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keepemflying Carb work and hydrostatic lock By Matthew Parkhouse #13272


Carb Work Summer is here! I try to do most of my airhead work outside, under our backyard trees. I have wound up one project, the “tight-


ening up” of the carburetors on our four airhead BMWs. I talked a little bit about this job last month and fig- ured I’d go into a bit more detail here. When my Slash Five was crossing the 200,000-mile mark, I noticed wear marks on the slide needles of the carbs. These needles are mounted in the slides and work with the brass slide needle jets. The slide and needle work up and down, operated by the rubber diaphragm and engine vac- uum. They are right in the middle of the intake passage and exposed to the fast moving air created by the intake strokes of the engine. Because the needle and jet are fixed in place, the same surfaces are constantly moving against each other. After 50,000 miles


or so, you can see a polished area on the needle. On a couple of VERY high mileage bikes, I have even seen a slight ovaling of the hole in the jet. When I replaced the four items (two needles and two jets) on my high-mileage Slash Five, it made a notice- able difference in how the engine ran and felt much smoother and “tighter” as I went down the road. My Slash Five now has 424,000 miles on


it, my “Mexico Bike” has well over 100,000 miles, the R 100 S has 47,000 on it, and Susanna’s R 100/7 has around 150,000 miles. I decided to service the carbs on all four bikes. The needles and jets are different on each machine, so I had to take a look at an online parts fiche. I had most of the parts for the job in my box of carburetor parts, but I did have to order some of them from a dealer. The needles and jets each run around five or six dollars each. Doing each bike’s carbs cost about $22 for the four parts. Replacement is straightforward. Going


in from the underside to replace the needle jet, unscrew the main jet carrier (a good opportunity to clean out any water or dirt in


there). When you have the brass jet in your hands, you can see the size numbers on the side. They are marked “2.66,” “2.68,” “2.70” and so on. They go in from the bottom. With the new jets in place, go to the tops of the carbs. Unhook the throttle cable, remove the choke cable adjuster with the cable, and remove the choke return spring (if there is one). Unscrew the two or four small bolts holding the top in place. The top now will lift off, exposing the slide and dia- phragm. Lift out the slide and check over the diaphragm. If the rubber part seems strong, it can be left in place. If it seems thin and stretched out, I would replace it. Early slide needles are held in place by a


spring clip inside the slide. The needle is grasped and pulled out while twisting. Pay attention to how many “notches” you feel as the needle comes out. The needle has four slots to allow the needle to be raised (richer) or lowered (leaner) in relation to the slide and the jet. The later Bing carbs also have needles with four notches but are held in place with a screwed-in plug. Unscrew the plug, and the needle falls out into your hand with a small clip attached. The settings can


I am explaining the workings of the slide needle and diaphragm to Monte, a local rider who is preparing to ride to Alaska in a couple of weeks. His well-preserved 1995 airhead has about 75,000 miles on it so it is a good time to make the needle/ jet exchange.


50 BMW OWNERS NEWS August 2016


After opening up the "downhill" carburetor, this is the oil I found. There is also visible oil in the needle jet and mixing tube area of the carb. With a good cleaning out, the R 100 S ran as it should and stopped smoking within a mile."


TECH


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