Fuel Supply – Airheads Beemer Club

Leaky Bing Carburetors

It is very important to turn off the petcock(s) when you park your bike. This is so even if you have a late model Airhead that has the fuel-shutoff solenoid valve located on the inside of the cover over the starter motor (many have been removed, but the same caution applies). It is exceptionally important if you park your bike in a garage where there is a source of gasoline fumes ignition …such as a water heater, ETC. We do NOT need to hear about BOOOOOM! We don’t want fires, either!

Bing carburetors are commonly known to leak in several ways. The most common is fuel dumping on your boot from a grossly overflowing carburetor, typically from a side vent. Sometimes there is just a slow weeping or a slow leak from the very small hole in the bottom of the float bowl.

Alcohol-Proof Float Kit

The MAIN advantage of these kits is that you will likely never have to replace floats. That does NOT apply to the float needles, of which there were two basic types. The VITON tipped ones have the same easy-to-lose fine wire clip as on the stock carburetors. That clip insures positive float needle operation when the needle should be leaving the needle seat and allowing fuel to flow into the bowl.

If the original floats (which DO AGE) are already really bad, and/or the float needle is already bad, things will certainly improve with PROPER installation of these PRICEY kits! Installation is, however, somewhat tricky. IN MY OPINION, installation of these kits when the old parts were worn, often considerably, is the HONEST & TRUE reason that SOME find improvements with fuel mileage after installing the kits.

Adjustment of these dual-independent float kits is more involved than with the stock floats. These kits can cause $$$$ problems, so, please read all of this article.

You must be a member to view complete articles on this website. If you are already a member, you can log in here. If you aren’t a member yet, you can purchase a membership here.

Synchronizing Carburetors

The following information specifically applies to carbureted twin cylinder two-valve BMWs, of the type known as “Airheads”, as manufactured 12/1969->1995+. Some of the information may be applicable to many other motorcycles, and even injected models, and to even single cylinder and two stroke engines. The method is applicable to BMW models before the above dates, and while the shorting method works for ALL, you CAN use the vacuum methods for those earlier models with, or install, vacuum ports.The author has used these methods on a wide variety of engines. The author is NOT responsible for any electrical shocks, nor other consequences, for any ineptness on your part in failing to understand and perform procedures properly and safely.

Quite a few decades ago many mechanics used a device called a Uni-Syn to balance carburetors. It was usable on almost any carburetor, and it was usable on most injected models (rare, then). Sometimes a minor modification was needed to allow the Uni-Syn to properly mate with the input area. The Uni-Syn was a specially machined or cast metal plate, rubber on one side to allow some pressure-sealing against the air intake side of the carburetor or injector, and an ADJUSTABLE venturi built into the plate, and attached to the venturi was a glass or plastic tube that had a floating ball in it. Properly used, this device would take accurate relative readings of the airflow through a carburetor or injection intake; then the device was transferred to the other carburetor (usable on engines with many carburetors too). I do not know if they are still available, I have one, and treasure it, as an antique! Treasuring some old tool does NOT mean I use it nowadays!

I do not! I have not seen one used, except in my own shop for some sports cars or 4 or 6 cylinder motorcycles, since the mid-1970’s, except to demonstrate its use. Mechanics had a lot of pride on their ability, without instruments, to listen to the engine; adjusting the idle mixture screw for a smooth idle, then setting idle rpm, and going back and forth until they ended up with the proper idle rpm and idle mixture adjustment. Some very experienced mechanics would even take bets on being able to synchronize a BMW Airhead engine without any special tools….and often made up stories about what they were doing….which, in truth was simply listening to the transmission internals rattling around at idle rpm (which they do, when the oil is hot). The throttle cables were synchronized by eyeball on lifting at the same time, and sometimes by transmission rattle. These methods are not good with multiple cylinder engines such as a V-6 or V-8 or inline 4, particularly those engines with more than two carburetors. The method is tricky to do with a BMW airhead boxer engine, and I do NOT recommend it. It is also NOT EVER as accurate as the vacuum nor shorting methods.

The primary method on the /2 bikes was to adjust the carburetor throttle stops for equal engine speed with first one plug cap, then the other, removed; you alternated between cylinders firing and not firing, back and forth. This was USUALLY safe on those magneto equipped /2 bikes because the magneto incorporated a safety gap, if the plug cap was pulled off the spark plug, the spark could jump across the safety gap. UNfortunately, sometimes the metal cap shell would give you an electric shock. This ‘lifting the cap’ method is still used today on the /2 bikes; although the shorting method is MUCH better.

You must be a member to view complete articles on this website. If you are already a member, you can log in here. If you aren’t a member yet, you can purchase a membership here.

Carburetors

Some time ago a very embarrassed Airhead owner contacted me about how to ‘fix’ his carburetors. He has Bing CV carbs, and replaced the diaphragms, floats, etc. himself. Frankly, he is a bit ham-fisted. He also hates to ask for advice. He proceeded to nearly destroy his carburetors. The many e-mails with him prompted this article. There is a LOT of information in this article, and lots more on the author’s website. Carburetor work is not very difficult, easy to learn, and once you have worked on your carburetors, perhaps at such at an Airhead TechDay, you will wonder what all the fuss was about and you will find that it is easy the second time, and you might hardly need to refer to this article …or one of my others on carburetors…again.

I have my own ideas about when one should consider working on your Bing CV carburetors (and fuel tank and petcocks!):

YEARLY: depending on mileage and use: clean/dump float bowl contents, check corner bowl jet to be sure it is clear/open, possibly replace float bowl gasket if poor; empty gas tank, remove petcock(s), clean tank, dry it, clean petcock screen(s), possibly service petcocks internally if they are getting quite stiff to turn; or, if leaking. Inspect fuel hoses. I consider these things part of carburetor servicing. Failure to service the fuel tank will eventually be costly.

5,000 mile intervals: synchronize the carburetors, but ONLY AFTER the valve adjustment and ignition timing adjustments are checked FIRST.

30,000 mile intervals: replace float assembly (if original one piece assembly). Always also replace the float needle and clean out the central jet assembly. Replacing the float assembly is UNlikely if you have the alcohol-proof separate floats that Bing now offers. However, if you have those, be sure to replace the float needle and

{mprestriction ids=”4,5,6,8,9,10,11,12″} check the adjustment of the flimsy brass floats bridge, info on the Snowbum website in its own article on these independent float kits:
http://bmwmotorcycletech.info/bingindependent.htm

You must be a member to view complete articles on this website. If you are already a member, you can log in here. If you aren’t a member yet, you can purchase a membership here.

Carburetor Rebuild Tips

“”My Bing rebuild kit arrived and I am having trouble removing the old slide needles from my 83 R100RT. Any advice on this and other Bing carb rebuild tips?””

There are two basic types of needle mountings. One type uses a captive hidden clip, and the other type has a screw that has to be loosened.

If you have to, use a pair of pliers and protect the needles with something like thin leather at the pliers jaws. BUT you might be able to remove the needles with clean and dry fingers or try soft thin leather gloves or kitchen plastic gloves. ROTATE the needle 90 degrees as you pull slightly downward, then repeat, until needle is removed. When replacing the needles, the same 90 degree rotation, with small upward pressure, is needed for each notch of needle adjustment/replacement. When done replacing the needles to what you think is the ‘correct notch’, be SURE the needles stick out of the bottom of the slides exactly the same amount…use a vernier caliper. I like to record that value. Mention of needle position is very commonly done by ‘notch’ number. The uppermost notch on the needle is

#1, and as the needle is adjusted to be higher and higher in the slide, the numbers increase, per notch, to #4.

Some needles, probably from about 1985, were made out of aluminum and the needle grooves (notches) tend to wear rather quickly. If the wear is high, that will allow the needle to be in the wrong position vertically; it can move up and down a LOT. All needles, aluminum or steel, will also wear on their sides, as does the associated needle jet. The needles are purposely free to move a bit angularly sideways, so normal vibration wears them, and their associated jet. Always replace needle and jet together when overhauling.

You must be a member to view complete articles on this website. If you are already a member, you can log in here. If you aren’t a member yet, you can purchase a membership here.

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!

You must be a member to view complete articles on this website. If you are already a member, you can log in here. If you aren’t a member yet, you can purchase a membership here.

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.

You must be a member to view complete articles on this website. If you are already a member, you can log in here. If you aren’t a member yet, you can purchase a membership here.

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

You must be a member to view complete articles on this website. If you are already a member, you can log in here. If you aren’t a member yet, you can purchase a membership here.

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.

You must be a member to view complete articles on this website. If you are already a member, you can log in here. If you aren’t a member yet, you can purchase a membership here.