounding plugs to the fins is really no big deal. You need to make positive contact when you touch the long-handled screwdriver to the spark extender and the fins. To by-pass the lower plugs, find a spare plug and put it back into the cap and use rope to tie the plug threads to the fins. Then just worry about the top plugs.

    I'm pretty sure your /7 carbs have the vacuum takeoffs. You could use something like a Twinmax (differential pressure sensor) or could even make a less-than-$10 manometer. Information is in the link in my signature line.
    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!

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    Personally I don't like the "grounding" out each side to do the opposite side's carb adjustment. I did it on my earlier /2 and then my /6 by (single plug per cyl) by creating a steel rod that would screw on the top of the plug in the engine, and then the other end was made so that the plug wire could connect to it. I could then use the mid-way part of the rod with a ground wire to connect to the cooling fins thus successfully grounding out that particular side.

    I also don't have a vacuum adjuster and frankly, I don't think one is necessary.

    However, I am "old school" and this it the way I learned to tune a twin cylinder engine with a carb on each cylinder when I worked at a Honda/BMW motorcycle dealership when I was in High School and then in college. Here is how the very experienced, and very good, head mechanic taught me to do it. I still do it this way for my own '78 R100/7:

    1) make sure all mechanical adjustments are done and are exactly the same. For example: valve adjustments, carb needles and jets are installed exactly the same. On my /6, the spark was controlled by the advance mechanism which had two cam lobes to control the points opening which causes the spark. However a BMW old timer told me that sometimes these two cam lobes are not EXACTLY 180 degrees apart. So, he suggested doing a static check and make sure both lobes open the points exactly at the same time. To achieve this, the "high" lobe must be carefully stoned (honed) down to make the points open exactly at the same exact spot on the engine flywheel.

    2) Here is what I do to the carbs: I lower the idle RPM down (as evenly as possible on both carbs) so that the engine will still idle. Then in a quiet area, I lie down and adjust one side idle screw so that I can hear any drop or rise in engine speed as I rotate the Idle jet screw. I turn it first in (clockwise), until I hear a drop in engine speed, noting the screw slot position, then I turn it out the same way and note the screw position when the engine again drops RPM, then I rotate the screw mid-way between the two positions. I then repeat the process to the other carb. Note, when doing this, I do not ground out either side's spark.

    3) I now re-adjust engine idle speed and to make sure both sides are the same, I get exactly behind the bike and Listen and feel the exhaust coming out of the pipes. They both should be hitting with the same pressure and sound. I put both hands behind the mufflers, one on each side, and the "feel" should be exactly the same.

    4) I go back and re-do the carb adjustments as described in my step 2 - trying to "hone" in on the best position of the idle adj. mixture screw.

    5) I now go back and re-do the idle balance adjustment as described in my step 3.

    Now comes the throttle cable adjustment:

    6) as someone described above, you can put your finger on one carb throttle cable connection, and while carefully watching the OTHER side, rotate the throttle until one or the other side lifts up the throttle. BOTH should begin moving at exactly the same time. This is one way to do it.

    However, I prefer what I had learned - using a slightly different method: Have a helper help you. YOU go behind the bike, and have your friend start rotating the throttle up at the handlebar. With your hands behind each muffler, one on each side, you will be able to "feel" one side or the other start to rise (feel will be a "heavier" pressure on one side over the other, AND, the feel will also be evident in the side rising first will get a little hotter at the instant that the first side tries to accelerate just off idle. Then adjust the lagging side's throttle cable adjustment so that it tries to "lift" up the throttle a little earlier. After doing this process through several times (3 or 4) you can get both sides hitting and rising at pretty much exactly the same time.

    There is one caveat that I have found with BMW's though. Since each carb has its own throttle cable, it is possible to have both carbs adjusted exactly correctly and the same at a certain RPM and engine speed, but as you turn on the throttle, the cable may pull one side up at a faster rate than the other. I especially had this problem with my /2. For example, both cylinders/carbs would be nearly perfectly balanced at one engine speed (let's say for example at idle speed) but at 3,000 RPM's, one side was hitting harder than the other - with its visible mis-balance. What I ended up doing in this case, I ended up making both sides hit exactly the same at the engine speed that I ran most at, and lived with a little mis-balance at other engine speeds.

    The reason for this on my /2 was due to the way the throttle up at the handlebar pulled on the cables. It allowed a little bit of "rocking" of the cables and thus rising and lowering of the carb slide would vary. Also, those old Bing carbs were aluminum with aluminum slides! Anyone knows that aluminum doesn't slide well on aluminum - especially when gasoline is introduced into the mix. I ended up machining my own exact duplicate slides out of brass, and having them chrome plated (like Honda had on their KeyHin (sp?) carbs). This drastically improved the smoothness and balance of the engine.

    The process described above takes some time, and one can get better at it with practice, but in my opinion, it is worth it.

    No fancy vacuum gages, and no worry about disconnecting the plugs or grounding them out with the possible damage to the coils. The only hesitation I would have, is you will need to do this when the engine is fully warmed up, and the danger is that you take too long letting the engine idle too long, possibly overheating.

    I know there will be some purists who disagree, but I learned from one of the best "old school" mechanics I have ever seen!

  9. #9
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    Jimmy -

    Duane Ausherman always talkes about using your ears to tell when things are balanced. I've tried it a few times and just don't feel that I'd ever get as close as using a "device" to help me out.

    Two things I'd offer as an improvement on what you suggested. First, during the idle mixture settings, after you've found the center of the two points where the bike stumbles, my suggestion would be to go 1/8 turn richer...CV carbs that's CCW and for slide carbs that's CW. This is a little better for the idling situation.

    The second thing has to do with when the carbs begin to pick up. If I read your post right, you said to work on the lagging carb by making it pick up sooner. I think you should work on the carb that is picking up too soon. The reason is that, assuming you started with some slack in the cables (1-2mm), tightening up the slack on the lagging carb could possibly eat up that slack and then you'd be in a situation where that carb would never find true idle speed because now it is hung up since all the slack is gone. Better to add slack than take it away.

    The ability to do this "by ear" is useful especially if you feel something has changed and you need to adjust on the road. I carry my shorting rods in my tool kit and feel comfortable doing it on the road if I have to. Done right, it's perfectly fine. IIRC, at the 2004 National in Spokane, Matt Parkhouse used this shorting method to balance carbs on his /5 at one of the seminars.
    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!

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    Your points are well taken. It is always safer to go richer, certainly better than the mistake of going leaner.

    About throttle cable adjustment (taking more slack out of slower carb side vs. adding slack to quicker moving carb) I should have first mentioned up in the "mechanical" section to make sure that both cables have proper amount of slack. The way I make sure of this is to move the handlebars side to side and make sure there is sufficient slack in cables at all positions. When I first purchased my bike, one of the cables would "pull" slightly when the handlebars were turned fully to the right. I had to correct that!

    But your point is also well taken - be sure that there is proper slack in both cables, and use the idle speed adjustment screw to adjust idle speed.

    I still am a "by the ear" guy. May not be perfect, but it is so doggone close that I am perfectly satisfied.

    I think the process takes time, and the ability to "hear" I think can be perfected. I know at first, I was not good. My real problem was patience. I thought that this process was somewhat mechanical and it worked just because I went through the motions. I found out that the "by ear" part is critical. along with patience.

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    Also, don't disregard the "pressure" (and heat) coming out of the exhaust pipes. With one hand behind each muffler, you can "feel" when one is hitting harder than the other. this is most useful as you have a friend start to "roll" on the throttle from idle. If cables are unbalanced, you will easily feel one side start to hit harder than the other.

  12. #12
    #4869 DennisDarrow's Avatar
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    As an old "shop" teacher, I would really like to point out the FACT that folks have different abilities, ways of learning, modes of manual dexterity, and most important, ambition to achieve. That said, ALL of the methods pointed out in this thread are really valid and real. Yes, being able to do it by hand/ear is an ability that MOST cannot achieve. Nor, do LOTS of us have the desire to spend lots of money on guages and equipment. Trying the above methods and finding out what works for YOU is the most important.

    Now, what I really wanted to say:

    When rolling on the throttle and feeling that bit of vibration or hearing the miss, adjusting that cable a bit stronger than the other has always worked for me. JUST A BIT......Perhaps 1/8 of a turn on the cable adjuster.
    When decelerating down a hill or down shifting for a curve and one gets a bit of popping going on, I get out the trusty roadside screwdriver and go in perhaps 1/8 of a turn on the air/mix screw of the carb that is making the noise.

    MOST important that no one has brought out in this particular thread is the REAL NEED to take a ride for perhaps 10 miles to really warm up that engine before making carb adjustments.

    Next thing is that engine gets really hot when you have it sitting there doing these adjustments.....USE the biggest floor fan in front of the bike that you can get your hands on. DO NOT spend more than perhaps 10 minutes, even with the fan, in doing your twiddling..........
    Anyway, just thoughts.........God bless.......Dennis

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    Quote Originally Posted by DennisDarrow View Post
    "As an old "shop" teacher, I would really like to point out the FACT that folks have different abilities, ways of learning, modes of manual dexterity, and most important, ambition to achieve. That said, ALL of the methods pointed out in this thread are really valid and real. Yes, being able to do it by hand/ear is an ability that MOST cannot achieve. Nor, do LOTS of us have the desire to spend lots of money on guages and equipment. Trying the above methods and finding out what works for YOU is the most important."

    Response: I agree that people are definitely different, and there may be someone out there who couldn't do it. However, it is pure nonsense to say only a very few can do it. As a shop teacher you ought to know that once you introduce that sort of idea, that is a SURE way to make a person think he/she can't do something. This carb adj. that I described above is not rocket science. A little practice can go a long way to achieving great and satisfactory results. Ater one tries it, and then again a week or so later, he/she will quickly gain confidence AND satisfaction in his/her own ability.

    "Now, what I really wanted to say:

    When rolling on the throttle and feeling that bit of vibration or hearing the miss, adjusting that cable a bit stronger than the other has always worked for me. JUST A BIT......Perhaps 1/8 of a turn on the cable adjuster.
    When decelerating down a hill or down shifting for a curve and one gets a bit of popping going on, I get out the trusty roadside screwdriver and go in perhaps 1/8 of a turn on the air/mix screw of the carb that is making the noise."

    "MOST important that no one has brought out in this particular thread is the REAL NEED to take a ride for perhaps 10 miles to really warm up that engine before making carb adjustments."

    Response: I guess you missed the part where I clearly said that the engine needed to be warmed up and in running mode.


    "Next thing is that engine gets really hot when you have it sitting there doing these adjustments.....USE the biggest floor fan in front of the bike that you can get your hands on. DO NOT spend more than perhaps 10 minutes, even with the fan, in doing your twiddling..........
    Anyway, just thoughts.........God bless.......Dennis"
    Response: I really disagree. Any ambient noise will compromise your hearing the engine (each cylinder) firing and and the sound of smoothness. A large fan producing a noise like the rushing of a jet engine will definitely harm your chances of accomplishing the very thing that you are trying to achieve: a smooth, balanced running engine.

    Better advice: Try doing the above method as I described in my previous post, and learn it so you don't have to waste time trying to "think" about it. Do the best you can the first time. Learn to do it and doing it with some "haste." After a few minutes, take the bike back out for a ride to let the air blow it off and help cool it down, then try again. Wait a week, and listen and feel how it performs as you ride it. Then, try the carb adjustment procedure again. You will be pleased with yourself as you gain confidence.

    There is, in my opinion, NO need to buy expensive equipment. This method I encourage you to do was the ONLY method available 30 to 40 years ago. It does work, and even an average person can do it!

    With the advent of 4 cylinder engines with a carb per cylinder, all of this became a LOT more complicated! I would advise against trying this sort of approach to any 4-cylinder engines!

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    Therein might lie a problem. It really is a good thing to use a fan to keep air moving across the engine while doing the carb synch. It should be hot to start with after a ride as suggested and now you're going to idle for maybe 5 minutes. IMO, that's too much time without adequate cooling. Probably would be OK, but the noob trying this the first time will try it and try it, taking 10 minutes or more. Now you're starting to get into a real danger zone.

    Not to mention trying to keep somewhat of an even temperature while doing all the tweaking. Not having a fan to offset the temps encountered during idling could affect the end results.

    If you experienced and can do all that in under 4-5 minutes, go for it. If not, use the fan to give yourself some time to go through the steps. I've been doing this for 30+ years on the bike, but you know, everytime I have the bike idling in the driveway while I make adjustments, it kind of freaks me out a little! I move quickly to complete everything, but the idling right there inches from my face and fingers can be unnerving.

    That's just me!
    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!

  15. #15
    #4869 DennisDarrow's Avatar
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    Quote Originally Posted by jimmylee View Post
    Response: I really disagree. Any ambient noise will compromise your hearing the engine (each cylinder) firing and and the sound of smoothness. A large fan producing a noise like the rushing of a jet engine will definitely harm your chances of accomplishing the very thing that you are trying to achieve: a smooth, balanced running engine.

    Better advice: Try doing the above method as I described in my previous post, and learn it so you don't have to waste time trying to "think" about it. Do the best you can the first time. Learn to do it and doing it with some "haste." After a few minutes, take the bike back out for a ride to let the air blow it off and help cool it down, then try again. Wait a week, and listen and feel how it performs as you ride it. Then, try the carb adjustment procedure again. You will be pleased with yourself as you gain confidence.

    There is, in my opinion, NO need to buy expensive equipment. This method I encourage you to do was the ONLY method available 30 to 40 years ago. It does work, and even an average person can do it!

    With the advent of 4 cylinder engines with a carb per cylinder, all of this became a LOT more complicated! I would advise against trying this sort of approach to any 4-cylinder engines!
    I really DO NOT understand the need to diminish other folks' advice, knowledge and recommendations; but each to their own needs.

    Yes, it is understood quite well about the learning capabilities of various types of individuals and the various styles of completing various
    "hands on" tasks. There are MANY ways to skin that particular cat. Next year will be the 40th year of the /6 750 and above engines that are equiped with carbs that have the capability of using vacuum guage equipment to adjust and synchronize. The method that YOU insist that all MUST use is a valid and real method that has been used on small engines of all types, cylinder configuration/number, and level of displacement. It is valid and real. All that I am trying to point out is that there are MANY ways to accomplish a task and NONE is wrong........

    No one is suggesting to bring in some kind of device to move air over the engine that is in a decible range that will drown out what the engine is doing. Here on our farm we have several "fans" that are in the 3' to 4' range that move LOTS of air. There is no way that they "drown" out the subtle sounds that are required to adjust the idle and sync by feel, touch, sight, and sound. WHAT is important is the need to keep that engine cool while one "learns" the various methods required to tune carbs or engines. Again, there is NO right nor wrong about this "tuning" business. It's just what works best for YOU, the situation, and machine...........God bless..........Dennis

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