Service

Service

CRANKY CHARACTER -> RESISTANCE REQUIRED -> TOOL TIME -> BEST USED BIKE

RAY NIERLICH

CRANKS AND TRACTION

Q: I noticed that the crank in a recent article comparing flat-plane to crossplane designs showed a crossplane crank where the two pistons on the left would be on the way up, while the other two were on the way down, hinting that they may fire an even 360 degrees apart (half the 720-degree cycle). How does this affect traction compared to the “big-bang” crank where all four pistons fired within a short 180 degrees of one another, breaking the tire loose yet allowing it to hook up again with 540 degrees of coasting?

CURTTRICARICO

CYCLEWORLD.COM

AI’ll do my best to channel Kevin Cameron: The first scenario ■ you refer to, two pistons rising with two falling, with even firing intervals, is a flat-plane design. It is up to the designers if they want to fire pairs of cylinders every 360 degrees (180 degrees in two strokes, à la the TZ750) or the conventional once every 180 degrees (90 degrees in two strokes). Way back in the 1970s and 1980s, it was typical to have “normal,” evenly spaced firing orders. Yamaha was first to figure out that firing pairs yielded better traction. Following Yamaha’s lead, the status quo for racing in the late ’80s changed to firing pairs of cylinders every 180 degrees (two-strokes).

Then Honda went to its “Big Bang” four-cylinder in ’92. This gave firing intervals 68 degrees apart, with 292 degrees of “quiet.” It worked. The longer “quiet” time allowed the rear tire to return to a wider footprint and regain traction in between firing pulses, and— voilà—everyone else had to copy Honda to catch up.

The four-stroke “big-bang” engine you describe works the same way. The longer interval between firing allows more time for the tire to hook up. Why is this important? Dredging up fuzzy memories from Physics 101,1 recall that the coefficient of friction for a static object is slightly greater than that of a rolling object (due to minor changes in the contact patch), and it is signifi-

cantly greater than a sliding object (the contact patch is greatly reduced as it skips across the high spots). In layman’s terms, odd firing pulses act for acceleration like antilock systems do for braking. The rapid grip and release cycling helps the tires remain in rolling state instead of sliding state, thus you get better drive out of the corners.

CLOGGED THUNDERCAT

I have a 2004 Yamaha YZF600R I * Thundercat that has sat for a ■ while. When I put gas in it and got it started, gas began running out the airbox tubes. I mean a lot of gas spilling everywhere. I haven’t taken the carbs off yet because I’m a little nervous about it. I have worked on my carbs on a CBR-F2, but the Yamaha is injected. Are they like regular carbs with jets? Anything you can do to help would be great.

JOHN “WOODSY" WOODS

ASHLAND, l(Y

A Not sure what you mean by ■ saying it’s injected but that you ■ haven’t pulled off the carbs,

John. But your YZF600 is definitely carbureted. It is not a sign of weakness to buy a shop manual. One or more of the floats have stuck or broken. Pull the airbox back so you can see which carb (or carbs) is flooding. Fuel will be pouring out, so be careful! You probably like your eyebrows. You could give the offending carb body a couple of sharp taps near the float bowl flange with a small hammer and punch, and see if it quits flooding.

I hesitate to recommend this approach because you can damage something expensive if you don’t know your limitations. Hit it hard enough to jar the stuck float, but not so hard as to ding the body or break any plastic parts.

Didn’t work? Pull the carbs and be prepared to give them a thorough cleaning. If your bike was fuel injected, you wouldn’t have all those slides, diaphragms, jets, needles, floats, and myriad drilled passageways to clean.

The incidence of varnished, stuck, or corroded fuel system parts has gone up exponentially since the advent of etha-

GOT A MECHANICAL OR TECHNICAL PROBLEM with your beloved ride? Perhaps we can help. Contact us at cwservice@cycleworld.com with your questions. We cannot guarantee a reply to every inquiry.

nol. I almost guarantee this is at the root of your problem. Dive in!

MORE ELECTRICAL WOES

fl I had the same problem as Michael Huss (Service, Jan./Feb.) ■ with a ’91H-D FXR. The spark plugs need to be the resistor type. The ignition was being interrupted by static feedback from the plugs that I had purchased from an auto parts store. I had bought non-resistor plugs and got the problem of intermittent shut-off. While at Daytona, when talking to an Aerocharger rep, he let me know they had solved the same problem. I am betting your ignition and plugs are on the same side of the bike, like the FXR.

CARL KELLY SAN DIEGO, CA

A ■ known Right on, for Carl. secondary Older Harleys ignition are ■ interference problems. Bikes have short plug leads, so even if they are resistor-type wire, they don’t have much resistance. Resistor spark plugs and/or ends help. Newer Harleys have better shielding and thus aren’t as sensitive to the wrong plugs being fitted, but why ask for trouble?

HOT-START NINJA

QMy * 50,000 1990 miles ZX750F has Kawasaki a hot-start with prob* lem, like the fuel-injected HarleyDavidson in the May issue, but my bike is carbureted. What could cause the same issue with my bike? After it sits for about two minutes the bike will then start up.

KELLY RANDALL PLEASANTON, CA

A ■ Almost reted engine always, such when as your a carbuZX750 ■ heat soaks (that is, all the parts that are cooler when running—like carbs—get heated up by lack of airflow when parked hot), it will go rich. Injected engines go lean. The symptoms are similar, but the remedy is not. In your case,

I suspect that spark is weak and most likely caused by bad coils (your bike has two). Less likely is that your carbs are flooding; ethanol makes rich hotstarting symptoms worse. Is your gas mileage lousy lately?

Before chasing your tail on any fueling problem, always check ignition first. You need a pair of insulated pliers to do a coil reserve-voltage test. It takes at least

15,000 volts to fire the typical spark plug in an engine, so the coil has to be able to provide that voltage plus a good amount in reserve.

Coils usually fail when hot, so get the engine warmed up. Then, at idle pull one of the HT leads off a spark plug and see how big an air gap you can make and still fire the plug. It must be able to fire the plug while jumping an arc across at least another quarter inch (preferably more). Check each plug in order. If the

spark can’t jump a gap, the coil is bad.

Also, check for arcing from the plug wires to ground by misting water on them (engine running). Water will encourage misfiring or arcing. Check for low battery voltage, low compression, correct spark-plug gap and insulator color, which should be nearly white on current fuel (dark equals rich). The most common cause of rich running are: a plugged air filter, broken fuel floats, and worn main needles and needle jets. CUM