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:
- Warm up the engine by going on at least a 15 minute ride. It’s very important that the engine is fully warmed up.
- 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.
- 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.
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(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.
- (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.
- (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.
- 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!
lunar cycle 