Clogged fuel filter(s). Nothing good can happen until you can get clean fuel flowing at the proper rate to your carb. Locate and verify that your fuel filter is good-to-go. It's in the carb nipple that the fuel line goes onto - sometimes it is found in the fuel line when you pull it off the carb nipple.

If you have a "bad" fuel tank, you'd better get a better fuel filter and keep check on it regularly.. If you find you don't have a filter at all - and you have carb issues - time to tear it all down for a "every single component and circuit" thorough cleaning..

 

So you've become CVK savvy. You've checked, cleaned, and verified all components for proper function. Still you have a problem. If your bike has had previous owners, there are "hidden" mods that could have been done that could be responsible for your problem. Needle jet mods by drilling out holes and/or drilling new ones - Drilled main jets - Drilled pilot jets - Drilled slides - Modified MAJ and/or PAJ with added restrictions or by being drilled. There are probably others I haven't remembered at this moment - I'll add as I can..

At some point, you will have to start measuring and comparing pictures to know what all you've got that "just ain't right"..

 

Air Filter: It's a no-brainer but must be said. Your filter needs to be new or like new in cleanliness. You can run it dry (street only) or with a very light coating of oil. Easiest way to lightly oil is with a spray-on air filter oil. It's things like this that have Murphy rolling on the floor laughing out loud..

Also note that your wrist is connected to the butterfly valve. It is not connected to the slide and needle (which fuel your bike). Your input is only a request for a power level. For instance: There is no such thing as 1/2 throttle as you can only control the butterfly valve - you do not control the slide and therefore the the fueling you get at your 1/2 "throttle" opening. And the fueling you get will vary. I.E. More if going uphill , less if going downhill..

 

There is a brass "collar" sitting above the Emulsion Tube. This is the Needle Jet. It protrudes into the carb bore and it is the first item the needle slides in and out of below the slide. It will likely fall out after you remove the Emulsion Tube. If you leave it out or install it upside down, it's "game over" until you go back in and do it right.

 

Running with the airbox lid on makes the carb setup easy - Such as a Stage 1 or 2 setup from a DJ kit (2206 or 2152)...

Running without the lid requires additional mods and a different carb setup to be useable and/or to extract the performance potential.
A performance slip-on, full exhaust system, or "uncorked" KLX300 exhaust system is mandatory. The main jet will need to be much larger - determined by altitude - and the needle will need to be set "conservatively" in order to not over fuel the low and mid RPM ranges with the larger main jet. The main jet affects fueling from 3-3.5k to redline.

Adding the MCM to any configuration should not require a good running setup to be changed, this assumes your good running setup is not actually on the ragged edge of being too lean. The MCM was my first mod on a brand new, totally stock bike - it ran great and had a noticeable power increase. However, the MCM will produce even more power with additional fueling from idle to redline.

 

FUEL BOWL VENT TUBE:
If, for any reason, your CVK does not have a open and clear vent to outside air, through the vent port and vent tube, you will never see proper engine/carb performance. With this venting restricted or worse, the carb cannot supply enough fueling for your bike. Your bike will exhibit crazy, possibly unpredictable, symptoms that you may not realize is simply because its' being starved of fuel under various conditions of operation.

 

Reserved for future information.

 

When you remove the metal top cover off the diaphragm, turn it upside down and notice the metal "tangs" that the slide spring fits onto.. These tangs index and stabilize the spring.. Now when you re-assemble the cover, make sure the spring is seated correctly onto the tangs on the cover..

Also before reinstalling, check the cap to make sure it is not bent, tweaked, etc, as it must be perfect for a proper seal on the diaphragm and carb casting.

If you don't do this, you will have aberrant slide behavior including complete sticking/freezing of the slide - usually at a high throttle opening..


It's one more item on the checklist to check off before you start bolting the bike back together.

Get it right the first time or re-do the project until you do...

 

I'll add that next time you have your 34mm carb off, check to see if the butterfly is precisely horizontal at WOT. If it isn't it's disrupting max CFM flow at WOT.

I noticed that just inside the engine side of my CVK there'd been some machining done at the carb factory, and there was a little ridge just in from the end a little. It's easily sanded away..

 

Understand that electrical and ignition problems can look, smell, and feel like carb issues on our little KLX's. One member, that I have been working with, has covered all the bases with the CVK - yet known working carb setups will not function properly. Only with the stock needle will his bike run correctly. We exhausted all knowledge, that we could find, regarding CVK issues..

Once we decided to look elsewhere, he pulled the coil, tested it, and remounted it. The persistent midrange stumbling was dramatically affected by this simple operation which indicates a direction for a "fix".. I will update this post as we solve his problem - but suffice to say, at a certain point in time, you have to trust that your carb is "ok" and start looking elsewhere.