new SmartCarb SC3 34mm advanced metering rod carb option

[Chuck78]

Looking into the newly released SmartCarb SC3 34mm, this carb very much appears that it will be a good fitment into the '95-'06 KDX200H & KDX220R chassis, unlike the completely non-fitting previous generation's SC2 36mm version, or even the older Lectron carb versions which are a poor fit (although they in fact can be run on the '95+ with mods/compromised mounting angle/stock reed cage only).

images-1.jpeg (37.61 KiB) Viewed 30868 times

The intake spigot looks to be 1mm shy of the diameter of the PWK33/35 OEM KDX carbs, but I'd imagine this would still be quite workable and clamp down just fine into an OEM intake manifold boot with VForce4 or a Boyesen RAD Vapve intake coupler.

It also appears they will require the long body adapter (Same as the 34mm Lectron Billetron Pro Series 38 carbs do. This is optional with the purchase.

SmartCarb_SC3_32-34mm_Dimensions.jpg (465.02 KiB) Viewed 30868 times

For $20 on their website, they do offer a 3D printed mock-up of these carbs for test fitting onto bikes which they have not yet verified fitment on.

https://smartcarb.com/product/32-34mm-s ... -3d-model/

With as frequent of requests they get for SmartCarb offerings to fit the KDX200/220 based on what they told me 1-2 years ago (they said at least once or twice a week they get a KDX owner contacting them wanting to buy a SmartCarb, but had nothing that'd fit the 95 & up KDX frames until now), I'd imagine at some point they are going to have a published fitment offering for our bikes, but for now, we can only wait and see. An inside connection told me that they were very much designed to fit the '95+-'06 KDX frames, as well as the 32 mm version which looks like it was designed to fit some other smaller enduro and trials type bikes. The 38mm & 30mm versions as well, which might make a very potent woods-modified KX112 SuperMini 2-stroke as well!





I think I'm going to pick up one of these this winter for a more Hard Enduro focused torque monster tractor engine build out of my spare 220 with a ported 70mm bored 223cc cylinder and a slightly modified KDX220SR head (come stock with much tighter squish clearance but a very wide squish band around 13.5 mm versus the Ron black carb at 9.5 mm which is close to stock but tighter squish clearance. I think I'll pair this with a stock pipe or Gnarly Woods pipe, & the SC3 34mm with its variable shaped Venturi should be a great carburetor for this engine as the low throttle opening area is much narrower since it is egg-shaped at the bottom, giving higher air velocity at low throttle for more low end torque and better throttle response when cracking the throttle open from lower/mid-RPM cruising position. In addition to that, I speculate it might be larger overall carburetor venturi bore area than a circular 34 mm carburetor bore, so it's likely it may flow every bit as much as a PWK 35 at wide open throttle, while having better throttle response down low than a PWK33.





These are a very advanced carburetor, and perhaps the best carb on the market as a whole now.
The Lectron Billetron Pro Series w/ Xcelerator metering rod is also a listed fitment for our bikes now. I was the first guinea pig for this after making connections with Matt Denecke who formerly worked at both Lectron and was the founder Apex cylinder heads, and now has his own company making prototype YZ400 2-stroke engines and YZ125/250 cylinder heads.
The Billetron Pro Series is an incredible carburetor that utilizes both the metering rod design and two supplemental thumbscrew adjustable jet circuits to make tuning a lot easier I need to change any brass parts.
The Smart Carb SC3 is even more advanced and has a few additional features, they have an internal venting design as well as spill proof design feature with a check valve so that no fuel will spill out when you lay the bike down on its side, as well as the advanced cloud forge venturi design I believe they call it, how it's narrower at the bottom for better air velocity and throttle response. Additionally, the Smart Carb delivers ALL of it's fuel off the full exposed height of the metering rod instead of having one or two supplemental jets as the Lectrons and the XTNG (doesn't fit the KDX200H whatsoever) carbs do.
With the fuel across the entire range delivered solely off of the metering rod and no supplemental jets to introduce larger less atomized fuel droplets like the standard carbs do, as well as no supplemental jet tubes that disrupt the laminar air flow through the venturi bore, a Smart Carb with the proper metering rod and proper adjustment will deliver the most complete combustion and highest fuel efficiency possible out of any carburetor, often surpassing that of modern fuel injection even. The Lectron Billetron Pro Series carbs have been said to outperform modern KTM fuel injection systems already... The Smart Carb when fully optimized promises to deliver even better.

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I have a PWK33, Ron Black modified 36mm Keihin PWK 35 Air Striker Quad Vent with divider plate added, an old style legacy era Lectron 34mm Power Jet carb, & Lectron Billetron Pro Series. The RB'd PWK35 Air Striker and the Billetron Pro Series Xcelerator metering rod carb are my favorite out of those, but I believe this SmartCarb SC3 34mm will be right up there at the top as well alongside the Billetron Pro Series Xcelerator. I definitely would like to get one of these this winter and run it on the hard enduro engine build. I may even weld an o2 bung into my expansion chamber + and exhaust head pipe temperature sensor if possible, so that I can test air fuel ratios and attempt to perfectly optimize the metering rod tune.

A purple anodized SC3 34mm may very likely find its way home to one of my machines this next season, if not the green anodized version...



We've now got TWO incredible options now for aftermarket carburetors thanks to these two good companies.

[brademan76]

great info and cool bonus that they come in different colors!

[Chuck78]

I read some comments online about the anodized body versions. These SmartCarbs are all made in the USA, however, they were not happy with ANY anodizing company that they had tested here in the United States (they tried more than a few), and actually send the carb bodies to Europe to be anodized I believe in Germany currently, so the custom color requests take quite a while to be delivered due to this. Unless it is something they sell a lot of and may have stock in a certain carb size and color, it may take a good 4-5 weeks to get something back, assembled, and delivered.

[kdxdazz]

All sounds great , my concern is the right metering rod being available, if no metering rod works then it's an expensive paperweight, only from my experience with the older style lectron trying to get it dialed in proved impossible, but a couple of years experience up my belt and the wideband O2 sensor means I might give it another go
I do like the smooth idle of the metering rod carb

[Chuck78]

All sounds great , my concern is the right metering rod being available, if no metering rod works then it's an expensive paperweight, only from my experience with the older style lectron trying to get it dialed in proved impossible, but a couple of years experience up my belt and the wideband O2 sensor means I might give it another go
I do like the smooth idle of the metering rod carb

100% my concern as well. I feel they definitely have the metering rods available, however, they are cost prohibitive to purchase several different metering rods to tune with,so you'd be stuck installing, testing & re-figuring richness vs leanness on various throttle positions, ordering another costly metering rod, waiting, installing and testing etc potentially going through a couple of rods before you're satisfied with having iseal exhaust gas temperature and AFR and ideal piston wash pattern (which only works with certain premix oils as I've noted, some are meant to burn extra clean and leave no carbon traces pattern on the piston crown, eliminating a very useful method of judging an ideal mixture for mid to upper throttle positions in the powerband).

I still feel the desire to buy one and get it set up for the ported 223cc cylinder and the KDX220SR head in order to have a very torquey responsive engine with primarily very strong low and mid-range grunt.

Given their "CloudForge" venturi design, I feel the SC3 32mm might be the total equivalent of the KDX220R PWK33 airflow area, as unless the 32mm version measures smaller than 32mm round (+ the extra cutout at the bottom that is more narrow adding more cross-sectional area).

I have a prototype SC2 36 body they sent me for KDX test fitment 2 years ago, which did not fit at all.
I'll have to measure it's i.d. and see if it's an actual 36mm + the extra cutout at the bottom that nakes their venturi bore an upside down egg shape, or if the 36mm is just the equivalent of a 36mm round venturi with the lower criss sectional area added to the remainder.

I'm sure with the 220SR head + cylinder being factory matched to an old fashioned round slide 28mm, the SmartCarb SC3 32mm would still do quite well on my proposed setup, although there may be almost no compromise in stepping up to the 34mm version of the same body for long term flexibility if I choose to run more of an RB'd head profile with it to support more upper RPM power.

[kdxdazz]

Just a thought on the keihin 28mm, you would need the matching carb boot and and plastic reed Insert, then you will need the smaller air boot to the airbox and ideally the smaller snorkel, all standard on the SR model , without these you will lose airspeed/venturi
My keihin pe28 is the Thai version that came out on the nsr150 so it's slightly different than the original Japanese version, it has holes in the emulsion tube and a replaceable needle jet, I think it's a copy version but here in Thailand parts are so cheap for it I can't go past using it, a new slide and needle is 7 dollars as an example of cost, needles are available from Japan through webike, also have the option of original honda nsr150 needle that costs just 3 dollars but that needle is too rich on the straight diameter
I think I will have to look at making a custom needle jet, stock is 2.60mm inside diameter but that is too rich for Thailand close to the equator, long term I'm hoping to get it dialed in, get the lectron dialed in in the other bike and do a back to back shootout on tight single track, Is a thousand dollars lectron really much better than a 50 dollar keihin pe28, thought that would make for a very interesting YouTube video

[Chuck78]

Just a thought on the keihin 28mm, you would need the matching carb boot and and plastic reed Insert, then you will need the smaller air boot to the airbox and ideally the smaller snorkel, all standard on the SR model , without these you will lose airspeed/venturi

Interesting thoughts there, kdxdazz. The airbox boot and inlet manifold would work well for a PWK28 carb or the Lectron Billetron mini as well, or the SmartCarb SC3 28mm or 30mm versions... I'd figured one would have to machine some adapters to fit them! I won't be trying this route though personally.

My keihin pe28...
I think I will have to look at making a custom needle jet, stock is 2.60mm inside diameter but that is too rich for Thailand close to the equator,
long term I'm hoping to get it dialed in, get the lectron dialed in in the other bike and do a back to back shootout on tight single track, Is a thousand dollars lectron really much better than a 50 dollar keihin pe28, thought that would make for a very interesting YouTube video

I don't believe I'd spend any money or any significant time on attempting to extract maximum performance out of a 1960's/1970's technology round slide carb like the PE28, & instead, I'd be looking at a PWK28 from the SuperMini motocross bikes or better yet from a trials bike application if you can find a deal (the needle might be more appropriate for the KDX220SR cylinder head squish band profile there from a trials bike application PWK28).
I several 4-cylinder racks of sought after high performance Keihin CR31 round slide smoothbore carbs ($1000 new), and 1 rack of the equivalent Mikuni VM29SS smoothbore carbs for my '77-'79 Suzuki GS street bikes, & at this stage of understanding performance engine tuning, fuel atomization and jet signal etc, I'm contemplating selling them all in favor of dropping big money on Mikuni RS34 flatslide carbs with accelerator pumps due to their superior jet signal and fuel atomization leading to better performance.


Surely a 28mm OEM take off carb vs a $749 Lectron Billetron Pro Series or $649 Smart Carb SC3 34mm shootout would make a great read, but due to the technological advancements from round slide basic old school carbs to the flatslide technology, it'd only be a novel read with minimal interest for most. A PWK28 vs those high dollar carbs, however, on a 220SR, would be a very immersive read in terms of a budget low speed technical trails carb alternative.
After the Lectron Billetron Pro Series carb came out, the first guy I talked to at Lectron was adamant about me experimenting with the Billetron mini on my 220R, statng that he'd dome the same on a KTM 250 XC-W with incredible results for low speed technical terrain... He insisted the Billetron Pro Series would not fit the KDX whatsoever. Well, I talked to an inside connection at Lectron, and got a scrapped Billetron Pro Series carb body to test, and with the manifold spigot diameter turned down to KDX PWK diameter in the lathe, and the airbox bell long body adapter extension turned down closer to the KDX airbox boot diameter, the Billetron Pro Series fits the KDX220/200H far better than their listed Lectron Legacy Power Jet carb fitment does, so I went that route and am very glad I did.

Adapting a PWK28 on a budget would be my direction for the 220SR if I were you, dazz... You're investing in old school technology (round slide) when you could have a modern more advanced proven superior carb design in place investing similar labor hours into with greater successes in the end. I hope that sits well with you. I know the PE28 carbs are plentiful there, but I'd want to strive for the best possible setup.

[Kawibunga]

You know, I had asked them for some very basic dimensions in the past so I could 3D print and test the fit, but they weren't interested in giving me the info......... kind of burns me I gotta pay for the 3D model when it costs them nothing, but it what it is......... let me know if and when you test fit one Chuck! I won't get to printing and fitting one until likely December sometime.

[kdxdazz]

You may be right on the flat slide PWK 28 being the better option, I'm just so impressed with round slide.carb I have, the way it responds to air screw mixture adjustments, I also have a copy pwk30 sitting on the shelf, I could connect that up to compare to the round slide and connect the wideband to check the AFR is dialed in. Anything genuine keihin is not really an option here in Thailand unless it comes from the older honda nsr150 and Kawasaki kr150

On a different note I did contact Matt denecke about 2 years ago to tell him the problems of dialing in a lectron close to the equator, he told.me that building up the needle with a thin layer of fuel proof epoxy can be done so that's what I will try in combination with the wideband sensor, I didn't have the wideband sensor last time I tried setting up the lectron so I'm much better prepared. I have a fuel resistant 3M product that can be thinned down with acetone to give a very fine layer on the metering rod

[Chuck78]

You know, I had asked them for some very basic dimensions in the past so I could 3D print and test the fit, but they weren't interested in giving me the info......... kind of burns me I gotta pay for the 3D model when it costs them nothing, but it what it is......... let me know if and when you test fit one Chuck! I won't get to printing and fitting one until likely December sometime.

This 3D printed model that you can buy from them for $20 per carburetor is a new thing which I have never seen. They also offer the 3D print file download for free if you want to print it yourself, which I believe you may have been referencing.

I'm pretty certain these will fit our bikes especially since they are similar to the Billetron and that they are a short body carburetor requiring a long body adapter, an extension on the airbox side, to span the gap. This also means that you can very likely have zero problem fitting the Boyesen RAD Valve intake or Moto Tassinari VForce4 reed block assembly, since both of those take up additional space over the OEM reed cage. The Boyesen RAD Valve with the SmartCarb SC3 34mm and an RB'd 220 head or KDX220SR head would make a great combination for low end torque if the SC3 34 will accommodate the extra thickness of the RAD Valve pushing the carburetor back several millimeters.

I believe one of the big design pushes for this size in particular was specifically for the KDX fitment which they were completely missing on their previous SC2 36 model. They had told me in the past that they would get one to two requests per week from KDX owners who would be disappointed in finding that they cannot fit the SC2 36 or any of the other available options back then to their bike.

The only issue I have is that something like just changing a pipe from a Pro Circuit Platinum 2 to an FMF Gnarly Woods will very likely require a different metering rod. Metering rods are not cheap like $16 OEM Keihin jet needles or Keihin jets. If one has the time to swap metering rods and wait for them in the mail and test, these carbs are absolutely going to be the best setup.
Perhaps one could negotiate to buy several different metering rods at a discounted price from SmartCarb in order to test them out for best tune.
As my friend found with his billetron Pro Series however, after having it dialed in on a Dyno and running it for a couple of seasons, not touching any settings, after doing a top end and gaining a lot of compression, the carburetor ran horribly! Luckily on the billetron pro series, you have full adjustment of the throttle range split into 3 different adjustments. But as Smart Carb will tell you, this is due to imprecise metering rod selection, relying on inferior jetted fuel delivery as a supplement on the low end and top end to find tune, which also adds airflow disruption from the jet tubes protruding into the venturi, disrupting the laminar airflow qualities... So a perfectly tuned Smart Carb on a setup that is not going to be changed is the best setup overall but takes potentially a lot more work to get dialed in to be absolute precision fuel delivery meeting the requirements of the engine, versus the Billetron Pro Series we'll call the number two best overall, but number one easiest to set up.

[Kawibunga]

Thanks for the heads up Chuck78! I didn't read close enough and thought you had to pay for the file itself. Already download the files the checking them out. I thought they were charging for the files, nice to see I was wrong!

[Chuck78]

Thanks for the heads up Chuck78! I didn't read close enough and thought you had to pay for the file itself. Already download the files the checking them out. I thought they were charging for the files, nice to see I was wrong!

In more assuring news, I was just digging through a messages with a semi-local guy here who has many industry connections, searching for our convo on local suspension tuners (Corey @ ProTune Suspension & Levi @ Bonzi Suspension), & stumbled onto our June chats on SmartCarbs:



"He (Cory@ SmartCarb) claimed that the 34mm will be a perfect fit (SC3 34mm on the KDX200H)!"



They recognize how many water cooled KDX200/220 bikes are STILL in service out on the trails, due to their frequent weekly+ requests for SmartCarb offerings from KDX200/220 owners / potential customers...
If they can nail the metering rod tune for the 200 & the 220 with the aftermarket pipes, I think they'll really reduce Lectron's Billetron Pro Series sales to the KDX community, but the SmartCarb tuneability added labir efforts will also deter some, who will ultimately bad-mouth the carbs simply because they did not want to take the time to play with different metering rods to achieve the ideal tune... Other than that factor, these really I feel are the ultimate carbs for woods riding.
I'm not selling my Lectron Billetron Pro Series nor my 36mm RB'd PWK35 Air Striker Quad Vent, however! All are excellent carb choices!

[Chuck78]

Already download the files the checking them out. I thought they were charging for the files, nice to see I was wrong!

Update - these carbs should be available for our KDX200 & KDX220 applications by the end of January 2026, just a few weeks away!
They are dyno testing them currently, to get their settings dialed in just right...

"We are within a few short weeks of having a SmartCarb kit ready for both KDX bikes, 200 and 220. As you know, the issue hasn't historically been the metering rods but rather the fitment - our best seller, the 36mm, is physically too large to fit without hard frame contact. As such, we've been waiting until the release of our 30, 32, and 34mm model SC3s - the 34mm size being targeted toward the KDXs to replace the stock 35mm Keihin.

That Keihin is an odd carb as far as external dimensions, but we've designed our 34mm to fit the exact same as it along with the VM and TM 34mm carbs in other applications.

See attached pic of a KDX 200 we have in-house for dyno validation these last few days.

IMG_4483.jpg (3.86 MiB) Viewed 20057 times

We'll reach back out to you in the next few weeks once we're ready to launch the new sizes with solid tuning baselines.

Best,

Nathan Dyess
Information | SmartCarb Fuel Systems

Office: (816) 255 2328"

It looks like they're using a Yamaha WR250 4-stroke fuel petcock on that KDX200, although that may be unrelated and may have already been a prior upgrade done by the bike's owner, however, the Lectron Billetron Pro Series carb that's similar size did not play well with the massive stock KDX OEM petcock, and the bottom outlet Yamaha YZ250X 2016 petcock allows the aftermarket carbs to fit far better with no petcock issues. The WR 4-stroke 2000's model, NOT the WR250Z 2-stroke petcock which is actually the same as the 2016 YZ250X petcock) , may work well on KX125 / KDX200/220 hybrid builds with the rearward facing side outlet vs the YZ250X bottom outlet.

[Chuck78]

I did contact Matt Denecke about 2 years ago to tell him the problems of dialing in a Lectron close to the equator. He told me that building up the needle with a thin layer of fuel proof epoxy can be done, so that's what I will try in combination with the wideband sensor, I didn't have the wideband sensor last time I tried setting up the lectron so I'm much better prepared. I have a fuel resistant 3M product that can be thinned down with acetone to give a very fine layer on the metering rod

kdxdazz,
I just thought of something else that I personally would prefer to try instead when modifying a metering rod.
I would protect the mounting end of the metering rod and clamp it lightly into vise jaws, hanging out the side, with the flat side down of course, and after it's been degreased well and hit with maybe two quick light singke passes of some fine emory cloth, wipe some flux only on the flat surface particularly where you want to build up the profile, and then heat it up with a torch and apply some solder to only the bottom portion, so that no solder migrates to the round remainder front and up to the sides of the flats portions. If the flux is only on the bottom, the solder will not travel onto the round portions. If any is accidentally smeared into the areas you do not want solder, a quick wipe with a rag with acetone or similar on it would help clean it off.


Now you got me curious as to how much of a difference some added height (lateral build up on the thickness of the rod taper) to the metering rod would make there I assume you're just building up the flat portions?

This brings up the question or curiosity as to how the metering rod actually meters. It is a flat backside ground into the rod at a tapered angle, so I'd imagine the only portion that performs fuel metering would be the amount of unrestricted open space in what you would otherwise call the needle jet in the carb body, versus the flat ground onto the back side of the metering rod. Therefore, grinding more off in a certain area with a nice smooth tapered profile will give more fuel delivery in that range, building up with fuel resistant epoxy or solder will reduce the amount of fuel that is allowed to wick or be siphoned up the metering rod flat backside due to the pressure differentials. The actual width of the flat doesn't have as much to do with it other than being a function of the amount of space between the flat and the backside of what will refer to as a conventional needle jet revised for these carburetors.


One should be able to do tests through the various throttle ranges at various RPMs using the single external metering rod height adjustment. If you can get it to idle good but the mid-range is lacking, you can adjust the metering rod to give a strong mid-range and note whether it's leaner or richer in the mid-range at the RPM range intended, and limp it along enough to keep it idling down low just for the test purposes. You can perform the same test for wide open throttle, and note whether higher/richer metering rod or lower/leaner metering rod fixes the individual throttle ranges at the specific Target RPM ranges desired. Then you can go back to Smart carb and tell them what metering rod height you are particular metering rod required in order to get a good idle, then tell them whether the mid-range or the top end was falling short on fuel metering requirements, and tell them whether the mid-range need richened up, and how much on the metering rod thumb screw clicker adjuster was required, and likewise with the top end. With this technique, it should be fairly simple to get the correct metering rod, although if your initial metering rod is not near perfect tune for your specific engine and exhaust configuration, then you may need to swap out to a different metering rod. They are costly, but in the end, this is the absolute ultimate fuel metering / atomization delivery setup for our bikes...


The SmartCarb SC3 34mm is an expensive carburetor, but it will be slightly more affordable than the Lectron Billetron Pro Series, and since Matt Denecke has left Lectron (solely to move where his wife's career took her), and Nathan Dyess and company are a very talented young fresh team of engineers who are very dedicated specifically to dirt bikes and enduro riders, the Smart Carb makes quite the excellent choice.
The Lectron Billetron Pro Series Xcelerator metering rod carb will be quicker and easier to tune, with very similar technology and benefits, but the Smart Carb is worth putting the extra tuning time into. I can't knock either one, each one has its benefits, these are both the best offerings on the market for carbureted enduro bikes by a longshot.

[Chuck78]

Already download the files the checking them out. I thought they were charging for the files, nice to see I was wrong!

Update - these carbs should be available for our KDX200 & KDX220 applications by the end of January 2026, just a few weeks away!
They are dyno testing them currently, to get their settings dialed in just right...

"We are within a few short weeks of having a SmartCarb kit ready for both KDX bikes, 200 and 220. As you know, the issue hasn't historically been the metering rods but rather the fitment - our best seller, the 36mm, is physically too large to fit without hard frame contact. As such, we've been waiting until the release of our 30, 32, and 34mm model SC3s - the 34mm size being targeted toward the KDXs to replace the stock 35mm Keihin.

That Keihin is an odd carb as far as external dimensions, but we've designed our 34mm to fit the exact same as it along with the VM and TM 34mm carbs in other applications.

See attached pic of a KDX 200 we have in-house for dyno validation these last few days.

IMG_4483.jpg

We'll reach back out to you in the next few weeks once we're ready to launch the new sizes with solid tuning baselines.

Best,

Nathan Dyess
Information | SmartCarb Fuel Systems

Office: (816) 255 2328"

It looks like they're using a Yamaha WR250 4-stroke fuel petcock on that KDX200, although that may be unrelated and may have already been a prior upgrade done by the bike's owner, however, the Lectron Billetron Pro Series carb that's similar size did not play well with the massive stock KDX OEM petcock, and the bottom outlet Yamaha YZ250X 2016 petcock allows the aftermarket carbs to fit far better with no petcock issues. The WR 4-stroke 2000's model, NOT the WR250Z 2-stroke petcock which is actually the same as the 2016 YZ250X petcock) , may work well on KX125 / KDX200/220 hybrid builds with the rearward facing side outlet vs the YZ250X bottom outlet.

Another local KDX guy here that has a lot of industry connections just called & talked to Corey @ Smart Carb, who said that they will have these ready to release within 2 weeks! I think I may be one of the lucky guys to get a discount on this carb beta tester / demo tester due to knowing David who's in contact directly with the guts at SmartCarb, and already having done some machining to an SC2 36mm carb body last year in attempts to show Smart Carb how they fit or rather DID NOT fit our bikes via my test fitting and sending them some videos last year, hence this new carb is designed with a smaller body for several bike models, but very much so for the KDX200/220 fitment.

[billie_morini]

Interesting update. Have a friend with a mid-1960's Triumph Tiger he restored. It's a beautify bike and fun to ride. My friend hopes to fit this Tiger with one of these "carbs."

[Kawibunga]

That's awesome news about the being a tester chuck. Looking forward to your results!

[Chuck78]

That's awesome news about the being a tester chuck. Looking forward to your results!

Well, the beta testing was really already done, on the dyno at the SmartCarb headquarters, with two generous KDX owners' bikes, but I was primarily referencing my efforts last year to work remotely with them to test fit and try and do machining to the body to get the SC2 36mm to fit the KDX's. I turned down the spigot to fit the KDX intake manifold boot, and even cut it offset sideways slightly to help in distancing it from the frame, but there was no way, as I showed them in my videos which I made for their fitment research purposes, that the very large bodied SC2 36mm was going to fit the KDX200H/220R without cutting and welding on the frame and significantly modifying the rear head stay.

I believe on a 1999-2002 or 2003-2006 KX125 hybrid build KX frame, the SmartCarb SC2 36mm clears with no issue, but that doesn't help the rest of us KDX owners on KDX200H/220 frame fitment, however, this SC3 34mm carb absolutely does! (FYI I believe the 94-98 KX frame hybrids can very easily have the shock crossmember modified like the otherwise virtually identical 99-02 KX125 frame, to fit the SC2 36mm with no issues, as the KX's don't use a rear head stay 94-02).


I'm sure every bike is slightly different, but I don't know if they've tuned / tailored the specific metering rod spec for a totally stock 220, a 220 with airbox lid removed and FMF Gnarly Desert pipe, or a 220 with a Ron Black modified rechambered milled head, a Fredette stock 220 chambered head that's been milled for high octane fuel and same squish clearance as the RB head, etc.... I'm curious as to the extents of their testing on the dyno of various parts combinations, not to mention any mild porting alterations and how that affects the fuel metering.

We'll divide the throttle range into 3 segments, low ("pilot"), mid throttle, & 70%-100% throttle, for simplicity sake.
Dialing in the metering rod height can fix one area of the throttle range on these carbs that may be off a slight bit, but may throw one or both of the other two ranges out of tuning parameters. I feel that playing with the metering rod height and recording the clicks vs your baseline setting as to how many clicks up or down it took to dial on one area vs baseline height adjustment, and recording where these different metering rod height adjustments are for each throttle range vs the baseline setting, will be the quickest way to having SmartCarb tech support be able to get you near perfect in metering rod specifications in just one or two tries if the metering rod shipped out does not fit your bike's air/fuel metering requirements perfectly on first try.

These are very very advanced carbs, which have a few advantages over the also very advanced Lectron Billetron Pro Series, but I also believe the Lectron Billetron Pro Series have a few advantages over the SmartCarb SC2 & SC3 models... They're both at the cutting edge of carburetor technology, exceling beyond the PWK Air Strikers and even the STIC metering block aftermarket add-ons for the PWK36/38 carbs. The only thing that comes close to the newest generation of metering rod carbs in overall performance would be the Yoshimura MJN technology, Multiple Jet Nozzle retrofits to existing carbs, primarily done in street bike carbs. It's a conversion of a standard needle jet / jet needle type carb fuel delivery into basically something that is the equivalent of a metering rod in the way it disperses more finely atomized fuel through the full height of the carburetor venturi bore below the slide, vs a standard jetted carb's fuel delivery beyond the pilot jet circuit being entirely delivered in larger fuel droplets coming out of only one metering orifice, the needle jet, which is variably restricted by the needle base diameter at low throttle vs the needle taper at higher throttle, and at wide open throttle is primarily only restricted by the sizing of the main jet due to the needle taper coming to a slender point on the bottom end the metering rod and the Yoshimura MJN both deliver more finely atomized fuel droplets through the full height of the carb venturi up to the bottom of the slide, clearly a better way to promote more full and efficient combustion.
On a dyno, or in the real world experiences, peak power will still be same either way, Lectron Billetron/SmartCarb etc vs a well tuned Keihin PWK Air Striker, but the fuel efficiency as well as throttle response throughout most of the lower and mid range portions of the throttle range will be noticeably improved with either of these newest generation of metering rod carbs.

What SmartCarb has done to create their "CloudForge" venturi design I believe gives them one advantage over the Billetron. The carb bore is shaped like an upside down egg, like a round 34mm bore (for the SC3 34mm), but the lowest area up to 15% or 20% throttle being machined out of the body further, I and below the typical 34mm round bore for a narrower carburetor venturi/bore, just for higher velocity airflow / jet signal around the metering rod at lower throttle positions in order to produce significantly better atomization and combustion at lower throttle openings.
I am assuming that the SC3 34mm is designed in this manner with the majority of the bore actually measuring 34mm, but then the narrower area cut out at the bottom will add additional total carburetor venturi/bore area, making it at least the equivalent of a PWK35, while giving low throttle position atomization and throttle response better than a KDX220R's PWK33 or even a Keihin PE28 carb...
Versus the Billetron Pro Series 38 which gains it's increased air velocity at liw throttle openings by being the same external dimensions and fuel delivery potential as a Keihin PWK36/38 Air Striker, but actually necking down the venturi bore at the slide down to a 34mm bore in order to give the added velocity over the metering rod. For this reason, I feel that for wide open throttle for MX use etc, perhaps the Billetron might not be as competitive in some riding terrain scenarios vs a PWK36/38 Air Striker, as it very slightly limits the maximize airflow into the engine, although not as much as you'd think, really...due to airflow physics, but for woods riders and tighter MX tracks, the Billetron Pro Series with the Xcelerator metering rod is going to be an incredible carb for all it's other perks and advancements. The Xcelerator metering rod literally does the equivalent of an accelerator pump when you rip the throttle open rapidly, and delivers an additional burst of fuel through a groove machined into the back side... Matt Denecke formerly of Lectron and Apex Technical Innovations, now Denecke Racing Technologies (Denecke Racing Technologies YZ400 2-stroke engine kit project is in the testing phases, 69hp so far!), is the former Lectron Lead Fuel Systems Engineer who invented the Xcelerator metering rod concept and design for Lectron.
The Billetron Pro Series also has both the adjustable metering rod, although it is not externally adjustable like the Smart Carb 's, but in addition to that, it has an adjustable thumb screw torque jet, basically a pilot jet, and adjustable thumb screw powerjet, basically like a main jet, both are supplementary to the fuel delivered off of the metering rod itself, to add just a little bit more enrichment in order to fine-tune the carburetor's fuel delivery without having to swap out a different metering rod.
The ease of tuning with the Billetron Pro Series as well as this Xcelerator metering rod breakthrough are it's big bonuses over simply just being an excellent cutting edge metering rod fuel delivery type carburetor.

Both of these carburetors are incredible advancements, I have two Lectrons, the original being one of the older power jet 34 mm carbs, which still runs really great and delivers awesome fuel economy and throttle response, but the Billetron Pro Series really has potential to deliver the ultimate version of that, four or five generations later in Lectron technology...