Service

SERVICE

Cold-hearted Yamma

I own a 1991 Yamaha FZR1000 with 16,000 miles on it. Rick Tomicic (the chap who built Reuben McMurter’s RC30 engines) has given it a full-ra-dius valve job, cleaned up the ports and degreed the cams. I then installed a Dynojet Stage 3 kit and a Yoshimura carbon-fiber exhaust canister.

The results were pleasing and gave no problems until recently, when starting became a 15-minute affair when the engine is cold. It does not matter what the outside air temperature might be; when the bike sits overnight, it is very reluctant to start. At first, it sounds like only two cylinders are getting any spark; and once the engine does start, it will not idle at all until the temperature gauge is about halfway, choke or no choke.

I used an ignition tester and was able to get a strong, blue spark to jump more than a full inch. I cleaned the carbs thoroughly inside and out, and made sure there were no vacuum leaks. I even installed carbs from another FZR1000, but to no avail. I then changed plugs, plug wires and coils, but the problem persists. Once the engine is fully warmed, it starts with just a stab at the starter button, and a recent, 126.5-horsepower run on a Dynojet dyno shows that it is still making great power. Can you help me before I burn up my starter motor? J. Ferrari Dearborn, Michigan

Your FZR ’s engine is not getting enough fuel at start-up, most likely because some or all of the chokes are not functioning properly. Actually, the cold-start device on each o f your FZR 's four carbs is not a choke; indeed, a choke is a mechanical valve in the intake opening of a carburetor that partially restricts the flow of incoming air so that intake vacuum will draw additional fuel up out of the float chamber. But each of your Yamaha ’s carbs (and those of most other Japanese engines) uses a separate mixture-enrichment circuit that is activated when you pull the fairingmounted knob labeled-for ease of user-understanding“choke.”

Based on your claim that an entirely different set of carburetors failed to cure the problem, it’s most likely that the cable and linkage connecting the choke knob to the four separate enrichment plungers are not properly attached or adjusted. That would prevent the engine from getting an adequately enriched mixture during cold cranking, thereby making it hardstarting and reluctant to run properly until almost fully warmed. You need to ensure that all four enrichment plungers are fully lifted when you pull the choke knob.

Flywheel fakery

The ’89 Honda Gold Wing I bought about a year ago has developed a problem that’s getting worse. When I close the throttle while riding along at anything above very low rpm, the bike slows so quickly that it feels like I’ve jumped on the brakes. If I do this while I’m riding in first or second gear, the bike slows down so abruptly that the rear wheel sometimes hops and chatters. I’ve ridden a few of my friends’ Wings, and they don’t act like this. It’s a long way to the closest dealer, so I’d like to fix the problem myself if I can. Barry Butler Valentine, Nebraska

Your Gold Wing s problem is in what Honda calls the Shot Air Deceleration Compensation System. Six-cylinder GL engines have an unusually small amount of crankshaft flywheel inertia, a design feature that helps the mildly tuned engine be much more responsive and possess quicker acceleration. But a lightness of flywheel also causes an engine to decelerate more quickly; and during development of the GL1500, Honda’s engineers found that the Six ’s deceleration could be excessively abrupt.

To compensate, they fitted the GL1500 engine with the Shot Air System, which dramatically leans out the fuel mixture on trailing throttle by bleeding air into the intake stream. (A very lean mixture causes an engine to “rev down ” more slowly than one with a normal or rich mixture.) Honda calibrated the system for just the right amount of momentary leanness to allow the engine a more conventional rate of closed-throttle deceleration.

Given the symptoms you describe, the Shot Air on your Wing either is not working properly or not working at all. Unfortunately, the procedure for diagnosing the cause of the problem is much too involved to describe here. It involves troubleshooting specific metering circuits within the GL1500’s complex, automotive-style dual carburetors; coping with some of the engine ’s nearly two-dozen separate vacuum hoses; and checking the electrical circuits that allow the Shot Air system to produce more “fake” flywheel inertia in the lower gears than in the higher ones. You could buy a f actory workshop manual and try to work your way through the problem yourself; but your best bet is to have the system diagnosed by a competent Honda dealer who understands that he should contact Honda s technical services department if he can’t find the proper cure on his own.

Think synch

I have a 1992 Kawasaki ZX-11 that runs great-except for one problem. When the bike is idling, the engine speed surges up and down, sometimes as much as 500 rpm. My local shop seems baffled, so any suggestions would be helpful. Douglas C. Searcy Fayetteville, North Carolina

Most likely, carburetor misadjustment is the culprit. If one or more of'the carburetors ' idle-mixture settings is off the mark, or if all four carburetors are not properly synchronized, a fluctuating idle could result. Proper adjustment and balance require a few special tools, such as a manometer, and the knowledge to use them properly. If you don’t have both, take your ZX-11 to a dealer or mechanic who does.

There’s an outside chance, however, that the uneven idling could be caused by a faulty ignition black box. If the box is allowing variations in ignition timing at idle, the idle speed would vary accordingly. You can easily determine if this is the cause of the problem by checking the ignition timing with a strobe light and watching to see if the timing varies noticeably as the idle speed fluctuates. If it does, replace the ignition box.

No diving, please

I enjoy old two-stroke streetbikes and recently purchased a spotless 1975 Yamaha RD200 two-stroke Twin. It works great except that the front fork dives excessively under moderate braking. At 155 pounds, my weight shouldn’t be the problem. Would it be possible to swap forks with an RD350 or 400 and solve this problem? Steve Lodien Cedar Rapids, Iowa

Before getting involved in a frontfork swap-which is quite possible but a lot of work and expense for what you d end up with-first try changing the fork oil to ensure that the front end is working with the proper weight and amount of oil. If that fails to produce the desired results, try an old box-stock production roadracing trick: Marginally overfill each fork leg, which increases the compression ratio of the airspace above the oil.

In simple terms, what this means is that the volume of the airspace above the oil at full fork extension will be slightly reduced, but that the air in that space will have to be squeezed into a considerably smaller volume by the time the fork reaches full compression. Thus, the overall spring rate (a combination of the rate of the coil springs and the rate provided by the air that is compressed as the fork moves upward) becomes progressively more stiff as the fork compresses. It will take some experimenting to find the right fork-oil level to meet your particular needs, but start with, say, an additional 40cc in each leg and go from there.

If you ultimately decide that a fork sw’ap is a better solution, an RD350 front end may be the better choice. The 400’s fork is a little more sophisticated than the 350’s, but it has more travel and softer spring rates. So, if you graft an RD400 fork onto your RD200, you 're liable to end up with just as much front-end dive as you have now, whereas the stiffen shorter-travel 350 fork will keep the front of the bike more level during hard braking.

CBR600F-toothier

I'm extremely happy with the ’92 Honda CBR600F2 I bought last June. But I’m ready to make some modifications, and Fd like to improve the bike as much as I can without going into the motor. My first move would be a change of sprockets, the front from 15 to 16 teeth, and the rear from 41 to 46 teeth. Is this too drastic, or will it give better overall acceleration along with increased top end? I also would like to change the exhaust system, but 1 noticed in your “Enough is enough” letter in the March, ’93 Service column that I could lose power with an aftermarket exhaust. Is the F2’s exhaust that nonrestrictive? Rod L. Gayda Merrillville, Indiana

Your proposed sprocket changes will lower the overall gearing by about 5 percent and almost certainly yield marginally better acceleration.

And because the stock gearing is a bit too tall to let the engine reach peak power rpm in high gear, the lowered gearing might raise the CBR ’s top speed by a mile per hour or two, although there is no way to be sure without bolting on the sprockets and seeing how fast the bike will actually go—a practice we don V recommend on the street. But rather than changing both sprockets, why don 't you keep the 15-tooth front sprocket and simply replace the stock 41-tooth rear sprocket with a 43? That would give you a 2.867:1 final-drive ratio, which is so close to the 2.875:1 ratio provided by the 46/16 combination that you d never be able to tell the difference. Either way, understand that this re-gearing will reduce your F2 ’s fuel mileage and increase engine buzziness when cruising on the highway.

As far as an exhaust change is concerned, we can 't tell you whether any given pipe will or will not increase your Honda s power output, because we haven’t tested any pipes on an F2.

But we can tell you that the stock exhaust system is extremely efficient, and that many aftermarket systems give you little besides more noise. Even some of the systems that will produce more peak horsepower do so at the expense of low-end and midrange engine performance.