Hey everyone. I'm not the only member to try this and I certainly won't be the last.

This thread will be used to serve as my tuning journal and discussion for the use of a TM36-68 on a modified 2009 KLX250S.

Mods are as follows:

• MCM
• FMF Q4 Slip on
• Airbox lid removed
• 13T sprocket

Initial fitment:

• VERY tight clearance between the bowl drain plug (why is it so damn big?) and the starter / starter cable.
• Tight clearance between the idle adjustment **** (removed the plastic **** fitting to make room - harder to turn now)
• It’s slightly too short in length from front to back. The rubber spigot boots fit, but just barely and take some loving to get set up properly
• Throttle cables have to be re-routed in a nonsensical way to reach, could potentially be risky - have to look at securing them or ordering longer ones.
• Throttle cable holder plate is cumbersome to use. The throttle cable hole is threaded and the return cable is not.
• The return cable fits easily with the use of the two nuts on the stock cables
• the throttle cable must be screwed into the threaded hole - this can be tedious and doesn’t allow for easy RE&RE of the carb. To remedy this, I simply drilled out the threads so it matches the the top holder and fasten it using the nuts again.
• The throttle seems to take less turning to open all the way - I personally prefer this since 250’s are typically WOT or not, you don’t generally modulate the throttle much like you would on a higher HP bike.

Initial set-up:

• I’ve dialed the accelerator pump back quite a lot. It begins to come on around 1/8’th throttle and finishes at ½ or so. I wanted to limit the amount of fuel coming into the engine as I knew the stock jetting would be too rich to start with.
• I’ve set the needle to the 1st notch - again since I knew it would be too rich given what I’ve read from other set ups using big bores.

Initial tuning:

• Pilot jetting is off - not sure which direction though (it was too lean)
• Turning the fuel screw in all the way causes the engine to bog and die - worse when warm
• Turning the fuel screw out about 3 turns yields the highest rpm for the give idle speed.
• turning the fuel screw out further doesn’t appear to cause the idle to dip - this confirms pilot jet sizing is out - too small or too big?
• Idle **** adjusted to 2k rpm - seems to help with the low end bog when accelerating below 3k rpm. - pilot jet issues again likely.
• mid end feels pretty good, a little flat around 4 - 5K, 6K feeling the most power, then flattening out again around 7k. Power delivery feels pretty equally flat through-out the range.
• After getting to operating temp, top end would miss a beat or bog a bit with snap to WOT - main jet seems too rich (stock is 130, so that makes sense).

First impressions:

• outstanding throttle response around 6k RPM. Power wheelies are pretty easy. sustained power wheelies in first gear are now possible on slight inclines. 2nd gear pop-a-wheelies (non sustainable) possible as well.
• Lost a lot of that low end bomb-proof chug-a-lug power the CVK had - hopefully the pilot jetting will resolve this. Some low rpm torque loss is to be expected with a larger diameter carb though.
• Top end wants to fly away despite being flat on power. Quite smooth power delivery all the way up past 9k rpm. Not as fast as the CVK with the 138 in it, but it’s also not jetted right yet.
• overall much more rideable than the CVK was with the 138 in it, but not quite as peppy as with the 128. Seems like a lot more power around 6k rpm now due to the AP, hopefully I can get the rest of the range performing the same.

 

August 6’th - Extended ride update

Current Set up:
PJ12.5, 9EBY01-50 Needle @ 1N, 568-P8 needle Jet, Main Jet 130. (stock parts)

Only change today was an adjustment of the A/P to come on sooner, and stop later. I thought this would help the fact that the pilot jet is a bit lean when rolling to WOT.

After the changes, the low end did feel much more responsive, I no longer have to be mindful of how much throttle I give it. That being said, it will still bog right out if I snap to WOT in first gear. Upping the pilot jet should resolve this issue a bit - but it's also going to be a characteristic of a flat slide carb.

The changes the A/P settings also helped with the top end lag when snapping to WOT, there is next to none now. Towards the end of my ride there was some stumble when snap to WOT and release, followed by a surge and back to normal.

aside from the Pilot jetting issues which I’ve masked with the A/P, the very start of the needle jet seems wrong (probably too rich). Tends to bog out under WOT load at 4K rpm…. nothing like the stumbling I had on my CVK though. If I do it right I can get the bike to get “stuck” at 4k rpm as it will not generate enough power to accelerate, instantly remedied by backing off the throttle for a split second and resuming WOT. I was moving too fast to smell if there was too much fuel or not enough. I’m not entirely sold on the needle being lean, but in a few week’s I’ll have an AFR meter on it as well as a leaner needle / needle jet to try. For the interim I suppose I could try putting the airbox lid back on - this would likely help with the lean PJ as well and if the mid range gets worse I’ll know it’s too rich. Given that most big bores are going leaner on their needle AND needle jet, I’m betting it’s too rich.

overall, the bike is a joy to ride. I lugged my way up a small hill in THIRD gear. Never done that before on that hill, it never even used to like going on in second from a slow start. Very nice!

 

August 23rd- 24’th - Workshop and Ride


Spent a day trying to dial in the carb before getting supremely confused and finally welding an exhaust bung for a wideband O2 sensor.

Arriving at 1200’ ASL elevation I opted to avoid riding the carb in the stock configuration before changing parts. Currently at PJ 12.5, 9EBY01-50 Needle, 568-P8 needle jet and a 130 Main jet.

Installed PJ 15, 9EBY01-51 (leaner) needle @ 3N, 568-P6 Needle Jet and 127.5 Main Jet. WIthout the gauge it was difficult to tell what was going on. The bike would almost not run from low RPM when slowly going to WOT. Had to back off the AP which didn’t help much. A couple runs up and down the road and I elected to go back to the stock needle 9EBY01-50 on the 1st notch. This helped the bike run better, but I would later discover that this was much too rich.

After fiddling with different needles and positions, I elected to install the Wideband AFR and gauge to properly diagnose the issue.

After installing the gauge the following observations where made:

Best idle appears to be around 12 - 13 AFR. That is to say that the middle of the dips is around this point. Running leaner resulted in 1/8’th throttle lean surging. This may be indicative of the AFR gauge reading richer than it should. It could also be causing surging in the carb which pulls extra fuel from the needle jet. Setting it around 13 for now (about 2 turns out).

9EBY01-50 needle seemed to be leaner under part throttle (at the 1st notch) - occasionally seeing 16.x AFR under 1/8’th - ¼ throttle cruise. Under ½ throttle cruise, AFR’s were rich at 10 - 11.0. Changing needles to the 9EBY01-51 resulted in a much cleaner 13.x AFR at ½ throttle but also an overly rich 1/8’th throttle cruise. Changing needle position from the 2nd notch to the 4’th notch leaned it out too much, again seeing 16 - 17 at partial throttle cruise. Dialing back to 3rd notch got me into a nice spot where I’d only see 10-11 AFR’s while under light load 1/8’th throttle cruise. 2nd Notch too rich. This is counter-intuitive to how the needle notches should work - take this section with a grain of salt.

Will need to try 568-P4 needle jet to try and get leaner partial throttle conditions and preserve richer mid-top range operation. Alternatively, trying to dial down the MAJ to 1.0 or 0.9 might be necessary but will require a smaller main jet.

Current main jet operation at 1200’ is about perfect at 12.9 - 13.5 AFR under load and WOT. Feels really solid. Moving to home elevation at 2000’ should dip the AFR into the 12.8 sweet spot as the air is slightly thinner.

Accelerator pump had to be dialed back as the bike began to get warm (warm enough for fan to engage). It was wanting to stall from an idle if you rolled onto the gas gently. Move the AP setting to engage at about 1/8’th throttle (almost as far back as possible) alleviated the issue and helped rescue some power from the bottom end. AFR gauge doesn’t update fast enough to accurately tune the AP so it’s based off of feeling and sound. It’s also tricky to adjust while it’s in the bike.


Overall impressions:
The mikuni and the KLX are extremely forgiving when running very rich. Seeing 10.0 AFR come up on the gauge while cruising and not having any major running issues aside from a small lack of power is pretty impressive.

Accelerator pump is difficult to dial in.

Pilot Jet is difficult to tune - can see why Mikuni suggests using the slow airjet instead. 12.5 to the 15 made it quite rich in operation under load. Despite everything being okay at an idle when adjusted for the middle of the dips. It was more responsive under load with the 12.5 but harder to start (and the fuel screw being 3.5 turns out).

 

August 26’th - Jets & ride


Changed the needle jet to a 568-P4.

Current set up: PJ15 @ 2 turns, 568-P4 Needle Jet, 9EBY01-51 needle @ 5N, Main Jet 127.5

This leaned out the entire operating range from 3k rpm to 7k. Part throttle operation was extremely lean now at 17:1 AFR. Half throttle, a touch lean at 14:1, ¾ - WOT around 13.5 - 14.

Adjusted needle position to 5’th notch (from 3rd). Part throttle afr’s now around 13, ¾ - WOT a tad richer around 13.5. Half throttle operation very rich around 10 - 11 at cruise around 6k rpm.

Will have to bump the needle back to the 4’th notch and perhaps go up in main jet size to the 130 to compensate for the narrower needlejet.

Mid range torque is awesome right now… good snap around 4k, lots of wheel spin. 1st gear low end rpm response is good and the AP settings seem happy right now.

Upper mid range torque takes a dip since it’s so rich, nothing terrible, but noticeable before transitioning to the main jet again around 7k.

Main Jet operating AFR's a touch lean at 13.5 - 14 once the input from the AP wears off.

It's important to note that the AFR gauge may not be very accurate due to the way its mounted. As such, the DIFFERENCES in the AFR values are what I'm trying to focus on for now. The top end seems to pull well with that value so I'm going to aim to see that value across the entire rpm range.

 

Do you have the AP shooting towards the bottom of the throttle plate?

I adjusted mine (on a KLX300) to start pumping just after the throttle started to open. And then adjusted the AP duration for the best throttle response from near idle.....blipping the throttle.

Ride on
Brewster

 

Sorry, forgot to mention...

the AP is aimed at the needle. There doesn't appear to be an vertical adjustment on it, it just swivels left to right.

 

August 29'th

Was having problems with the 9EBY01-51 needle being too lean around 3k rpm under part throttle, or too rich around 6k rpm under part throttle. (4'th and 5'th notch respectively)

I've switched back to the 9EBY01-50 Needle (richer), and set it at the second notch. It's MUCH better now. Much more even distribution across the entire RPM range. I'm happy staying with this set up currently. Might go back to the 130 main jet as I'm seeing 14.0 afr at WOT but it's cooler today and still pulls fine up top.

Current and tentatively final set up:
PJ15 @ 2-2.5 turns out // 127.5 Main Jet // 568-P4 Needle Jet // 9EBY01-50 Needle @ 2N.

Again this is on a stock bore KLX250s with a Mikuni Tm36-68. FMF Q4 Slip on, MCM, and no airbox lid.

Feels great and AFR's are staying between 12 and 14.5 across entire RPM range once warmed up.

It's extremely important that this carb is fully warmed up before taking impressions of the changes. I cannot stress enough how critical this is. I actually have reservations about this carb for people who frequently change elevation or experience large temperature swings - I'll keep reporting on mine as things start cooling off for the winter.

 

130, 130, 130, 130...

 

pwjm, I believe those are the components I've been using with perhaps the exception of the .9 air idle jet. You're definitely right about the engine needing full temp. On the altitude, when I go to 10K or higher for more sustained riding like around Silverton, CO, I drop the needle one notch and the main jet one step leaner(125), but that's only for some really sustained high altitude riding. Otherwise I just drop the needle and leave the 127.5 main in place. I'm going to Utah in a few days for about 2.5 weeks of riding in the mountains around Moab and Monticello. I'll hit some occasional 11K-12K heights, but it will be quick and won't need a main change...just a needle drop for me.

Edit: I've been thinking about my experiences of high altitude operation between the CVK34 and the TM68. I was always highly impressed by the CVK40 that was on my KLR650. It had a huge latitude of operation as far as altitude and jetting. My KLX250S was not that impressive. Yes, it's a smaller engine (air pump), so it's going to suffer. Still, I'm not that impressed with the CVK34 for high altitude latitude. I had to rejet the main and needle just like I do on the 36 pumper for really consistent high altitude. And oddly, where I would have assumed the accelerator pump to cause more difficulty at altitude...it didn't.

 

Update

Put the 130 main jet back in and it's helped the top end decently. It was quite cool today - about the limit i'd be riding at and once the bike warmed up it was around 13.5 AFR under most circumstances. I imagine once it warms up again it should be closer to 13.0.

I'm pretty happy with the bike like this, I think I'll be leaving it this way till the end of the season.

I wonder how our friend snowdrifter is progressing with his TM?