carburetor – Airheads Beemer Club

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

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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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