Measuring the Carburetor Fuel Float Level

What the heck we talkin' about here?

On the BMW R75/5 model motorcycle, produced from 1969 thru 1973, are two Bing CV (Constant Velocity?) carburetors. There is an elaborate procedure for making sure these carbs are set up properly. One part is to set the fuel level in the carburetor bowls. The Bing carb manual (revision 6/96) states on page ... (wait: it doesn't have page numbers!) "it is imperative that an exact pre-determined fuel level is maintained." This is done by adjusting a tab on float mechanism such that it just touches the float needle, a valve which, when closed, cuts off fuel flow into the float chamber, while the tops of the floats are parallel to the bottom of the carb body.

This is all fine and dandy, especially for way-back-when these carburetors were designed. Since then alcohols have been added to gasoline and the alcohols have attacked the shellac coating protecting the plastic floats. Gasoline can get into the floats and cause them to lose bouyancy. As a result, even with the careful application of the float setting procedure, the float level will be too high.

The recommended solutions involve purchasing new floats, for a perty penny, which will soon enough exhibit the same problems if you can't guarentee that you are only burning alcohol-free gasoline. Alternatively, Bing has a new float mechanism which they designed to have improved fuel level regulation when the carburetor is accelerating forward. As a bonus, the floats in this newer float mechanism are supposed to be alcohol proof. But this new mechanism comes with an all new float, associated hardware, and a new float bowl. Expensive!

And we still haven't determined if we even have a problem! I've heard that if your floats are brown, they are too heavy. Also, there is a way of removing the floats from the carb and seeing if they have the necessary bouyancy by putting them in a coffee can full of gasoline. They can also be weighed to see if they have become too heavy (I believe 10 oz is the correct weight for the floats).

I'm still not satisfied. I'd like to determine the float levels directly, regardless of the condition of the floats. Then I can set the float level correctly with current hardware, and without spending much money. Besides, I, in an earlier state of idiocy than I am in now, or a previous owner, managed to put the float pivot pin in backwards, making nearly impossible to remove the float in such a way as it can be put back!

Summary

Remove the Bing carb tops, slider/needle, and the brass jetting hardware to clear out a path to the float bowl while leaving the floats, float needle, and float bowl in place. Snake a thin clear plastic tube through that path and use it as a manometer to directly observe the fuel level in the float chamber. The carbs, as well as the bike itself, must be leveled for accurate measurement. The absolute level that the fuel should attain is unknown at this time, but could be found by someone with known good floats.

Float Level Measurement

Warning! This procedure involves sucking gasoline into a flexible plastic tube. While not as potentially unhealthy as siphoning gasoline (you aren't going to suck it all the way through the tube), it is still an inherently dangerous activity. Gasoline and gasoline vapors are very nasty in the lungs. Not to mention the fire hazard!

Tips to prevent inhaling gasoline vapors:

• Use a long enough piece of tubing. One and a half feet is barely adequate for the measurement, the rest is for safety.
• Suck from as high up as possible within the limits of comfort. If you are sucking from below the level of the gasoline on the other end, it'll run right into your mouth! If you suck from high up, it won't run into your mouth unless you suck too hard.
• Make sure there is liquid gasoline in the bowl before you start sucking. If it's empty, you'll get a lungful of gasoline vapors with minimal effort, regardless of how long the tubing is or how high you suck from. (Does this one sound like it comes from experience, or what?)
• Suck only from one end of the tube; mark it to keep track of which end it is.
• Drain the gasoline in the tube after each measurement; then blow it out from the marked end by standing up and with the tube hanging down from your mouth.

Before you start, the following will need to be acquired:

• some clean space or some plastic baggies for hold tiny parts you don't want to get dirty.
• one or two pieces of 3/16" OD clear tubing about 3 feet long each.
• the BMW tool kit (or appropriate tools instead)
  • 8 mm wrench
  • 10 mm wrench
  • flat head screwdriver
• needle nose plyers
• a small bubble level
• a ruler marked in mm up to the end of the ruler. Make sure the first (or last) mm is accurate.

No carb kit parts are required to measure the float level, but since we will be going deep into the carb, having parts like diaphragms on hand might make sense, especially if you haven't replaced these parts recently. While you're inside your carbs, check the parts to see if they are the correct ones. This table applies only to stock R75/5 models!

years	left carb	right carb	main jet	needle jet
'69-'??	64/32/3	64/32/4	140	2.73
'??-'73	64/32/9	64/32/10	135	2.70

Measurement procedure:

1. Get the bike level:
   1. Turn the bike off; turn the fuel petcocks off.
   2. Park the bike on its centerstand.
   3. Make sure the bike is level. For front to back, I've got no idea how to check this. For side to side, I used the bubble level on the lower of the two main frame lateral tubes that go above and below the driveshaft boot near the transmission. Road crud must be cleared off this tube for accurate measurement. If the bike isn't level, use pennies or something under the centerstand to level it; alternatively, you may need to move it to a more level location.
2. clear out a path to insert the 3/16" tubing into the float bowl
   1. Drop and drain the carburetor bowls. I just dump the gasoline back into the tank.
   2. Remove the throttle cable, at least from the butterfly valve lever, if not from cover as well. I just popped it off the lever, so as to preserve the previous carb balance (which may not be any good after completion of setting the float levels).
   3. Remove the enricher (choke) cable from the cover, if not also from the enricher lever. I did fine just getting it off the cover.
   4. Remove the four screws holding the cover onto the carb body. If you've never done this, you may find that these screws are really stuck. Penetrating oil and an impact driver are wonderful tools for helping these screws come out.
   5. Slide the cover straight up and out, and set aside. The slide may or may not come out with it.
   6. Remove the slide if it didn't come out with the cover. Take heed of the orientation of the slide relative to the carb body: there are little vent holes in the slide which must go on the engine side of the carb. Also, there is a tab on the rubber diaphragm that must mate up with a notch in the carb body. While we're here, check out the condition of the diaphragm -- this is a part which must be replaced every few years or so, and if you haven't done it now's a good time.
   7. Take a peek down to where the needle rests when the slide is all the ways down. That brass part you see is the atomizer. It sticks up a little bit from the rest of the carb body around it.
   8. Good time to measure the length of the needle on the slide. I think I have it in the 3rd clip on each side, at any rate both were 38.5 mm long, so I left them alone.
   9. Remove the main jets and their washers with an 8 mm wrench. While we're here, check the table to see if it is the correct main jet for your R75/5.
   10. Remove the jet stock and O-ring with a 10 mm wrench. When it comes out, the needle jet will also (hopefully) drop out, as might the atomizer: catch 'em as they fall. Make sure you understand how they go back in: you've got one more carb to watch carefully on how things came out. While we're here, check the table to see if it is the correct needle jet for your R75/5. Also, check the condition of the rubber O-ring on the jet stock. This is a part that comes with every carb rebuild kit; I'm sure you could find the part at a local hardware store if you needed to.
   11. Reinstall the float bowl.
   12. If the atomizer hasn't dropped out with the needle jet, tap it out with the handle of a screwdriver. It'll fall into the float bowl, and you should retrieve it from there. If it just won't come out, see if you can snake the 3/16" tube through it.
   13. Put the float bowl back on again.
   14. Do the other carb, too. Don't get the parts mixed up! Not all of them are interchangable. I'd leave the parts of the carbs on the side of the bike they came off of.
3. Level the carb bodies
   1. Loosen the clamps on the air tubes on either side of the carb. No need to loosen more than these two (or three, on mine) screws on each side, as we are not going to remove the carbs. To measure the float level, the carbs should be as much as possible in the orientation they will be in when used.
   2. Use the bubble level on the top of the carb body. Twist the carb body within the tubes until the carb body is level side to side. Measure with the two side screw holes as guides for the bubble level. If the bike itself isn't level, you'll be able to measure your float level, but the carb won't be properly oriented when you are using it out on the road.
   3. Retighten the screws holding the carb body onto the air tubes. Make sure the body is still level after the body is resecured.
4. Stick one end of the 3/16" plastic tube down thru the center of the carb body into the float bowl chamber. Drape the line to fall below the float bowl and then come back up above the float bowls again.
5. Fill the bowls with fuel by turning the petcocks on.
6. Suck fuel into the lines, keeping portion of the tube below the top of the float bowl. Be careful! Gasoline is *very* bad for the lungs. Pulling the fuel up will be hard, then easy as it flows down to the level of the bowl again. Stop sucking after it passes the lowest point on the tube. The fuel should come to rest in the tube, where the interface between the fuel and the air in the tube is at the same height as that in the bowl.
7. Measure the level.
   1. Using your ruler, measure the distance down from the bottom of the portion of the carb that sticks out opposite the butterfly (throttle) lever. If you've got the later carbs, it's the structure that has the vacuum take-offs on it, but the bottom of those is about 1mm higher. (one of these days I'll add a picture).
   2. Don't let a lot of gasoline into the line and then pull the 'mouth' end of the tubing up to drain it back into the bowl. That'll raise the level of gasoline in the bowl, possibly affecting your measurement.
   3. Measure the lowest point in the arc that defines the air-gasoline interface inside the tube.
   4. Raise 'mouth' end of the tube until the gasoline flows out into the bowl. Pull the tube out of the carb and blow it out, maybe twice, letting the tube just hang down. Get those gasoline vapors out of there!
   5. Turn off the petcocks.
   6. Drop the carb bowl and drain the gas back into the tank.
   7. Might help to repeat the measurement a few times to make sure you got it right. I found it quite repeatable to well within 1 mm, maybe I could've gotten the precision to half of a millimeter if I had an easier to read scale.
8. adjust float tabs, and remeasure. Use a needle nose plyers on the float tabs where they touch the valve, near the hinge.
9. reinstall parts. (you saw how they came out. Put 'em back.)
   1. stack the atomizer onto the needle jet and that onto the jet stock. Screw the stack back into the carb body. The atomizer likely will not want to go back to its correct position without a little help from a pen or narrow screwdriver blade from the top.
   2. Watch the tab on the diaphragms. Get it oriented correctly into the slot.

Results:

I found the right float at 15mm and the left at 14mm. I repeated the measurements several times until I was sure I was doing it repeatably. Since the left side was higher, and I was pretty sure the floats were correctly set via the prescribed 'parallel' method before I started, I should've surmised that the left side float was heavy, and its lost bouyancy needed to be compensated by lowering the float into the fuel so as to make it shut off the float valve sooner. For reasons that now escape me, I worked on the right side and after a few attempts, I got both sides at 14mm. Ah well, rich is better than lean, right?

On another /5, I measured the left side at 9mm and the right at 12mm, but I did not attempt any adjustment. Also, on this bike, the centerstand was quite screwed up, so while level side to side, it was not pitched with its front wheel off the ground like my runner, so I did not attempt to even adjust this one.

After I rode the bike about 30 miles, I balanced the carbs with a water manometer. After this, the bike felt better than I can ever recall, especially from mid to full throttle. At the time of this procedure, it had 120,000 miles on it and maybe 10,000 miles on a complete upper end job two Winters previous: first overbore pistons, rings, valves, valve guide sleeves, etc. The carbs saw a rebuild kit, including diaphragms, maybe the Summer before that.

Concerns:

The point of measurement, under the butterfly valve, is not at the same front-to-back position as the center of bouyancy of the floats. If the bike is pitched differently, the numbers will come out differently. I have yet to identify a good place off of which to measure fuel height appropriately further back along the carb body, nor come up with a good place on the frame for measuring the level of the bike itself front to back.

I have no clue as to the absolute correct float level; with this technique I can only see (for now) if the carbs are correct relative to each other. I'd need some carbs with good floats in them to calibrate my measurement, and an improved measurement procedure (see above concern) to get the float levels repeatable, before I can quote a correct absolute level. (I'm hoping you, gentle reader, can do this! :)

Even if the float levels are matched, or even set to the absolutely correct level, whatever that is, the floats may still be too heavy to perform correctly under dynamic conditions of cornering and accelerating. But there's now hope that if they're only a little heavy, you could at least get the static condition correct. Seems a better starting point for the dynamic performance than a fuel level too high.

My right carb was pretty far off from being level side to side: it was twisted in the air tubes. Fixing this maybe was the key to improved performance, rather than matching the float levels.

Return to Roger's techpage.
Return to Roger's homepage.