Well, I was going to say air leak, until I watched your video. :-)

Something else to consider, double check your gas tank is venting well. It's possible that as fuel is consumed, if it's not venting in fresh air from the outside, there's a vacuum building up which is keeping fuel from flowing as it should and as you said, it seems to happen after running at highway speed for a while and then you stop, it could be getting some fuel starvation - sounds plausible. Try even loosening the gas cap a little bit to test it, if you cannot reproduce the problem when the gas cap is not sealed, then that might be an indicator. If so, it could be the gas cap, maybe the vent tubes need cleared out, or similar.

 

I'm running a 40 over here in AU with the stock carb, what makes you think it's running lean on the pilot? You have the miniki carb right?

What stock jet settings were you running? to rule out vacuum just replace all of the lines, vacuum hose is cheap as chips. I would almost say you are running too rich, my bike has a pretty crap idle aswell but I'm adamant on the idea that my bike is running to rich

That's just my opinion, put up your jet sizes if you know then

BTW I wish my girlfriend would appreciate bikes as much as you

 

Ain't that the truth!

LJ: you've been battling this for a while. I hope it gets figured out. When it does, please let us know.

 

Maybe this is obvious, but...... what if there is just something wrong with the carb? Chances of anything mass produced being messed up are pretty good. Brand new broken things are sold every day....

 

...Except, like I said, it's done this exact same thing with both the stock carb AND the Mikuni, and with different jetting with both.

Initially it ran great with the stock rejetted, but then it went all downhill from there. I've replaced the carb AND had new rings put in. The only thing I haven't changed yet is the valves, which I know need new shims.

 

lauriejennifer, since the problem is still there with a different carb It sounds as though something isn't quite right further down the line. Intake manifold (such as it is)? Air injection system?

 

Problem is not carb related, nor air leak. Problem is exhaust valves. Changing shims next week, as soon as I get a chance. Tight valve clearance = inconsistent compression leak, especially once at full operating temp.

It explains two symptoms I've been having that I didn't think were related:

1- Acted like clutch was slipping at higher RPMs under hard acceleration. Would give "false" RPM revs with no power for a moment before "taking off" again. Clutch is confirmed 100% fine-plates, springs, cable, etc. It's not the clutch at all. It's the leak at the exhaust valves.

2- The horrible erratic idle issue from the inconsistent compression. As it affects power under acceleration, so it affects idle.


Failed compression and leak down test. We thought it was the rings, but the rings are new(er) and not the issue. The design of the big bore kit will give a false reading of excessive leaking past rings, especially on a cold engine. Checked valve clearances and exhaust valves are "a hair tighter" (.001m) than they should be to be in acceptable range. Changing shims next week and then will do another leak-down test and report.


In the mean time, I'm riding my pit bike around:

Monkey Business - LJ going in circles on a 50 - YouTube


**EDIT**

If shims doesn't cure it, guess who's getting some new valves. :-/ Oh, and for those who are wondering: Bike has over 28,000 miles. And, yes, it's been raced (road race track) and wrecked and all sorts of loved. I bought this thing to ride and I'm getting all my money's worth and then some. For all that I've put this thing through (snow, mud, track, road trips, commuting, stairs, curbs, crashing, big bore, other mods, etc.), I'm stunned at how well it has held up.

 

While your changing the shims, make sure your ACR unit is loose and not stuck.

 

Bike is with a coworker right now, who's doing the valve adjustment for me for a very reasonable fee. He's a talented mechanic, so I feel much better paying him to do it, rather than trying to save a buck and fumble with it myself.

Here's what he discovered:

The exhaust valves were more out of spec than I originally thought. So, those have been adjusted. Otherwise, everything looks great, BUT...


The decompression spring is totally shot. Ergo, the hard starting whether hot or cold.

The exhaust valves explain the erratic idle and "flat spots/false RPMs." The bad spring explains the hard starting. All this time I've thought both were due to jetting issues, so I'm at this point going to have the bike just dyno tested once these issues are resolved, as I'm pretty sure I've been making the jetting worse, not better. It's like trying to tune a guitar by ear. You can actually make it worse. LOL So, having solid numbers to go off of will make jetting a breeze. A few dyno runs is only $60 local here.

That's the latest update. We're waiting on the spring. It's a less than $5 part, but we didn't have one in stock. Should be here the middle of next week and then I'll give another update.


I just know what it's like to search online for a problem your bike is having and then find a "dead thread" that never resolves, so it just leaves you in suspense and doesn't help solve your own problem. I know my solution will not be universal for all the same symptoms, but I want to keep you guys updated in case a future KLX 351 owner comes across the same issues and, like me, didn't think to check valve clearance.


Cheers, guys! This forum rocks.


Oh, and by the way, the KLX now has close to 30,000 miles. Here's last summer, crossing the 25k mark:

Kawasaki KLX25,000sf!!!!!! - YouTube

 

Okay, just kidding. The bike ran amazing for a day after the valves were adjusted. The next morning it died on me on the way to work. Now we're back to square one.