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.
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.
lunar cycle 