Service

SERVICE

That synching feeling

I took my ’90 Honda CBR600 in for a tune-up a few weeks ago and the dealer said he probably would have to “sink” the carbs. When I picked it up after the tune-up, the service manager said they had to “balance” the carbs. When I asked him if sinking and balancing were the same thing, he said, “Well, sort of.” The bike runs a lot better after the tune-up, so I have no complaints about the work. But I still want to know what carb sinking and balancing are, and whether or not they’re different from one another.

Carter Pell Tuscaloosa, Alabama

The correct spelling of this unusual word is “synch,” which is short for “synchronize.” And when used to describe certain aspects of carburetor tuning, the words synchronize and balance mean the very same thing.

The final step in a proper tune-up of any multiple-carburetor engine is to

ensure that all of the carbs are adjusted equally for idle mixture and idle speed, and that all are operating in perfect unison from idle to full throttle. Carbs that are not so adjusted can cause an engine to idle irregularly, to

hesitate and stumble when the throttle is opened, and to produce less than full power at wide-open throttle.

To perform this synchronization, the mechanic first attaches a set of vacuum gauges-one gauge for each carburetor-to fittings on the intake manifold. The gauges allow him to adjust each carb’s idle-mixture and idle-speed screws until all the carburetors pull the same amount of intake-manifold vacuum when the engine is idling at the proper speed. This ensures that all of the individual carbs ' idle mixtures are fairly equal, and that no carbs are supporting more or less than their share of the idling responsibilities.

When that step is complete, the mechanic then adjusts the throttle linkage so that the butterflies (on constantvelocity carbs) or the slides (on slideneedle carbs) all begin to move o ff of idle at precisely the same instant as the throttle is opened. The final step is to hold the throttle wide-open (with the engine off, of course) and use any of several methods to ensure that all four butterflies or slides have reached the fully open position.

Front-brake friction and fiction

The single-rotor, four-piston Brembo front brake on my ’92 BMW R100R is, according to all reports I’ve read, an excellent brake. Mine, however, has been anything but. Corresponding to no external conditions that I can determine, it exhibits the following symptoms: It grabs inconsistently, growing stronger and weaker with each revolution of the wheel; it makes a clicking noise with each revolution; it squeaks. My dealer first said this was simply the result of having only one rotor on the front instead of two. He later said it was the result of having two sets of dual pistons, which, he claimed, gripped unevenly. He then thought there was a leak in the caliper seals, so he replaced the entire caliper, but the problem returned in a couple of days. As this is the same dealer who replaced my sparkplugs with the wrong heat-range plugs three times in a row, I have little faith in his diagnosis. BMW had a brake-pad recall for this bike, and mine were duly replaced. At high speeds, the brake’s behavior isn’t so pronounced, but at low speeds (5-15 mph), the inconsistent braking renders the front brake almost useless. Since I live in Japan, I spend quite a bit of time at these speeds. I would appreciate any advice.

Embry A. McKee APO, Japan

Your Beemer’s front-brake disc either is slightly warped or, more likely, has developed a hard spot on its surface. Brake discs often develop these kinds of disorders because of the tremendous heat generated by stopping, and by the enormous number of rapid-fire heatingcooling cycles a brake disc must undergo under normal use. The heat tends to loosen somewhat the molecular bonds of the disc’s steel material; and as the steel cools, the molecular structure can be altered just enough to provide local differences in surface hardness. With any one occurrence, the differences are small; but over time, they can be great. And while those differences might be very small in terms of, say, the Rockwell scale of hardness, they have enough impact on the material ’s coefficient of friction to noticeably affect the rate of stopping at any given lever/ pedal pressure. In extreme instances, the brake disc can even get hot enough to permanently distort.

The solution is simple, if not cheap: replace the brake disc. And the dealer.

Un-LTD spending

I have a 1981 Kawasaki KZ750 LTD and am contemplating a major overhaul. This is my first streetbike and I love it, but is it worth rebuilding and sinking $500 to $700 more into on top of the $900 I already have in it? Besides, it’s just a 750. James McClure Tornado, West Virginia

It ’s unclear whether you re looking for someone to talk you into or out of rebuilding your LTD. We also don’t know anything about the bike’s overall condition, or the going price for Kawasakis of that ilk in Tornado, West Virginia. So, you have asked a question that is-from a financial standpoint, at least-impossible to answer. Your best bet is to determine how much your LTD is worth by visiting a couple of dealerships and asking them to estimate its retail value, either with or without a rebuilt engine. Then just use simple arithmetic to determine if the overhaul is worthwhile in terms of dollars and sense.

In the end, though, whether or not you should invest that kind of money in a 12-year-old motorcycle depends upon how emotionally attached you are to the bike. If your 750 LTD truly gives you great pleasure and satisfaction, $1600 is not an outrageously large investment by any means.

Bleeding hearts

A couple of my friends and I have a question about methods of bleeding brakes. Until just recently, we’ve usually bled the brakes on our bikes just by pumping the lever or pedal and turning the bleeder on the caliper, but a lot of shops now use “power bleeders” that force fluid into the brake system through the bleeder screws. I know that these power bleeders do a better job of getting air out of the brake system, but I’ve asked a few mechanics why and never gotten a satisfactory answer. Can you provide one?

Dennis Hackett Bound Brook, New Jersey

Unlocking the mysteries of brake bleeding is simple once you understand two key facts: 1) air bubbles are lighter than brake fluid and therefore try to rise to the highest part of the brake system: and 2) the bleeder screws are on the calipers, which generally are at the lowest part of the brake system.

With the manual-bleed method you and your friends have always used, any air in the brake system is unlikely to get pumped all the way down to the bleeder screw unless that air is trapped in the caliper, which is close to the bleeder; insufficient fluid is displaced during any one bleeding cycle to move the air very far. So, unless the air can rise unrestricted into the master-cylinder reservoir, where it can escape the fluid and join the small volume of air above the fluid level, it will remain trapped in the system and cause some degree of mushiness in the feel and operation of the brake.

A power bleeder, however, works in the opposite direction by forcing fluid through the bleeder screw, up through the entire brake system and out through the master-cylinder reservoir orifice. This tends to sweep air bubbles up and out of the system.

Even with a power bleeder, though, air can still get trapped in the high spots of little nooks and corners—such as those in some brake-line fittings and junction blocks-thereby preventing the bubbles from rising into the master cylinder. In those instances, the simplest solution is to remove those f ittings or junctions from their mounting points and rotate or reorient them in a way that sets the air bubbles free so they can rise into the master cylinder reservoir □