5 Essential Tech Tips for Rebuilding Mikuni Round-Slide Carburetors

Mikuni round-slide carburetors are some of the most common and reliable carburetors found on classic motorcycles and powersports equipment, and rebuilding one is definitely a gearhead right of passage. If you’ve got a bit of mechanical sense and a basic set of tools, you can definitely get the job done, and finally have that old bike running again.

We’ve rebuilt enough Mikuni carburetors to field an entire army of powersports equipment, and have a few tricks up our sleeve for getting the job done right. It’s a pretty simple affair, and we’ve outlined the whole process in this how-to video below using some really crusty VM34s from our 1972 Suzuki Titan 500. If this isn’t your first rodeo with a Mikuni round slide, we’ve outlined five key areas that could improve your carb-rebuilding game.

Fuel Bowl Seems elementary, I know, but there are several ways to screw up removing a Mikuni round-slide fuel bowl. Mikuni uses four JIS (Japanese Industrial Standard) screws to retain the fuel bowl. JIS drivers and screws aren’t designed to “cam out” at a specific torque like Phillips screws are. In most cases, you can get by with a standard Phillips, but the correct tool would be a JIS No. 2.

Once the screws have been removed, you’ll notice the bowl will remain stuck to the carburetor body, and a few smacks with the handle of your screwdriver will loosen it up. This is a point where it pays to take some extra care. The original bowl gasket on the Mikuni is pretty tough, and you can almost always reuse it. However, the gasket can sometimes stick to the bowl and the carb body, and it can tear if you continue removing the bowl.

If this happens, run a small straight blade along the bowl gasket surface to separate the gasket from the bowl. If the gasket isn’t torn, it’s fine to reuse it as long as your gasket surface is clean.

Float Assembly Your Mikuni round slide will have either brass or composite floats, and the assembly is a little different for each. With either setup, you’ll have an arm that interfaces with the needle and seat; brass floats are usually affixed directly to the arm, while composite floats extend from the bowl and push on the arm with small pins.

The float arm pivots on a pin that goes between posts on both sides of the needle and seat, and you’ll have to remove the pin to access the needle and seat. On this carburetor from our Suzuki Titan, the pin doesn’t have a head on it, and simply fell out after jostling the carb. On many other Mikunis, this pin will have a head on one side, and the holes in the posts are actually tapered for a tight fit. Make note of what side the head is on, and be sure to reinstall it in the same direction.

We’ve found a light-duty, spring-loaded center punch to be the tool of choice for removing tight float arm pins. While the spring action seems harsh, we’ve never broken a float arm post off using a spring-loaded center punch. I can’t say the same for a standard punch and a hammer.

Needle and Seat Assembly You can rebuild Mikuni round slides over and over and not have an issue with the needle and seat, but when it finally happens, it’s a headache you won’t forget. The needle and seat both have a tapered surface where they interface with each other, and it’s imperative that this surface is perfectly clean. If there are visual imperfections, it’s usually junk, and sometimes even a perfect-looking needle and seat assembly can leak fuel.

If your machine is hard starting, wants to flood out or has gas leaking from the carburetor vent lines, the needle and seat could be your issue. If cleaning it doesn’t work, you’ll have to replace it, and we opt for the viton-tipped needles whenever possible.

If you suspect the needle and seat is leaking, there’s a simple way to test it. With the carburetor fully assembled, suspend a suitable fuel reservoir a foot above the carb with a half-gallon of fuel in it and connect a fuel line to the carburetor. If the needle and seat are leaking, you’ll notice fuel in the carburetor body after a few minutes. This process is simple (albeit somewhat of a moot point) on a motorcycle with a gravity-fed fuel tank, but if you’re working on something with a fuel pump, it saves you a bunch of kicking, pulling or cranking, as the fuel bowls are now pre-filled.

Pilot Jet The pilot jet is the smallest jet in a Mikuni carb and has tiny orifices, and as such, it’s almost always clogged. The pilot is hidden in the tube forward of the main jet, and you need a very specific screwdriver to get it out. My weapon of choice is a 1/8″ Craftsman straight blade, and if memory serves, I even took a little material off the sides on the widest part of the blade.

You’ll want to apply lots of downward pressure when extracting the pilot jet, as the jet is made of brass and the slot is small. If you strip it, you’re in a world of hurt.

Emulsion Tube The emulsion tube is hidden underneath the main jet, and can be a real pain to remove given its tight fit in the carb body. Many tinkerers opt to leave the emulsion tube in the carburetor for cleaning, but there are a couple of good reasons to remove it.

For one, there’s always some old fuel trapped along the side of the emulsion tube, and if left for too long, it’ll turn gummy or crusty, making the tube more difficult to remove. With the emulsion tube out, you can clean it much better, and you’re able to read the part number and verify that it’s the correct piece. Remember that emulsion tubes and jet needles are matched, and swapping to a different jet needle profile may require a different emulsion tube.

After removing the main jet, we’ve found the best tool for removing the emulsion tube is a small deep-well socket—usually something in the neighborhood of 4 mm. Pick one that doesn’t interfere with the emulsion-tube locating pin, and remember not to hit the threads, or they’re toast!

Want to see the process in depth? Check out the video below to see our VM34 go from barn-fresh to minty.

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Speed Read: A modern Honda Motocompo concept and more

Remember the adorable Honda Motocompo? One industrial designer wants to bring it back, so he’s rendered a modern Honda Motocompo concept that uses the Monkey as a base. We’ve also got a tidy  Triumph Thruxton from the Czech Republic, a BMW K75 from The Netherlands, and a BMW R1100GS from Germany.

Honda Motocompo concept We seldom feature digital design concepts on Bike EXIF, simply because the story of how a design is brought to life is often more compelling than just the design alone. But the internet is crawling with highly imaginative motorcycle concepts—all you have to do is sift through the silliness to find the gems.

This one is particularly interesting—not only because it’s extremely well-rendered, but also because it’s rooted in reality. It’s the work of Boston-based industrial designer Mitch Silva, and it asks the question; “What would a modern-day Honda Motocompo look like?” And if Mitch has his way, it’ll eventually become a physical custom build.

Mitch has over a decade of industrial design experience, with a client roster that includes the likes of Bose and Rivian Automotive. He recently launched Bonedog Industrial Design—a design consultancy focused on helping custom motorcycle builders visualize their ideas. He rendered the neo-retro Honda Motocompo concept as a personal project, to add some extra motorcycle flavor to his portfolio.

Honda’s mini-bike range is packed full of nostalgic bikes, but they all reference historic Honda models from the 60s. Believing that the lineup lacks some 80s radness, Mitch envisioned a modern Motocompo built on the current Honda Monkey platform.

If you don’t remember the Motocompo, it was a foldable scooter that looked like a briefcase and could fit into the trunk of a car. Mitch’s concept isn’t designed to fold, but it does transform the Monkey platform into a worthy reimagining of the iconic Motocompo.

The concept wraps the Monkey in enclosed bodywork that traces the classic Motocompo silhouette, but with a more angular vibe. The vents cut into the side of the body, blocky headlight, high handlebars, and wraparound taillight are all clear nods to the Motocompo. The sharpened-up lines and black inlays draw inspiration from another 80s Honda—the Honda Elite scooter.

The lower half of the render shows the Monkey’s bones, with the engine blacked out to match the Motocompo’s finishes. The swingarm is modeled after the Honda Grom, and there’s a Yoshimura exhaust hanging off the side. The Motocompo had cute three-spoke wheels, so Mitch designed wheel covers, with grooves cut into them in sets of threes.

If the stars align, Mitch will bring this bodacious Honda Motocompo concept to life. He plans to build it as a plug-and-play kit, ideally using the Grom as a base rather than the Monkey. Challenges will include building a hidden fuel cell, and finding a strong material that’s also affordable so that he can keep the costs down. [Bonedog Design]

Triumph Thruxton by Rod Motorcycles Building a custom motorcycle is easier said than done. The owner of this 2010-model Triumph Thruxton found that out when he decided to take his bone-stock café racer to the next level. He ordered a smorgasbord of top-shelf bits for it—but he eventually ran out of knowledge and time, and the bike was relegated to the corner of the garage.

Two years later, he saw a photo of a Honda that had been customized by David Zima at Rod Motorcycles in the Czech Republic. With a renewed interest, he dropped the bike and parts off at David’s workshop to get the build over the finish line.

With Öhlins suspension, tubeless Alpina wheels, and Brembo radial calipers already purchased, the build was off to a good start. Rod’s task was to take those parts, add a whole bunch more, and tweak the bike’s styling. The Thruxton is a handsome enough café racer in stock form, but the client wanted a more purist look.

Step one was to swap the Triumph’s chunky OEM seat for a fiberglass tail unit. The subframe was trimmed to accommodate it, and the seat was trimmed in stylish Alcantara. A small Lithium-ion battery sits under the tail bump, along with a control unit and keyless ignition from Motogadget.

Next, 74 hours were spent simplifying the Thruxton’s complex wiring system. The Rod team removed everything they didn’t need, then installed a nifty Motogadget gadget that let them get around the bike’s CAN bus system to swap out the speedo.

Other mods include new clip-ons, levers and push buttons, new rear sets, and tiny Kellermann turn signals. Lower down you’ll spot a carbon fiber front fender, K&N filters where the airbox used to be, and twin Arrow exhausts.

As modern classic British café racers go, it doesn’t get more quintessential than this. [Rod Motorcycles]

BMW K75 by WiMoto It’s not easy to work with the goofy, angular lines of the K-series BMW, so we’re usually impressed when someone pulls it off. This one’s the work of Wido Veldkamp at WiMoto in The Netherlands, and it’s a mild custom job by his usual standards—but that doesn’t mean he cut any corners.

The project came from a client that had started taking her bike apart, but needed help welding a subframe. So she took it to WiMoto for a little mild fabrication work and ended up just handing the whole project over to them.

The tail section is an off-the-shelf item that she’d already bought, so WiMoto stuck with that. But rather than simply fettle the subframe to fit it, the team fabricated an all-new subframe, angling it upwards at the back and sinking a taillight into it. It’s a clever design; this café racer might not have the flat bone line that so many builders crave, but its contours follow a consistent pattern from front to back.

Next, WiMoto adapted the forks from an Aprilia Tuono to the K75, with adaptors to run twin 320 mm brake discs. The front fender and radiator covers are custom parts, and the battery now lives in the tail section, leaving the space under the seat open. The BMW also sports new handlebars, risers, grips and mirrors, and a custom-made license plate bracket that also hosts the rear turn signals.

If you’re looking for the front turn signals, they’re part of the LED headlight unit. A three-into-one exhaust system sits lower down, with Avon Roadrider rubber wrapped around the stock rims.

Finished in a dark turquoise hue, with pops of orange to add contrast, this K75 is as tidy as they come. Most custom builders ignore the older three-cylinder K-series Beemers in favor of the newer and bigger models, but we’d give this plucky street tracker a go any time. [WiMoto | Images by Mark Meisner]

BMW R1100GS by Himora Motors The BMW R1100GS was quite a radical machine when it first hit the scene, but it was soon overshadowed by its successor, the world-famous R1150GS. You can pick the older 1100 up for pretty cheap these days—but while it’s a tough and capable bike, it’s no looker.

Kay Riemann at Himora Motors in Germany agrees, so he gave this 1994-model BMW R1100GS one heck of a makeover. As an industrial and graphic designer, dragging the old boxer’s aesthetic into the modern age was a high priority.

Kay started by swapping the R1100GS fuel reservoir out for one from a newer BMW R1150R. Then he ditched the stock plastics, dressing the bike in a set of smart carbon fiber pieces instead. A two-piece seat sits out back, perched on a modified subframe with a sharp fender to finish off the tail.

The changes weren’t only cosmetic—Kay took care of the bike’s functionality too. He refurbished the wheels, then spooned on new Heidenau K60 tires and upgraded the brake discs. Going deeper, he rewired the bike around Motogadget components and swapped out the EFI unit for an R1150GS system.

The crash bars, handlebar risers, and headlight brackets are all custom parts, the headlight and heated grips are from Koso, and the hand guards are Barkbuster items. The speedo, mirrors, turn signals, and switches are all from Motogadget, the throttle is from Domino, and the footpegs are from Lonerider. An SP Cobra exhaust muffler finishes things off.

Between its purposeful stance, muscular bodywork, and aggressive black and red color scheme, Kay’s R1100GS sure looks like it means business. Parked next to a stock 1100, it’s streets ahead. [Himora Motorcycles]

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Built to Shred: Best Street-Legal Dirt Bikes for 2024

Boasting track-caliber suspension, powerhouse thumper engines and just enough equipment for street legality, today’s 350 to 500 cc dual-sport motorcycles are truly a hooligan’s dream come true. Often descended from real-deal cross-country racers, these 2024 street-legal dirt bikes pull double duty so you can leave the ramp and trailer at home.

Don’t expect stellar road manners or comfortable long-range highway cruising, but if you’re after a machine with enough dirt prowess to make you feel like Ricky Carmichael, and a license plate for the trip back, these bikes from Honda, KTM, Husqvarna, Beta and Christini are worth considering.

Honda CRF450RL Based heavily on the modern CRF450X cross-country racer, the Honda CRF450RL is the street-legal triple threat that Team Red riders were demanding. The 450RL is completely unchanged for 2024, but Honda’s always been an if it ain’t broke sort of brand.

Before the 450’s debut in 2019, Honda had a sizable gap in their street-legal lineup between the 250L and XR650, and riders were begging for a more aggressive option. Instead of a bigger, badder 250L, which is what many anticipated, Honda delivered a real 450 cc class contender, borrowing all the best stuff from the 450X and 450R motocrosser.

Despite being softer all around than its off-road cousins, the 450RL brings the fight to the Europeans in a big way. The 450 cc Unicam engine puts out 41.7 hp with all emissions equipment affixed, and its adjustable Showa suspension returns around 12 inches of travel at both ends. It’s also quite a bit cheaper than the competition, priced at $10,099.

It’s hard to go wrong with a Honda, especially when it comes to build quality, maintenance costs and an extensive dealer network—all reasons why I have a 450L in my garage. [Honda]

KTM 450 EXC-F Six Days Any off-road machine with a license plate affixed is somewhat of a compromise, but if your compass points toward the extreme, Team Orange has a little something for that. KTM’s 450 EXC-F is one of the most aggressive bikes in the segment, and checking the box for the special ISDE-inspired Six Days version gets you a machine that KTM assures you is 100% ready to race.

KTM’s entire EXC range receives extensive updates for 2024, and the 450 EXC-F Six Days is said to have 95% new components. It all starts with an orange chromoly frame, with new 48 mm WP XACT fork and XPLOR PDS rear. With a peak power output of 57 hp, KTM’s 449.9 cc engine is seriously potent and weighs just 64 pounds. Fit with a new clutchless Quickshifter, the Six Days four-stroke models can be shifted a wide-open-throttle without depressing the clutch.

Inspired by the International Six Days Enduro event, the 450 EXC-F Six Days is littered with enhancements for the roughest riding. The standard saddle has been swapped with a special gripper seat, and you get Metzler Six Days extreme rubber, a special map-select switch, high-strength wheels from GIANT, a composite skid plate and a host of signature orange off-road components.

Choosing between the 450 and 500 EXC-F Six Days machines is all about preference, as both bikes weigh about the same, with the 500 dishing out an extra 7 ponies. While we haven’t thrown a leg over the big 500, we found the 450 to be lethal in the dirt and well-balanced. But it’s your checkbook that will need balancing, as the Six Days 450 sells for $13,949. [KTM]

Husqvarna FE 501s The big 500 cc thumpers from KTM and Husqvarna are one of the final steps up you can make in the street-legal, pseudo dirt-bike world before you take a big step up in weight. Beyond looks, choosing between the two really comes down to the rear suspension—and while it’s splitting hairs—we’re admittedly linkage guys.

Husqvarna’s FE 501s received much of the same updates as KTM’s EXC line for 2024, including a new chromium molybdenum steel frame, new subframe, die-cast aluminum swingarm and WP XACT suspension front and rear with 11.8” of travel. The lightweight and compact 510.9 cc engine delivers north of 60 hp, which is the sort of grunt you’ll definitely notice in the saddle if you’re used to a 40 hp bike.

Offering brutish power, premium suspension and a competitive dry weight of 258 pounds, the FE 501s is one of the least compromising street-legal machines money can buy. As such, you’ll need to pony up $13,049 to get one. [Husqvarna]

Beta 390 RR-S All the displacement in the world doesn’t mean anything if you can’t control it, and when the going gets rough, there’s something to be said about a well-balanced machine. Italy’s Beta is well-versed in enduro competition, and their catalog of street-legal dual-sport machines offers four different power levels from 350 to 500 cc.

The street-legal RR-S line shares many of the same components across the board, including new ZF Sachs forks, progressive linkage rear suspension, titanium intake valves and integrated traction control. Closely related to the prior year’s RR race-edition machines, the RR-S models are sold in basically race tune and only gain about 4 pounds from their DOT-friendly equipment.

One of Beta’s best sellers is the 390 RR-S, a bike that’s not afraid to break convention. There are no published power figures available, but the 390 RR-S is probably in the low 40-hp range and weighs just 241 pounds (wet, without fuel). Given the similar spec between models, Beta’s larger 430 and 500 RR-S models offer more power without an appreciable jump in weight, but again, that’s not always the name of the game.

Sold on a BYOB (Build Your Own Beta) basis, pricing hasn’t been announced for Beta’s street-legal dual-sports just yet, but expect the 390 RR-S to sell for around $11,599. [Beta Motorcycles]

Christini AWD 450DS Every list is best concluded with a wildcard, and even among such racy company, the Christini 450DS has an unparalleled trick up its sleeve. Concealed within the frame and fork of this street-legal dirt bike is a trick all–wheel-drive system that delivers game-changing performance off-road, without sacrificing street manners or suspension travel.

While it sounds a bit like reinventing the wheel, Christini has been perfecting its AWD motorcycle formula since 2002, and the system has reached a new level of seamlessness. In a nutshell, power is taken off the drive sprocket by chain up to a gearbox in the frame. From there, a driveshaft runs to the head tube where opposing bevel gears are housed. Then a pair of gears and chains in the lower triple clamp power two telescoping driveshafts along the fork tubes to the front hub.

Sounds complicated, but an untrained eye wouldn’t even notice the system, and the big-name publications have reviewed the quality of Christini’s AWD parts favorably. Of course, this system leads to a whole different experience off-road, most of which is good, but that’s a bit outside of our scope here.

Christini’s bikes are getting better and better, and the AWD 450DS would nearly stand on its own without the trick front drive. The 450 cc liquid-cooled single makes a competitive 42 hp, and the adjustable Fast Ace front end is good for 12” of travel. You’ll pay for the AWD tech, as the bike sells for $16,500, and it weighs as much dry (288 pounds) as the competition weighs at the curb, but you have to keep things in perspective here. It’s an all-wheel-drive 450 cc dirt bike that’s 100% street-legal—what a time to be alive. [Christini]

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This custom build is the Yamaha R9 we deserve

Rumors of a new Yamaha R9 have been percolating for a while, but there’s been no official word from the Japanese marque… yet. The theory is that Yamaha is planning to transplant the magnificent 889 cc tripe-cylinder engine from the popular MT-09 into a sportbike—as they did with the YZF-R7. But some Yamaha devotees are growing impatient.

Seb Hipperson is so tired of waiting for the factory Yamaha R9, that he’s literally taken matters into his own hands and built his own R9 prototype. We hope Yamaha’s designers are taking notes because it is glorious.

Based in London, Seb is a full-time fabricator focusing mainly on furniture, but often dabbling in other interests. He’s also a motorcycle enthusiast, customizer, and racer—hence the burning desire to see the R9 concept come to life.

“I’m pretty sure I’m not the only one who has become impatient while waiting for Yamaha to put its lovely ‘cross-plane’ triple into a chassis and body suited for the track,” he tells us. “The vision I had in my mind was influenced and inspired by the Yamaha GP two-strokes of the 90s, like the TZ250 and YZR500, mixed up with the design of the current Moto2 race bikes.”

The project kicked off with a low-mileage Yamaha MT-09 donor bike, which was then unceremoniously stripped down to the absolute basics. Seb then set to work building a bespoke frame to house the MT-09 mill. Using the cradle from a Yamaha TRX850 as a cornerstone, he TIG-welded together an elegant trellis steel frame to construct his R9 around.

Technically, Seb welded three frames together, because it took a little trial and error to achieve the right geometry and aesthetics. The swingarm is from a Ducati 749, the rear shock linkage comes from Corse Dynamics, and the shock itself is a custom-built unit from Mupo Race Suspension in Italy.

Seb grafted a set of Ducati Panigale forks on at the front, but not before he upgraded them with Bitubo internals. Five-spoke alloy wheels from an Aprilia RSV Mille went on next, along with Brembo brake calipers at both ends.

With the rolling chassis in place, Seb turned his attention to the bodywork. The MT-07 features plastic panels over a hidden fuel reservoir and a few other key components, but none of that was going to work for what Seb had in mind. So he covered the chassis in foam blocks and started shaping his R9.

Once the parts were formed, Seb used the foam to create fiberglass molds. The front fender, fairing, belly pan, and tail unit were all shaped by hand out of fiberglass. The tank is a two-piece affair; a fiberglass airbox sits up front, with an aluminum fuel reservoir at the back, complete with a built-in sight glass.

The bike’s silhouette is flawless, and it’s easy to forget that you’re looking at a one-off special rather than a retro factory superbike. Carefully considered details abound, like the muscle car-inspired vent on top of the airbox. It’s not just there for show either—it directs air to the airbox, as does the vent at the front of the fairing.

With over 110 hp on tap and a throttle response second to none, the MT-09 is as lively as they come. But Seb still added a couple of non-invasive mods to make sure it runs strong and smooth. For starters, the ECU was flashed on a dyno to squeeze the maximum performance out of the custom airbox and exhaust system.

The exhaust itself consists of Lextek headers and an Arrow muffler, with a custom-made pipe connecting them. Other track-specific mods include a thumb lever for the rear brake, a protector for the front brake lever, and crash bobbins. Clip-ons, aftermarket levers, and a GPS-based speedo round out the spec sheet.

Seb’s Yamaha R9 would be impressive enough wearing nothing but a coat of primer, but he’s wrapped it in a livery that pushes it into the stratosphere. It’s hard to argue with Yamaha’s iconic red, white, and black ‘speedblock’ design, and the raw aluminum tank gives the build a factory prototype vibe.

Seb didn’t just build this to look good though. He’s already spent a season shaking it down on the track, and he plans to take it racing. That’s why the only light on the entire bike, is a small track-approved LED taillight.

If an official Yamaha R9 is coming, it’s likely to be announced at EICMA later this year. But as far as we’re concerned Yamaha should just make Seb an offer on his design, add lights to it, and put it into production.

Seb Hipperson Instagram | Images by Sam Napper

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