Page 9 – Airheads Beemer Club

Early R75/5 Bing CV Carburetors

This article deals with problems with early versions of CV carburetors used on the R75/5. This includes getting the bike to start and to idle correctly as best possible.

A basic mini-overhaul can be done with the carburetors in place on the engine. This may be enough. Three articles, the one you are reading, and http://bmwmotorcycletech.info/bingcv.htm ….and http://bmwmotorcycletech.info/bingcv-2.htm are not just about how to overhaul Bing CV carburetors. These articles are specifically designed to fully inform you what you need to know, as an Airhead owner/rider, about Bing CV carburetors.

While it may appear that the information in the below article does not apply to the later carburetors (except for information as to how the /9 & /10 and later carburetors are different from these /1, /2, /3 and /4), that is not so! The information in this article should be read by anyone that also works on later model CV carbs …as some things are good to know ….such as about the springs that might be added, changes in parts, and problem area.

Before working on the R75/5 carburetors to cure what might seem to be carburetor problems, such as high or irregular idle rpm or inability to properly adjust the carburetors, it is important that the engine valves be set correctly, & the ignition timing be correct. Be sure there are no vacuum leaks at the throttle shafts. Be sure the hoses between carburetor & cylinder head are tight & not leaking vacuum (spray with any volatile solvent …must not be idle rpm changes). Never work on carburetors unless you are sure all else is OK!

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Bing CV Carburetors – Part 2

A basic MINI-overhaul can be done with the carburetors in place on the engine. This is often quite enough. Two articles, the one you are reading, AND http://bmwmotorcycletech.info/bingcv.htm are not just about how to overhaul a Bing CV carburetor. These two articles are specifically designed to fully inform you what you need to know, as an Airhead owner/rider, about these carburetors.

The one exception is if you have an R75/5, which came with a particularly troublesome version of the Bing CV carburetor. For those R75/5 carburetors, you should review the above linked articles; AND, in particular, review http://bmwmotorcycletech.info/earlybingR75CV.htm

1. Starting problems:

It was in November of 1971 that BMW first …and almost lastly….recommended OPENING the throttle during cold or very cold starts. We all know that this can be a necessity depending on the bike, temperature (does NOT have to be very cold), & how the carburetors are adjusted. That is still true for later models, but BMW has dropped the recommendation of an opening throttle. SHAME. While SOME Airheads will start OK with throttle shut off and enrichener turned on or fully on, many need a bit of throttle playing AS THE ENGINE BEGINS TO START.

If your Airhead starts OK, enrichener (choke) in use, ….then seems to run out of fuel, after perhaps 15-30 seconds:
Inspect for a partially clogged jet at the bottom of the corner of the bowl well. If OK, check the bowl gasket and also the enrichener gasket/screws problem. The gasket may get sucked inwards a bit, check all 4 sides; check the 4 screws too, they tend to loosen. There is also the possibility of a wrongly assembled enrichener. If the bowl corner jets is clogged badly enough, bike might not start at all. Both situations are often much worse on cold days. Check the tube for cracks…this tube leads downwards from the carburetor body into the bowl well. Very early Bing CV carburetors have pressed-in float bowl enrichener jets, not screwed-in. THE enrichener jet in the float bowl MUST BE CLEAR, NOT CLOGGED! Check all these items for BOTH carburetors.

WHY does the float bowl gasket make a difference in starting? Because sealing of the float bowl is involved in how the enrichener in the body of the carburetor is able to lift the fuel from the float bowl well.

Check if you might have installed the enrichener parts backwards, or mixed-up left and right parts. NOTE that the punch prick mark on the enrichener shafts have been seen Bing-factory-installed 180° off! That is, the punch prick marks are WRONGLY DONE for the shafts of the enrichener as installed by Bing. See http://bmwmotorcycletech.info/bingcv.htm; or Part 1 of my two Bing articles on this airheads.org website.

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Bing CV Carburetors – Part 1

For a well-done overhaul article…step by step…with 44 pictures….see BMWMOA magazine, BMW-ON (BMW Owners News), March 2003, for an article in great depth by Gary L. Smith. You can probably get that back issue at http://www.bmwmoa.org. From that main page, click on left side for Country Store, and then on the Country Store page, go to Back Issues.

I have SOME nitpicking on that article:
1. See my notes in the article you are reading.
2. Use faint amount of silicone grease on all O-rings & on enrichener parts (do NOT overdo this, you do NOT want to plug jet holes).
3. If doing a complete overhaul, which involves removing the butterfly to replace the throttle shaft O-ring (be sure to silicone grease that one & be SURE to MARK the butterfly for correct refitting: something like TOP, OUTER; or TOP, facing REAR…or something of that sort….BEFORE you remove the old one. HOPE that the previous workman did not install yours backwards, because it can be fun to figure out unless you know what it is supposed to look like. In the article you are now reading I have photos of what it IS supposed to look like….. & NOT look like.
4. The article you are reading, below, has considerably more complete information about orienting the enrichener parts.

A basic MINI-overhaul can be done with the carburetors in place on the engine. This is often quite enough. Two articles, the one you are reading….and http://bmwmotorcycletech.info/bingcv-2.htm (which is article 3B) are not just about how to overhaul a Bing CV carburetor. These two articles are specifically designed to fully inform you what you need to know, as an Airhead owner/rider, about these carburetors. The one exception is if you have an R75/5, which, if stock, has a particularly troublesome version of the Bing CV carburetor, as it was the first generation of these carburetors. For these carburetors, you should review both of these above two linked articles AND, in particular, review myarticle #6: http://bmwmotorcycletech.info/earlybingR75CV.htm

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Bing Carburetor Floats and Float Needles

Bing carburetors FLOAT NEEDLES, whether the pure slide type carburetor, or the CV type carburetor, both as used on BMW Airhead motorcycles, come in several varieties. In general, TWO present-day available versions will fit all the Airheads carburetors. Very early carburetors had a solid metal needle. That needle was available in more than one ‘style’, but for all, there was NO rubber tip nor a Viton tip, tip was simply part of the float needle. The later type of needle, that most of Airhead owners have, is similar to the photograph below. The needle shown in this photograph has a spring loaded plunger with a hole in it, with the easy-to-lose wire clip, and RED tip material. GRAY-BLACK tipped material will also be seen. I suggest purchasing the stock type needles from your BMW parts supplier, and not Bing themselves.

The all-metal needle used on early models is a different size, and will not properly fit later carburetors, and, vice-versa. Be sure to get the correct needle. If you want to install a Viton-tipped needle in a carburetor that used the Bing all-metal needle, you can, per Bud Provin (TheNickwackettGarage.com), who says to use the very common float needle used on many old Amal carburetors, as used on old British bikes. The part can be found on a simple internet search, for float needle 622/197; or, as 013, 622/197.

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Poor Fuel Mileage, horsepower, etc.

Why do motorcycles get such poor fuel mileage compared
to many cars, which are vastly bigger and heavier?
How much horsepower and/or torque increase does it take
to improve performance? What about RAM AIR?
What is the best cruising RPM?
Premium fuels versus mileage and combustion temperatures.

All objects …..cars, motorcycles, bicycles …..and you! …..when moving through air …..create friction and drag. On Earth, the weight of the atmosphere, held to the Earth by gravity, causes a higher pressure at sea level, than it does on a mountain top. The higher you go with your motorcycle, the faster you should be able to go, theoretically, due to less friction with the air ….or; conversely, the less power from your engine you would need for the same speed as at sea level; due to the reduced air drag. While friction and drag are not the same, you can regard them as the same for this particular bit of reasoning, which, other things not considered, is true about the speed, etc. If your motorcycle (and you!) are not moving, but if you are exposed to the wind, that creates friction and drag. Of course, if the wind is moving towards you while you are riding, that effectively also creates drag.

As you ride higher in altitude, the oxygen content of the air is less (by weight), and your engine will have less power as you go up in altitude, which will reduce performance increases from less air drag and less friction. In some instances, fuel mileage will increase with an altitude increase, especially so on fuel mixture compensating engines, such as those with fuel-injection. Mileage may decrease or increase, especially in hilly or mountainous riding. Sometimes supercharging or turbocharging can improve fuel mileage, particularly with small engines with smaller combustion areas (cylinders and heads). Mileage can also go down, if you use more and more throttle. Many factors enter into fuel mileage.

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Petcocks, Fuel Filters, Choke, etc.

Each petcock is fastened to the tank by both right hand and left hand threads. As you rotate the large “nut” CW (facing from below), that tightens the nut to the tank while at the same time it brings up the petcock. The petcock large nut has to be FIRST engaged the proper amount onto the petcock BEFORE screwing it upwards to begin to engage the proper amount of tank threads. If you have never done this before you will have to fiddle with this until you get the right amount of petcock and nut threads assembled …to start …and finish…the petcock-to-tank fastening process. Once you learn, it is easy. You want approximately the same number of threads engaged on both the petcock and the tank after fully tightening.

BMW has used numerous petcocks over the Airhead production years. There are five types of petcocks normally seen on our Airhead motorcycles. All except the /5 bullet types are easily re-buildable. Usually this means that you unscrew some knurled or slot “nut” at the handle area (after removing a black trim cup, if it is there on your version), and you can replace a gasket that might be bad, or clean and very-faintly-lightly silicon grease the moving parts. You then have a much smoother operating petcock. On some there are one or two dimpled discs, which can be repaired with a shaped punch, lightly hit, for a better detent action; I usually don’t bother. These discs have a locating tab. They can confuse, so take notice of how they were assembled as you take things apart. You cam figure it out, if you forgot to take notice.

Our petcocks have a Reserve function. Except for the bullet type, photo below, the handle long portion upwards towards the tank is the reserve position. Horizontal (either way) is always off. German “AUF” printed on the body/cap means on, not off. Handle lever downwards means on for the main part of the tank.

Early /5 Bullet Style Everbest Petcocks were different in how the handle indicated fuel flow, & the rotating handle affecting gas flow from was in the opposite way from standard piping flow.

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Fuel Caps

Removing/replacing/rekeying other locks in the later Airhead (and Classic K bike), luggage, etc., will be found in Snowbum’s article 75A on his website, http://bmwmotorcycletech.info.

WARNING! ….BMW SCREW CAPS SHOULD NOT BE OPERATED LIKE AUTOMOTIVE CAPS! AVOID spinning these motorcycle fuel caps to the ratcheting point when tightening them! The ratcheting parts WILL eventually wear out, and WILL eventually cause you problems in trying to remove the cap. BMW Airhead fuel caps are not constructed like car caps. On most modern car fuel caps, the cap is designed to be rotated to the ratcheting point. That is done because the car caps must fully seal a complicated fuel fumes venting system; if a car cap is not rotated enough, the cap can leak fumes, which will likely be detected by the car’s computer monitoring system, and a “Check Engine” light will illuminate on the car’s dashboard. On our Airhead BMW’s, the cap for the earliest Airheads is simply designed to keep liquid fuel from sloshing out. Later models had cap vents and even later ones had simple venting to direct fumes to the crankcase. These last versions had a more complicated fuel cap, called a SHED cap, but it is best not to rotate them to the ratcheting position; even though that is often done by those locking their caps. If you do tighten to the ratcheting point, try to ratchet only one notch. Much more on these various caps with the ratcheting mechanisms further down this article!

Fuel caps and venting methods for the BMW fuel tanks vary by year & model. For information on the Pulse-Air system, the fuel tank venting system, & other associated items, please read: http://bmwmotorcycletech.info/pulseair.htm

It is common to hear about an Airhead “running out of fuel”….stumbling, loses power, etc. The problem will often be the tank vent …or the fuel cap. Both problems can be identified by loosening the fuel cap and usually hearing a whoosh of air entering. Within 15 seconds, the engine will then run OK. Engine stumbling from this venting problem usually happens much more quickly with a more-filled tank and also from high speeds. While this fuel delivery problem can occur on the models with the fuel solenoid mounted on the underside of the starter motor cover, that is less likely.

There can be other types of problems, such as fuel leaks at the caps, etc.

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Why Diode Boards Fail

At 51,000 miles, I noticed the alternator light on my ‘92 ‘GS glowing at idle. Time for a new diode
board (again), I thought. When I removed the board, I found that both of the upper rubber mounting posts were broken–the stud end bolted to the board had come unbonded from the rubber. It was clear from the discoloration of the metal that those studs had gotten very hot. This occurrence is not uncommon as many Airhead owners know, but others who have written on the subject seem not to have noticed what I have.
The problem has been discussed extensively, but my observation suggests that solid mounts and a
bunch of ground wires with soldered lugs are not really going to fix it.

In the world of electricity, current times resistance equals power. If the resistance is in a bad
connection, power is wasted there as heat. Power from the alternator is supposed to replace the
power supplied to the bike and therefore keep the battery charged. But the bad connection I refer
to is at the point where the diode board is bolted to the studs.The voltage drop there causes the
the regulator to flog the alternator more to maintain the desired voltage at the battery. And that makes even more heat at the bad connection.

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Wherle Electronic Voltage Regulator

This modification is for the plastic case electronic voltage regulator that Wherle made, with its name on the case. I have only modified a few of these. IT IS POSSIBLE THAT YOURS MIGHT LOOK DIFFERENT INSIDE, as other versions could have been made.

This modification will increase the charge voltage to better charge the battery and give longer life. This will be useful in instances where the voltage at the battery terminals is found to be at or below 13.9, at approximately 70°F at the regulator. This modification will ensure better battery charging, and longer battery life, especially for in-town commuters. This modification essentially turns your voltage regulator into the pricier BMW-sold high voltage Authorities (Police) regulator, very cheaply. TWO methods are described, so that at least one of the parts needed will be available locally. All Wehrle regulators are not the same. Be sure yours is the same as the one described here, before modifying it.

Cost: I will assume you have the tools, and will do the labor yourself. The parts cost will be from 10 cents to $3.00, depending on what part(s) you purchase.

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Testing Voltage Regulators

If you have a charging problem, you may want to find out if the VR (Voltage Regulator) is the problem. A faulty VR can cause various problems, the most common are no charging or too high a voltage during charging. NOTE that the stock factory VR internal setting is such that the battery terminal voltage is typically 13.8 volts during cruising (the higher voltage Authorities…Police…regulators are about 14.2). The 13.8v is a reasonable compromise for water use on flooded fillable batteries, but really too low for best battery life, particularly on AGM/VRLA batteries.

There are TWO very easy ways to test a voltage regulator that do NOT involve the technical methods in the rest of this article. These two methods are:

1. Substituting a known good voltage regulator (any 1970+ BMW airhead regulator, or any three terminal automotive regulator that fits the airhead plug). If whatever the problem was now disappears, then the VR was faulty.

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