Bing CV Stumble Off Idle

I have a ’77 R100RS with 40 mm Bing CVs (94/40/105-106 to be exact).  After sitting at idle for more than a couple seconds, the engine would stumble and occasionally die when the throttle was opened, for example, to take off from a stop light.  The idle was set at about 950.  My temporary workaround was to very lightly twist the throttle open every second or so, while waiting for the light to turn, since this prevented the stumble.  The bike was completely tuned-up, running very smooth, except for this one issue.  I had recently rebuilt both carbs, replacing floats, float needles, diaphragms, main jet and needle, gaskets and o-rings.

 

I sought help from the members of the Airheads Beemer Club, and received a bunch of great advice.  If you’re not already a member, you should definitely join.  Here’s what I did to solve the problem:
 
I checked for air leaks first.  I checked the vacuum port screws, intake tubes, carburetor top, bottom, and choke housing.  The right side vacuum port screw moved a tad bit as I tightened it, but nothing was leaking.  My fiddling (putting fingers tightly around crevices and listening for changes in engine tone, as well as spaying WD40 on crevices) verified that there were no leaks.
 
Initially I thought a richer fuel mixture might solve the problem, but it didn’t.  Both sides had the mixture screw set in the middle, where it runs best, between the two points where the engine starts to slow.  To test a richer mixture, I first turned both mixture screws counter-clockwise by 1/8th turn.  The stumble was still there.  I then tried 1/4th turn.  The idle dropped after this adjustment, so I readjusted the idle, but the stumble was still there, although reduced, sort of.
 
Next, following Tom Cutter’s suggestion with the bike on the center stand, motor recently turned off, petcocks having been open for a while, I closed the petcocks, and dropped the float bowls on both sides.  I have 40 mm carbs, and the fuel depth as measured from the base of the circular well in the bottom center of the bowl was 27 mm, for both sides.  Great, since this is supposed to be between 25-27 mm for 40 mm, and 21-24 mm for 32 mm carbs.
 
Next, I followed Snowbum’s advice.  I lightly pushed up on the floats and opened the petcock.  Then I slowly let the float drop until fuel began to dribble out.  I did this a couple times, on both sides, and the floats were always parallel to the bottom of the carburetor body when the fuel began to flow.
 
Next, I read page 14 of the Bing manual.  The symptoms described there are different than mine.  Page 14 describes things like hard starting, stumbling until above half throttle, coughing through the carburettors, and an unresponsive mixture screw.  It’s a very nice write up, but I don’t have these symptoms.  (The Bing manual should have included my symptom though, since the solution on page 14 is what solved my problem.  They suggest cleaning the idle circuit with carb cleaner, air, and small copper wire, which is softer than the carb body, and smaller than the orifice you’re attempting to clean.  They even point out the very tiny pin holes which they describe as being between the slide and the cylinder head side of the carb.  I would describe them as being behind the butterfly valve, a.k.a. throttle plate, and only visible when the valve is open.  One of the Airhead members called these transfer ports, and I think these were the source of my trouble).
 
Next, I removed the intake tube between the air filter and carb, and checked the slide operation by gently pushing it up and down with my finger, as Snowbum suggested.  They were smooth, on both sides.  And when I let go of the slides, they fell down on their own weight, as they’re supposed to.
 
Shucks, I really didn’t want to take the carbs off again, but I did.  Once off, I confirmed that the butterfly valve (a.k.a. throttle plate) was moving freely.  At this point, I’m thinking the advice I got about the transfer ports being blocked was probably spot on.  I removed the float and needle, idle jet, mixture screw, and vacuum port screw.  I cleaned the idle circuit with B12 carb cleaner, a strand of copper wire (where applicable), and air.  I counted a total of 5 ports in the carb body near the butterfly valve.  2 of the ports are very very small, and sit just behind the butterfly valve (you can’t see them unless you open the valve).  I believe these are called transfer ports.  These seem to be connected to the port where the idle jet sits.  I covered the other 3 ports with a piece of electrical tape, and blew air through the idle jet port, and confirmed that the 2 very small ports behind the butterfly valve were clear (blowing air).  Actually, they’re right next to each other, and very tiny, so all I really know is that at least one of the two was blowing air.  Next I checked the 2 small ports that you can see even with the butterfly valve closed.  One is connected to the port where the vacuum port screw sits.  The other is connected to the port where the mixture screw sits.  Both were clear.  The 5th port was the big one, which was definitely not blocked.
 
I reassembled the carbs and put them back on.  I verified that the butterfly valves were still operating smoothly before connecting the cables, fuel line, and intake tube between the cylinder head and carb.  Then before putting on the intake tube between the carb and air filter, I once again verified that the slides were operating smoothly.  I also verified the float level a second time.  After carefully adjusting the choke and throttle cables, I started it up, and went for a 15 mile, 20 minute ride.  It was somewhat smooth without being sync’d, since I was careful not to change the original settings too much during cleaning and reassembly, but what really made me happy was the fact that the stumble was gone!!  I brought it back to my garage, and sync’d the carbs.  I set the mixture screw back in the middle, where it runs best.  I adjusted the idle screw using the shorting-method, and then verified this with the crouching-method (crouch down by the exhaust pipes, and listen to and feel the exhaust).  This time I set the idle back to 1050, which is where I used to keep it, but had tried 950 in an attempt to get rid of the stumble–some of you are probably wondering why I lowered the RPM in my attempt to fix the stumble, because you know a higher RPM would probably eventually fix the problem, but I didn’t know that, and I also don’t like the idle to be higher than 1050.  An idle of 1000 or slightly above is better for oil pressure, especially when the engine is still cold, since the idle can drop a little too low when the choke is off.  Finally, I adjusted the throttle cables.  I loosened the right cable adjuster, took it off the center stand, sat on it, twisted and held the throttle at 1450 rpm, and turned the cable adjuster clockwise and counter-clockwise while listening and feeling the vibration to find it’s happy spot.  I then locked down the adjuster and confirmed that the engine was still smooth, and that I had sufficient cable free-play.  Ok, helmet back on, and out for another ride.  Very smooth, and no stumble!!!
 
Perhaps not worth mentioning, but there was a very, very small sphere of mercury that I believe came out of the vacuum port after I removed the screw and blew air through it. However, the carb had been disassembled and I had used B12 to clean things, so that itsy bit of mercury could have been anywhere.  A previous owner probably had a mercury carb tuner, as I have never used one.  So the idle circuit was either blocked by the typical crud that everyone finds in their carbs, or it was blocked by this sphere of mercury.

 

The moral of this story is that when you rebuild a carb, it is very important to ensure that the idle circuit is not blocked by blowing air through the ports.  You need to be certain that you can blow through them.  And when you reassemble the carb, double check everything as you put it back together.

The art of synchronizing carbs on a 70’s airhead

A BMW airhead motorcycle with balanced and properly adjusted carbs will startup beautifully and accelerate smoothly.  Adjusting them takes practice, and patience, while you learn what needs to be done, but later becomes surprisingly quick and simple.  There are three key factors involved:  1) the fuel/air mixture, 2) the idle stop setting, and 3) the cable adjustment.  There are a number of postings online about how to adjust the carburetors, but they don’t emphasize (enough) an important piece of information.  The key to dialing this in to near prefection is listening to the engine, and feeling the vibration of the engine when it is sitting on the ground, off of the stand.  Sound and feel are very important tools in adjusting carburetors on airheads.

 

By now you probably know that there are two generally accepted methods for tuning the carbs:  the shorting method and the vacuum gauge method.  When I got my first airhead, I spent about six months tweaking things trying to get it to run smoothly.  I don’t have a vacuum gauge tool.  I have always used the shorting method.  But instead of relying on sound and feel, I used a tachometer (actually, a timing light with a digital tachometer).  I could never get it right at first, and always assumed something else was wrong, like the timing or valve clearance.  But now I know the trick to getting it right, and will try to pass that on here.

 

The best information on how to do this is at http://bmwmotorcycletech.info/synchcarbs.htm, and for the shorting method, the tools you need can be purchased at http://www.northwoodsairheads.com/Tools.html, or you can run down to the hardware store.  Adjusting the carbs is the last thing that you do when tuning the bike.  The valve clearance, timing, spark plug gap, floats, and cable free-play must be adjusted first.  I won’t go into general tune-up procedures here, but the floats are part of the carburetors, so I will quickly discuss the floats.  The floats are easy to adjust with the carbs on the bike.  Just turn off the fuel petcocks and remove the bowl from beneath the carburetor–be careful, there’s gas in there.  Now with your finger, hold the float so it is level with the bottom of the carburetor body and turn the fuel petcock on.  Gas should not flow when the float is level with the carburetor body, but as soon as you allow the float to drop the fuel should begin to flow.  The best way to do this, as Snowbum has pointed out, is to gently hold the float so fuel does not flow, and the slowly lower the float just until the fuel begins to flow, and make sure the float is parallel to the body of the carburetor.  If it’s not adjusted properly, you’ll have to use a screwdriver or something similar and carefully bend the metal tang on the float where it presses against the float needle.  You will need to remove the float in order to do this.  Another way to confirm proper level is to measure the depth of the fuel in the bowl.  With the bowl in place, open the petcock so that fuel flows into the bowl.  It should stop when the float rises and float needle seats.  Now with the petcock closed, carefully remove the bowl.  Because the petcock is closed, the vacuum will prevent any more fuel from falling into the bowl when you remove it.  Now measure the depth of the fuel.  The measurement is taken from the base of the circular well in the bottom center of the bowl.  For 32 mm carbs, it should be between 21 – 24 mm.  For 40 mm carbs, it should be 25 – 27 mm. 

 

Now I’ll assume you’ve read everything you possibly can about how to tune the carbs and you’ve tried to do it, but you’re still not happy with the way it runs.  Here are my steps to bring it to perfection: 

1. Warm up the engine by going on at least a 15 minute ride.  It’s very important that the engine is fully warmed up.
2. Bring it into the garage and be careful not to lean it over on the kickstand while the petcocks are still open, in order to keep normal levels of fuel in the bowls.  Use the center stand.   Don’t allow the bike to over heat.  Use a fan, or take it out for a ride again if you’ve been working for more than 8 minutes.
3. I like the idle to be slightly above 1000, say 1050 rpm.  If you’re not in this range, go to step 5 and come back to 4.  Otherwise, proceede directly to step 4.
4. (Adjust fuel/air mixture) With the bike on the center stand and the engine running, turn the fuel/air mixture screw clockwise by 1/8 or 1/4 increments and listen to how it is running on that side.  Turn it clockwise until it starts to run rough (this can be subtle, but the engine will sound different, like it’s struggling a bit, but when you find the spot in the middle, it will sound peppy, quick, without hesitation).  Now turn it counter-clockwise again until it runs rough.  You want to set the fuel/mixture screw in the middle, where it runs the best.  I find that from this spot, I can typically turn it 1/4 turn in either direction and it will run rough, or if I turn it just 1/8th in either direction I can’t really tell a difference.  Anyhow, in the middle, it runs the best, without struggling.  Do the same on the other side.   On the Bing CV carbs, the mixture circuit is a fuel circuit, so turning counter-clockwise will yeild a richer mixture.  On the Bing Type 53 slider carbs, the mixture circuit is an air circuit, so turning clockwise will yeild a richer mixture.
5. (Adjust the idle stop screw) With both tires on the ground and you sitting on the seat, feel the vibration and put your ear nearby each cylinder, one at a time.  If the idle stop is adjusted correctly, it will be smooth and sound the same on each side. If they’re way out of balance, the tachometer will vary up and down and the bike will shake.  You can also crouch down low behind the bike and listen to the exhaust in stereo, assuming that you have two good ears.  They should sound the same, and the stength of the exhaust exiting the pipes should feel the same on the palm of your hands.  Adjusting the idle stop screw is probably the hardest of the three adjustments to do by sound and feel alone, but if you detect a difference, turn the idle stop screw on one side and attempt to find the balancing point.  It may help to go clockwise by 1/8th turn, and then counter-clockwise by 1/8th, to compare.   I like to set the idle slightly above 1000, say 1050 rpm.  You may need to adjust the mixture again, and then readjust the idle stop, if the bike was drastically out of tune.  If this is necessary, you will also probably need to go for a short ride to cool the bike down.  If you don’t feel comfortable listening and feeling for the right balance, this is a good time to use the shorting-method.  I find that the shorting-method is really only necessary for adjustments to the idle.  I don’t think it should be used for adjusting the mixture or the cables.  And I’ve never used a carb tuner/vacuum gauge, for any reason.
6. (Adjust the cables) As with the idle stop screw adjustment, the cable adjustment is done with both tires on the ground and you sitting on the bike.  When you’re doing this, be sure that you have enough free play.  When the cable enters the jacket at the adjuster screw on the carburetor, you should be able to lift the cable about 1/16th of an inch without affecting the idle.  When adjusting the cables, if you don’t have enough free play to be able to turn the adjuster out, go to the other side and turn the adjuster in.  Anyhow, to adjust the cables, I start by loosening the adjuster on one side (or both if you prefer).  And while sitting on the bike, I turn the throttle until the RPMs are about 1450 and feel the vibration.  Now with the throttle still at 1450, you can push sideways or pull a bit on the cable on one side.  Did it get smoother?  If not, try doing that on the other side.  If both cables are pulling evenly, the vibration will be minimal.  You can turn the adjuster in 1/4 or 1/2 increments and continue to feel the vibration when the RPM is at about 1450.  When you find the perfect spot, tighten the adjuster up a bit to ensure it’s more or less set, and then turn the throttle and see how smooth it is in the range between 1000 and 3000 RPM.  Take careful note of how it feels at about 2500 RPM.  If it’s set right, this will be very smooth.  If not set right, try turning the adjuster on one side 1/4 or 1/2 clockwise.  Did that make it worse?  How about 1/4 or 1/2 counter-clockwise?  If you find the ideal spot, moving in either direction by 1/2 turn will make it noticeably worse.  When you finally lock down the adjuster–don’t over tighten it–don’t allow it to move from the prefect spot.  When complete, you should be able to twist the throttle and feel a very smooth engine in the range between 1000 RPM and 3000 RPM.  Take careful note of how it feels at about 2500 RPM.
7. You’re done! The bike should run very smoothly now.  Just remember there are three parts to this: 1) fuel/air mixture, 2) idle stop setting, and 3) cable adjustment.  And the most important tool to getting it right is sound and feel.  Listen and feel the engine while sitting on the bike with both tires on the ground.  It takes time to get good at this, so the first couple times, you might need to go for a short ride in the middle of all this, to keep the engine cool.

Good luck!