Total Motorcycle Community Forums

Hello all,

I've been working on this one for a while, and I'm out of ideas.

I have a '96 Kawasaki ZX-9R (900B3) with about 12000 miles on the clock. It's been trouble free for the years that I've owned it. Recently, I noticed a slight miss when running at part throttle. RPM doesn't matter, it misses from idle to almost redline, but only when the throttle is in the first 5% of it's range.

I did the full routine, carb cleaning, valve adjustment (ugh...shims), air/fuel/oil filters. No change.

At more than 5% throttle, there's no miss, and it pulls very strong. At full throttle, I don't have the guts to see the end of 4th gear.

Tempurature doesn't make a difference, hot or cold, it runs the same, although when the choke is on (cold) there's no miss (but a high idle). Turning on the choke after it's wamed up it will bog and die (like it should).

I suppose I could have missed something when cleaning the carbs, but usually, a little bit of choke will fix that. Not in this case.

There's no hesitation when opening the throttle, and under high loads, any RPM, any gear, it runs fine.

So, I'm at a loss. I'm open to suggestions.

Thanks in advance,
Hawk

Have you synched the carbs?

Twice, just to make sure.

I'll admit that the carbs being out of sync might produce the symptoms I'm seeing.

I'm not very familiar with this particular model of carb, but it seems like the transition circuit is clogged (if these carbs have that circuit).

Check for vacuum leaks first - carb boots, vacuum lines, etc. Also check the CV diaphragms for cracks, splits and pinholes. Then, I think I'd re-check/clean the pilot jets. Then, if that doesn't help, I'd open the idle mixture screws a bit - maybe 1/8 to 1/4 turn to get a little richer mix.

You might also try shimming the needles. It often helps in lean off-idle transition.

Is there a idle mixture adjustment screw on these things? I was under the assumption that the screws were plugged. As far as I can tell, it appears to run rich at idle already, at least is smells like it.

I did inspect for vacuum leaks, and everything seems fine. I did just recently replace a craked o-ring between carb number 2 and the airbox, but I doubt a leak there would effect off idle response.

As for shimming the needles, I've never been down that road before. Are there shims made for that purpose?

Is it worth springing for an A/F gauge? I might make diagnosing this a bit easier.

Shimming the needles is usually done to cure a lean condition, not a rich one. Hardware stores usually have stainless steel washers that are about .020 thousanths thick, and they raise the needle, enrichening the idle to low speed fuel circuit. If you're already running rich, this is not the way to go.
Unless there is a problem internally, Japanese bikes tend to run lean rather than rich.

See, that's the problem. At idle, it smells like it's running rich, but while riding it, it seems like it's running a little lean. The idle mixture (I think) is rich if anything, but the transition is lean.

At his point, I'm thinking it's trial and error.

OK, then. Maybe we're not fighting a "lean surge" here. That's kinda where I was headed and I'm still not totally convinced that was the wrong way to go, but anyway...

Just for future reference, you can indeed adjust the idle mix, but first you have to remove the plugs to gain access to the screws. Use a small drill bit, maybe 5/32, and just barely pierce the thin cap of the plug. Then you can usually just pry out the remains of the plug, just be careful you don't damage the brass screw underneath.

When you did the carb cleaning, did you happen to check the float levels (otherwise known as "fuel service levels")? If those were within spec, then make sure the choke is operating correctly - if it isn't returning to the stop...well... you get the idea.

I did in fact check the float levels...within spec. Made a slight (very slight) adjustment.

Choke operates properly. Returns to full off position properly, without any sticking.

Although I'm new at this particular carb setup, I'm not new to carbs in general. I'm honestly stumped.

I did the carb sync again today. It's as perfect as it can be.

The bike did sit for over a year, with no fuel in it. After chargin the battery, and putting new fuel in it, it started right up. I cleaned the carbs after that, but the miss was there before it went into storage.

I had a previous bike, a VF700, that had a horrible miss at idle. That turned out to be a burnt valve. The ZX doesn't have nay of those symtpoms, so I can't think of anything but a fuel problem.

After messing with this for a few weeks, I'm seriously considering a fuel injected bike. The New ZX10 looks enticing. I just don't know if I'll be happy commuting on something so sporty. The ZX9, being a '96, is heavy, but comfortable.