Throttle & Clutch Cables

In my shop I saw many control cable failures from these things.

1. Throttle cables on the Airheads: Left cable failing at the carburetor, due to the throttle cable being bent as owners checked the oil dipstick. Do not bend the throttle cable at the left carburetor when checking your oil. There is no need for the oil dipstick to be overly-tightened. Bending the left throttle cable is a prime cause for that left cable to have increased friction, possibly spread some coils on the wrapped sheath (& making that carburetor difficult to synchronize, if bad enough), & eventually you might break an inner strand …usually where you can see it between the throttle lever on the carburetor, & the sheath. A single strand found broken (You do inspect these cables regularly, don’t you?), will usually cause other strands to eventually break from the same reason why the first strand broke ….this will …or can …result in total cable failure in as little as few hundred miles or so.

2. The bushing at the clutch lever at the handlebars is a replaceable plastic part and as it wears the result is the lever can move up and down & also allow angular motion. If worn enough, the stranded core of the cable will start rubbing, or even catching, on the sharp edged guiding slot in the lever. Eventually a strand breaks, failure comes soon as more strands break. The bushings are easy to replace and not expensive. If your new bushing does not finger press into place, heat the lever first. The Nylon-like bushing is 32-72-1-232-662 and has been used from 1976 onwards. That bushing may need light reaming for a good fit to the pin. If you do not have a 8 mm tapered ream, you can use very carefully selected drill bits, to progressively remove a quite small amount of material, a few thousandths at a time …until the pin fits properly …an easy, but not loose, push-sliding fit. The lever has a recess, and in that recess must be a waverly washer, 32-72-1-230-871. I recommend the sharp edge of the slot in the clutch lever be filed smooth. Be sure the crimped end of the cable that fits into the clutch lever at the handlebars is not fouling the lever.

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R100GS Flip-A-Lever Throttle Lock

BMW has a complete line of accessories for motorcycles destined for police use, and one of the handier of these is a friction throttle lock. The idea is simple: It screws into the throttle grip housing and rubs on the flange of the grip to resist the pull from the carburetor butterfly valve return springs. Screw it in tight enough and you can take your hand off the grip and the bike will keep running at the same speed. Back it off a bit and the weight of your hand will have the same effect, and it will be easier to change throttle openings to compensate for hills and such. Most useful on long trips to let your hand take a break once in a while, it’s also nice to be able to take your hand off the throttle to adjust your face shield, open a vent in your jacket, or wave to someone at the side of the road.

This little doodad has been around for years, for most of BMW’s motorcycle line (the K1200 series is a notable exception).

Simple as it was, the BMW screw took some fumbling with to get set just right, and if you wanted to change it you had to take your hand off the grip and reach under the bar to make the adjustment. Usually this resulted in the bike lurching down to full closed throttle as the grip snapped shut. At least it did until Bill Schneider got to thinking about a better way. The result of his thought experiment is pictured above.

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Airhead Lighter Carb Springs

Almost since I’d purchased the R100GS it was clear that the throttle pull was heavier than it needed to be, even after replacing the worn throttle cables with new BMW items. I had a drawer full of carb springs left over from my Norton days, but somehow the idea of converting something from a screen door to fit one of BMW’s finest left me cold. The idea kept rattling around in my head, manifesting only as a line on my “todo” list until I stumbled across an ad at the IBMWR Motorcycle Marketplace for lighter airhead carb springs. I posted off an email, and was promptly put in touch with Tony, a middle age entrepreneur with a passion for older BMW twins. Tony was curious about the GS, as his test mule was a /7, and he wasn’t sure his replacement springs would fit. In a “it’s a small world” type coincidence, Tony lived just over the hill from Dublin in Castro Valley, so it wasn’t long before we were rubbing elbows and comparing springs. Sure enough, the prototypes didn’t have enough reach on one end, and measuring tension revealed that they were just a bit stiffer than the stockers off my GS. Tony was having the springs custom wound by an old tool’n’die man, and offered to come up with a new variant that would take care of the GS. I had an extra spring to use as a template, and a week later we had springs to test. The new samples were made from stainless steel wire and were easier to stretch between the fingers. When I got ’em home, I set up a test rig with an RCBS trigger scale to find out just how much lighter they were.

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