Your videos no longer work, but your prob sounds almost identical to what I began experiencing after drilling my slide as I described in the "Massive Fueling Differences on N1TC" thread. The 1N problem I described at the beginning of the thread is not related to the 4K sputter, it was caused by some sort of faulty re-assembly on my part-- but understand that the reason I tried the 1N to begin with was because of the 4K sputter at either 2N or 3N (dont remember)

My problem is a hard stumble/sputter starting at 4K and extending to about 6K, then the bike takes off like a rocket. As you stated earlier, when cold, the problem is either absent or much less noticeable.

When good and hot, if you go WOT in a taller gear from say 2.5K or so, the bike will accelerate until it hit 4K, then it will fall on its face, sputtering, sometimes taking SEVERAL seconds to pull out of that stuttering range-- when it hits about 6K its all good. This is happening at K148, N1TC 1N, lid on, DG-O Series with 1" baffle, no snorkle + 16- 1/4" holes drilled in the lid plastic to give a little more flow and minimize the grawl.

Today I took the carb off and completely down, inspected everything from float level to emulsion tube to diaphragm to needle, etc. Found nothing.

SO, I replaced the N1TC, which was running at 1N, and replaced it with the stocker needle, shimmed with the brass shim that usually goes on top of the N1TC. This makes the stock needle sit about 1.5 to 2 mm lower than N1TC at 1N.

To check, I examined the position of the bottom of the needle after installation by pulling the MJ/emulsion tube and compared the two. N1TC at 1N sits with the bottom of the needle pretty much flush with the bottom of the threaded aluminum that the emulsion tube screws into.

The stocker with the shim sits about 1.5 to 2mm below the bottom of the threaded aluminum the E tube screws into.

When I reassembled all, the problem seemed to be gone, I was certain that the stocker would not overfuel. However, after about 10 minutes of putting in the yard, popping up on small berms on the side of my driveway culverts, I took off down the road try some taller gears. Sure enough, it was still there-- about the same as before.

Now, before I go any further-- I want to make it clear that I did not EVER notice this problem prior to drilling my slide. Only thing that makes me not even more certain about it is the fact that I both drilled the slide bleed hole (7/64") AND went from a K138 to K148 at the same time.

Granted Im leaning hard towards the slide, I do find it strange that the problem occurs at exactly the same RPM range regardless of N1TC 2N, 1N, or stock needle shimmed (which is about equivalent to N1TC 0 or -.5N).

I ALMOST put my K138 main back in when I had the carb apart, but stopped short-- heeding the cardinal rule of making 1 change to the carb setup at a time.

The videos you posted dont seem to work anymore-- can you re-up them? Ill see if I can rig up my sons Ipod to provide a video of my bikes problem.

 

Snowdrifter and PWJM both "fixed" their issue with a TM36-68 replacement. Snowdrifter sent his carb to me and it worked perfectly on my bike.

Originally, Snowdrifter was wanting to blame his MCM, PWJM did mention blaming the 7/64 slide lift hole at one point, but then after "handling" his coil the stuttering stopped for a while..

Your stuttering has appeared, gone away, and reappeared..

I worked with PWJM on google docs and on the phone for about a month, then he posted here for more ideas.. No "fix" was found for his bike. It would run well at sea level but not at his altitude..

Both issues went away with replacement - Snowdrifters "bad" CVK ran my bike without any problems - Solution for this is still unknown.

 

Folks have been drilling the lift hole for years now - the DJ 2206 supplies a bit and directions to drill it.. A properly drilled lift hole simply can't cause this - we would have known of such an issue years ago..

 

^^
Had the problem not occurred immediately after drilling the slide (and upping the main) I would agree. But I made the changes and it appeared. If its not either of those things, 1.) Its not carb related and 2.) It is something else with the engine, valve clearance, etc that happened at a very conincidental time.

Who knows, perhaps I had a bad drill bit (though it was brand new), or perhaps the bit could have wobbled slightly during the drill and opened the hole slightly too much (I was careful, but until I find the culprit, all bets are off, not ruling out anything).

lm working on an ingenious (or just plain ignorant, depends) way to test the slide bleed hole theory.

I just removed the slide from the carb, I then cut out a appropriately shaped and curved piece of plastic from a milk jug and I have set and glued it using industrial strength silicone based glue, over the bottom saddle of the slide. Once it is set, I will use either a 3/32" or 5/64" bit to drill through the jug plastic over the existing slide hole.

Of course I will also have to drill the hole for the needle as well. If I can re-create the original diameter of the bleed hole (can anyone comfirm if its close to 3/32"?) and the setup holds up, I should be able to confirm or rule out the bleed hole thing.

Of course, next after that if it doesnt work is to go down on the main. If the glue fails and the plastic gets sucked in, Im pretty confident it will get burned up and spit out without much risk of damaging anything.

 

No, I wish it were that easy.

The piece of milk jug plastic I glued underneath the slide has NO holes in it at present time. I must create TWO holes in it. One for the bleed hole (which will be drilled LESS than 7/64" to effectively go back to stock), and one for the needle to pass through.

 

Remember that on one occasion when you "handled" your slide assembly, the problem went away.
It's ashame the installation of the stock needle didn't also "cure" the problem..

One thing is for sure, combining all comments, both recent and old, it seems possible that this is aberrant slide behavior in a small part of its' travel in certain situations. A camera set into the air box looking down the carb throat, with some kind of lighting, could verify this..? Certainly it is sticking, fluctuating wildly, etc, if its' a slide ops problem..?

 

It's frustrating, but try to not forget that you are running a "known working" setup with (K148/N1TC@1N, 7/64 lift hole, lidless, slipon) - it worked perfectly on your bike, for a time..


Quote:
Well, I really dont know what happened the first time out with the #148 and 1N, because I dropped to 1N tonight to help with the midrange overfueling and it worked great. No more sputtering/hesistation when wicking the throttle down low. No starvation either. 30 minute ride and it performed as good as Ive ever seen it.

Really good feeling lows, decent mids (absolutely no sputtering or detectable bog) and hard hitting top end. None of the fuel starvation, 50mph top speed the last time I used 1N. The only possible explanation, as KLXster alluded to before, for the previous starvation at 1N was that there was some kind of vacuum leak and/or the diaphragm was stuck and couldnt pull up to let the fuel up. As I said before, when I moved off of 1N back to 2N after experiencing the starvation problem, the diaphragm did not appear to be out of place or pinched.

In any case, Im extremely thankful I have a good working setup.

Now, the power on this bike still comes on really hard right at 6500 rpm, even with WOT say from 3000 rpm-- so this surge of power can only be explained by the dynamics/harmony/resonance of the intake and exhaust /cam setup. Its very , very obvious that something special happens at that rpm-- I checked it several times in several gears watching the tach and the surge ALWAYS comes on at the same time, ~6500 rpm.

My Mojaves simply do not do this, they have a much more even and smooth powerband, despite a nearly identical engine. Im getting more and more tempted to try the MCM.

 

^^

I will say this about the above-- when I went back to 1N the second time (to try and stop the sputter I got at 2N), it was only a local ride, no woods, and short-- 20 minutes or less-- its possible I didnt test it enough before I posted the "good news".

Now, another update:

So I had to work OT the last few days and just got around to trying my "milk jug" slide modification tonight. Popped a hole small enough to accomodate the needle in the center of the jug plastic, then drilled a 3/32" hole over where the bleed hole is. Not sure if 3/32" is exactly stock, but its quite a bit smaller than 7/64".

So, the bike fired right up with the slide mod, revved freely and quickly in neutral-- in fact, putting around the yard, there was absolutely no difference I could tell from the drilled slide as far as response.

So, I take off the airbox lid completely just to maximize air flow (my lid already flows freely with no snorkle + many 1/4" holes, but still). While bike was cold, no stutter at 4K when wicking throttle from low RPM in high gear when cold (just as before). I ran around the yard a bit and took off down a gravel road across from my house. It hits a chip-seal road a mile down where I can run it hard.

Unfortunately, still get the stutter and bog at 4K once the bike is hot. The slide made no difference. I once again played with the choke and it makes things worse, sometimes the sputter will start around 3 to 3.5K with choke.


3 things I about this problem I am absolutely certain of now--

1.) It IS a fueling problem. Prob gets worse/more pronounced after engine warms and/or choke **** is pulled.

2.) It is NOT needle related. Going to stock with a shim put the needle quite a bit lower than what would be 0N on the N1TC and still had the prob.

3.)It is not related to the 7/64" slide drill. The milk jug mod worked, and problem was still there. BTW-- I'll see how it held up when I pull the carb off next time. I really think this could be a legit way to fix a slide that has been overdrilled or if someone wants to return to stock. Only thing I need to nail down is the best glue or epoxy to use to attach the plastic.

SO-- next time, before I touch the carb again, Im going to try running with no air filter at all-- this should give maximum possible air to the carb with the idea it might be enough to overcome the presumed richness I hit at 4K with WOT. Just a quick test to see how the issue responds.

My next step will be to drop to a K138 main-- when I went to #148 I drilled the slide at the same time-- Ive seemingly eliminated the slide drill hypothesis, next is the smaller main. If I still get it at K138, Ill drop further, probably to either a DJ 126 or K132-- Im 100% sure the issue was not there with the DJ126 (which I ran prior to getting my DG) and Im 99% sure it wasnt there with the K138+DG.

One of these tries Im going to nail the bastard-- Im not giving up! If things really get hairy, I have some stock Mojave needles which are shorter and thicker than the N1TC or stock needle. I might throw one of those bad boys in there along with the Mojave needle jet just to see what happens.

 

It would sure be nice if you can actually find the problem and fix it. While it seems this issue is relatively rare, it has no known solution..

As you have found, this is not a tuning issue, it is a malfunction. Since Snowdrifter and PWJM cured it by replacing the CVK, perhaps it is a malfunction of the carb.

So, it would seem to be a "fuel dump" starting at 4K. If the slide is functioning correctly, it should be beginning to open @ 4K WOT from a 3K snap - so it's not possible for that to cause a fuel dump. Just thinking out loud that if it is stuck fully open or near fully open, there would be a fuel dump when the butterfly snaps to WOT, until engine RPM increases enough to handle the rich AFR of a fully or near fully raised slide - which is, oddly enough, about 5.5K rpm..

So if this fuel dump is coming from the emulsion tube, and the needle/seat is not causing flooding, it can only be a stuck/sticking slide..?

 

My theory right on on it is this:

1.) The slide is not sticking, but going full open too quickly, before the engine RPM is ready for the full main and/or when the slide first gets to that point, a surge of fuel that is not atomized enough to combust properly is being sent to the cylinder. The atomization/vaporization of the gas in the bowl is achieved by the combination of the MJ, emulsion tube, needle position, needle jet being used, and of course, the velocity/neg pressure of the air in the carb throat. Could be one of many or a combination of things.

I am confident the slide is not sticking because when sitting in neutral, I can control throttle with quick snaps or gradual roll ons and the engine is very quick to rev up or down. A stuck slide at any point would prevent such responses.

2.) The slide is going full open when it should, but the K148 main is just allowing too much fuel in too early, despite the needle and needle jet. A smaller main may cure the problem, at the expense of upper range/top end power loss. No idea on why many do not have this problem but my bike does.


Since I refuse to pull the carbs back off my good running Mojave's ( they are 3 times more PITA to pull than the KLX), I been looking for a spare carb. Last night I sniped one one Ebay for $68 shipped. It comes off of a KLX 300 which means it includes an N1TC AND the OEM KLX 300 needle jet, which most of us 250 owners do not have.

When I get it in I will check the jetting, etc. and throw it in (assuming I dont have the problem fixed yet). Things should really be getting interesting by then...