Nearly stumped, need a little help - 2000 KE 100 running poor at high RPMs

Working on my first motorcycle - a model year 2000 Kawasaki KE 100. It's got a serious issue. The engine will break up at high RPMs. It's kind of a stumbling/stuttering behavior in which the engine will sometimes surge and buck and feel like it's misfiring. At slower RPMs the bike runs great still, will pull hard (for a 100) through all the gears. Winter or summer, the behavior persists. Seems to do it worse when the engine is thoroughly warmed up though. While the behavior is more persistant at sustained high RPMs, problems around the shift points are common as well. Breakup does not seem to occur unless the engine is being loaded although this cannot easily be verified without (in my opinion) endangering my engine.

This is what I have done so far, in approximate order:

1) New plug, clean, well gapped to .030, within the gap range for this bike. Quality NGK plug, correct number for OEM spec.

2) Verify fuel quality - run tank to nearly empty (about 140 miles of riding) and refill with brand name premium gasoline.

3) Clean petcock fuel strainer. Verify gasoline supply to carb. Replaced fuel line at this time as it had shrunken up badly and was pulling the crankcase grommet loose.

4) Install inline paper element fuel filter to reduce possibility of sand ingestion into carb.

5) Carb is clean. Idle, main, needle and seat all inspected and are in perfect working order by all accounts. Float level was adjusted higher to allow more gasoline into carb.

6) Main jet fattened up from 80 to 90 size jet. Flat level raised still more. Behavior becomes worse.

7) Float level set back to previous parameters. behavior persists as it did before step 6.

Float level lowered from original parameters. No change in behavior.

9) Air filter inspected and pre cleaner is cleaned. Reinstalled, no effect. Removed air filter and rode briefly up and down driveway but behavior still persists, absolving the paper element of any oil blowback.

10) Spark arrester removed, cleaned (was already clean), and reinstalled. Exhaust header removed and exhaust port cleaned and inspected. Was already clean anyway. No change when put back together.

A few other notes in no particular order:

The bike is equipped with electronic ignition, and to those unfamiliar, Kawasaki's system has a rep for being extremely reliable. I don't think this problem is spark related although I have not yet put an inline checker on it to be sure. Reason I don't think this is obviously because the engine seems to need a load to exhibit the behavior I described.

The bike doesn't need the battery to run so don't go there. It has an isolated magneto coil. Even if it did need the battery, the one in it currently is guaranteed to be in good shape - recently serviced.

There are no real signs of the bike running rich or lean on the plug. For the most part they're all a healthy brownish color with the slightest amount of two stroke oil fouling pretty common on these bikes. There are 29000 meticulous miles on this motorcycle and I really doubt the head gasket or crank seals. No signs of leaking from the head gasket.

The bike idles rock steady. I can leave it idle while I walk into the garage to grab a 10mm wrench and it will be running 40 seconds later just as happy as a clam. It idles like no other two stroke I have known, possibly for minutes unattended.

Dud probably to EPA/CARB restrictions the idle mixture screw is blocked off on this bike. I may unblock it and see what's going on there but I have my doubts that this is a jetting related problem.

The Oring for the intake manifold is in good shape, seems to seal tight although I am trying to brainstorm a way to verify that it is in fact OK.

One thing I haven't double checked lately is the oil pump sync, but the engine is smoking a healthy smoke - a tad cloudy with the choke on and very little unless you count the puffs between gears. In other words, I'm going to check it, but it's most likely fine.

Even though it has good compression, with the header off I got a bore scope and took a peek inside and everything looks just like a healthy engine should. No rust rings, no pitting, and a normal amount of carbon on the piston.



So at this point I am beyond the realm of my experience. I can't even be entirely sure if the bike is becoming too rich or too lean at high RPMs. Originally it seemed consistent with running lean but the bike never showed a tendency to overheat. I now have a few hypothesis but am open to others. My ideas include:

1) The seat on the enrichener "choke" circuit is leaking, but not severely enough until the high vacuum loads of high RPM riding actually draws the fuel through the choke circuit. I've heard of something similar happening before on an old Mikuni round slide carb, the difference being the incidence I heard of was about a hundred years old, not 8. (yes, I'm being hyperbolic about being a hundred years old).

2) There is an issue with the rotary valve system that is somehow vibrating or not sealing tightly against the crankcase, causing the engine to lose at least some of it's fuel/air intake charge and lose power.

3) The oring for the carb and intake manifold is not sealing tight. NO idea how to check this other than perhaps to buy another seal and test it. Probably ain't cheap, but doubtful if it's more than 6 or 8 bucks...

4) God really does have a sense of humor and is working in some mysterious way, driving me partially insane in the process.

I've been as thorough as I think I can be. Any help or assistance you might lend, from personal experience or otherwise is appreciated.

Check for intake leaks by spraying water from a windex bottle around the carb while the bike is idling. I realize this will be a pain, given where the carb is located on the KE, but if it dies while being sprayed, you have a leak.

Also try some plug chops at high RPM's to see the true condition of your mixture at high engine speeds.

Look to see if your in-line fuel filter is the problem. Those things do not flow fuel very well, which is why I don't recommend them. Put a piece of hose in it's place and see if the problem persists.

One more thing...I'm not sure what you consider high RPM, but take a look at your exhaust pipe. See the expansion chamber? Not a high RPM pipe. You may just be asking more of the engine than the pipe will allow it to do. If you want the formulae for building the perfect two stroke pipe, let me know. If anyone on this forum has the ability to follow through with that project, it's you. :)

The only thing I have to respectfully disagree about is the inline fuel filter. There is one (exact same NAPA part number) installed on my dad's FXDXT harley, KLR 650, XL 600, and even the neighbor's truck equipped with a 327 Chevy. They flow plenty well for a motorcycle if you pick the right one. :) I've seen a lot of the funky little conical nylon mesh fellows clog up though. I keep a pocket knife and a spare filter in my pack too just in case with mine, but I haven't had an issue since I took the god forsaken think off my Suzuki and put in an automotive unit...

In spite of my dad telling me as much I tried without the fuel filter anyway and no dice. Behaves the same.

I was searching for a solution to this problem some more last night and on the 16th page of google results I did find an individual who solved his problems by cleaning out the exhuast behind the spark arrester which I have not actually tried. His was an older model though in which the baffles were accessible. On mine I have not a clue how to access the baffle mesh to give it a good scrubbing. The procedure in my shop manual does not appear to apply to my very late model bike.

High RPM is difficult to call on this bike. There is no tachometer, but the redline on this engine is specified at 10,000 RPM. The theoretical posted top speed of the motorcycle according to its overall gearing is 62 mph. Seeing how the machine once ran perfectly at the speeds I ask it to go I'm not so convinced that it's fundamental design flaw so much as it is something else I'm overlooking. I don't normally ride it beyond about 45, maybe 50 if I'm trying to get a run on a hill somewhere. I've clocked it at 58 indicated, 55 actual but it just isn't designed to sustain that, and I really almost never try to put it there. That just isn't fun with a 19 inch knobby front tire and a 28 inch seat height with a guy my size on it.

See if you can beg, borrow or steal a peak voltage meter. It has an inductive pick-up that clamps around the plug wire, and will show how many volts are travelling through the wire at any given time. These are also handy for diagnosing jetting problems, oddly enough. Low voltage on one cylinder from a known good ignition system indicates a rich condition, high voltage indicates a lean condition.

Cool, huh?

If the high RPM plug chops don't tell you anything, try this. Remove the exhaust and shoot it full of EZ-off oven cleaner. let it sit, and then rinse it out with water. I've cleaned a lot of Tomos and KE pipes this way.

That sounds cool, although I wonder how to establish a baseline voltage. Even if you found another identical model I imagine the voltages could vary by a few hundred volts between two identically designed systems.

I believe my dad's Fluke or OTC multimeter may have such a function, although it's been a while since I've been through those owner's manuals. They remind me of college texts.

Didn't get to fixing the bike today and I still need to put fork seals in it (Dennis Kirk sent me the wrong ones so I'm waiting for replacements). Right now the left fork is dry so getting it through all the gears for a plug chop is something I'm not really willing to do.

The left fork is dry because I tore the left fork apart, got the package from DK, and muttered a number of expletives realizing they sent me the wrong part. The listing being wrong according to their sales rep. So I broke out the calipers, measured off the correct sizes and they said they'd send the correct ones. That was a few days ago, and they appear to be due to arrive on Monday so I'll probably try some of this stuff then.

Ordering LeakProof brand seals for the lifetime warranty and the irritating habbit this bike has of blowing fork seals (they seem to blow ever 1300 miles or so, even the originals).

You're looking for variations in voltage, rather then a specific number. Most manufacurers won't publish outputs based on RPM's because they can vary from coil to coil, but if the voltage drops off radically at a certain throttle setting, you can start looking at the mixture as your potential problem.