Source: Bike Bound
Twin-Turbo Scrambler: “Manic Melina” from Doctor Motorcycle…
Introduced in 1977, the Suzuki GS400 may have looked much like its middleweight twin rivals on the outside, but the engine boasted dual overhead cams, 180º firing order, and a counter-balancing system to dampen vibration. Cycle World called it “the smoothest machine in the class,” and while 400cc twins might not have the sex appeal of DOHC fours, the GS400/450/500 series would earn a reputation as some of the most underrated, most reliable bikes ever built.[1,2]
“It’s truly amazing how Suzuki could come up with such an outstanding first-time effort in a market they had never really entered before. If one were to argue that the GS400 was the best machine in the class, they’d have more than just a leg to stand on.” –Cycle World
Of course, if you’re going to turbocharge a bike, particularly a vintage one, it pays to start with the toughest donor possible, and applying to forced induction to motorcycles is the specialty of our new friend James Kauder of Arizona’s Doctor Motorcycle.
Like many of us, James got his start on a boyhood dirt bike…although it blew up right after he got it. Fortunately, James had a grandpa with a black thumb, who tore it down the top end and sent off the cylinder for re-plating.
“My grandpa re-installed it and had it running the next day. I just thought that was the coolest thing ever, and that was the beginning of wanting to learn how to wrench.”
James spent his teens and early 20s buying non-running bikes, getting them roadworthy, and selling them on. From 2015-2020, he worked on bikes as a sideline to his position as a CNC machinist, and during the pandemic, he founded Doctor Motorcycle and his highly popular Youtube channel (180,000+ subscribers!), where he specializes in turbocharging bikes — Sportsters, Buells, Viragos, and more.
The ’77 Suzuki GS400 you see here is one of his personal bikes. While his street setups usually run 8-9 psi, James had been wanted to build a fuel-injected, intercooled twin-turbo system to allow for higher boost levels.
“I’d always wanted to convert a motorcycle to EFI, and my GS400 seemed like the perfect candidate as it’s mostly an off-road and dune bike, so it doesn’t have to have good street manners.”
To prepare the engine for boost, James lowered the compression to 8.6:1, with a ported head and widened ring gap. He details the full turbo setup below, but highlights include a Microsquirt ECU, twin VZ21 turbos, intercooler, Ninja 650 throttle bodies, ZX10R injectors, antilag launch control, on-the-fly boost control, and even a quickshifter.
The whole system is much more sophisticated than most of the turbo setups we see, particularly for a vintage bike:
“We also utilized a flex fuel sensor, so you can run anything in between straight 91 pump gas and 98% ethanol, and the tune will adjust itself accordingly. On 91 pump gas you can run 12psi of boost safely, and on E85 you could probably get away with 20psi, though I keep it at 15-16psi so that I can ride it hard.”
James estimated 65 hp — nearly double the factory output — and a wet weight of less than 350 lbs. As for the suspension, he’s running Race Tech springs, heavier weight fork oil, and 14″ rear shocks, with 5.5″ front / 5.2″ rear of travel.
“People on the trails are genuinely surprised at how quickly this bike can be hustled around. Suzuki GS400’s and 450’s are an excellent choice for scramblers, as they’re already sub 400lbs from the factory.”
The aesthetics were more of a secondary concern to the performance and riding experience, but “Manic Melina” looks mean as hell with the turbos hanging off the sides…and she’s one hell of a hoot in the dirt.
“It’s a grin machine! Everyone that rides it comes back with a cheesy grin on their face.”
Below, we talk to James for the full story on this GS400 turbo scrambler, and we highly recommend you subscribe to the Doctor Motorcycle YouTube channel for more forced induction fun.
Turbo GS400 Scrambler: Builder Interview
• Please tell us a bit about yourself, your history with motorcycles, and your workshop.
I became interested in motorcycles after my first dirt bike blew up right after I was gifted it for my 12th birthday. The previous owners didn’t clean the air filter and it was just slowly ingesting dirt. The top-end seized, and I thought it would never run again. My grandpa tore it down, sent the cylinder out to be replated, and the day the cylinder showed up, my grandpa re-installed it and had it running the next day. I just thought that was the coolest thing ever, and that was the beginning of wanting to learn how to wrench.
I spent my teens and early 20’s buying non-running bikes, getting them running and roadworthy, and selling them. All sorts of bikes, ranging from SV650’s to dirt bikes and quads. As the years went on, and I’d gotten a few dozen bikes going, I had the urge to buy an old Japanese bike for dirt cheap and make it my own — a full custom build. So in 2015, I bought a 1982 Suzuki GS450 from a friend of mine for a few hundred dollars, and over the course of a year, turned it into a Brat Style bike. At that time, I was a CNC machinist. I didn’t have enough experience or money saved up to start a business, so I spent the next five years continuing to work full time as a Lathe Setup Specialist and work on bikes on the side.
The pandemic was the beginning of my business, and the Honda CM185 Scrambler build that’s on my YouTube channel was my first build video. After 4 or 5 months, the video suddenly took off, and I went from 30 subscribers to a few thousand, as was making a few hundred a month from YouTube. And that’s how Doctor Motorcycle came to be.
• Why was this bike built? (Customer project, company promotion, personal, etc.)
The bike was built because I’d always wanted to convert a motorcycle to EFI, and my GS400 seemed like the perfect candidate as it’s mostly an off-road and dune bike, so it doesn’t have to have good street manners.
There were also some other things on my bucket list that I wanted to cross off, one of those being a functional, high-boost twin turbo setup (I usually just run 8-9psi) with an intercooler. The intercooler allows me to sustain higher boost levels and ward off detonation. This bike is also on E85 as well as having lower compression, so I feel pretty comfortable running 15psi as my base boost level.
• What was the design concept and what influenced the build?
I go into each build with a performance feel in mind, the way the bike looks is almost an afterthought. My main goal with a scrambler is to make the bike actually ride and handle well off-road, everything else is secondary. It’s why my scramblers often end up super bare bones, stripped down to nothing but the essentials.
• Tell us about the build and setup.
Turbo Setup – Twin VZ21’s (IHI RHB3), intercooled, Tial blow-off Valve, a manual boost controller (to adjust the boost on the fly), antilag launch control, quickshifter. Finished bike was on E85 and 15psi of boost.
Engine Setup – Original untouched bottom end, new piston rings with opened up ring-gap, the head is ported and the combustion chambers were opened up to get the compression down to a calculated 8.6:1. Estimated power at the crank is 65HP.
Suspension / Chassis – Race Tech fork springs, 20WT fork oil with raised oil height to stiffen it up. The shocks are 14″ long, which necessitated the removal of the passenger pegs. Suspension travel is just under 5.5″ up front and 5.2″ out back. The chassis is de-tabbed as you’d expect and we welded in a frame loop.
Electrical / EFI – We converted the bike to EFI with a Microsquirt ECU, Ninja 650 throttle bodies, ZX10R fuel Injectors, a 180lph fuel pump, and an AEM wideband o2 sensor. The rest of the sensors (intake air temp, MAP sensor, etc.) are GM sensors, and the ignition coils are GM LS2’s.
The crank trigger for spark control was the hardest part, we ended up using a VR sensor with an 18-1 trigger wheel that we had to modify to get to work, including machining a spacer to move the wheel out an inch (this was shown in the build video). We also utilized a flex fuel sensor, so you can run anything in between straight 91 pump gas and 98% ethanol, and the tune will adjust itself accordingly. On 91 pump gas you can run 12psi of boost safely, and on E85 you could probably get away with 20psi, though I keep it at 15-16psi so that I can ride it hard.
The bike remains kick-start only.
The rest of the wiring is bare-bones; this is primarily an off-road bike so it has a rear running light and a headlight, and that’s it. In place of a speedometer and tachometer is the wideband Gauge, which uses our 3D-printed mount, and a boost gauge. In fact, this bike is mainly a dune bike, and we originally planned to run a paddle tire on it and take it to Glamis for the final build video, but we have some other projects right now that can’t wait, so it will happen early next year. So look forward to some pretty epic riding footage of this bike in the dunes early next year!
• Where does the “Manic Melina” nickname come from?
The bike is manic to ride offroad; as soon as the boost gauge swings over to full boost it just lights the tire up — it feels like riding a 250 2-stroke MX bike. So I knew I wanted to call it manic, and give it a girl’s name. Melina was top of mind for whatever reason.
• Can you tell us what it’s like to ride the finished bike?
It’s best on 12psi or so for off-road riding in dirt and sand, any more and it just doesn’t hook up at all. The boost is adjustable on the fly, so when I hit the dirt I turn it down from 15psi. Even with full-on non-DOT dirt bike tires, it doesn’t get traction at full boost until 4th gear. The power is actually linear at half-throttle and less — just putting around, it’s relatively docile. Even at ¼ to ½ throttle, it’s only making 5-7psi of boost — it pulls similar to how a 650cc or 750cc parallel twin would pull.
It’s not until you go wide open that the boost gauge swings over and it takes off like a scalded cat. Ripping shifts with the quickshifter is a blast, and you get the occasional pop and fireball out of the exhausts which is pretty cool. The rolling antilag sounds like gunshots, so I don’t use that all too often. It’s a grin machine! Everyone that rides it comes back with a cheesy grin on their face.
I’m running Race Tech springs and thicker oil up front, and in combination with a 130/90/18 tire up front, it’s relatively easy to ride in the sand, which is where I intended this build to be ridden. I spent a lot of time on the rear shock valving as well, and finally settled on a shim stack that’s nice and firm.
People on the trails are genuinely surprised at how quickly this bike can be hustled around. Suzuki GS400’s and 450’s are an excellent choice for scramblers, as they’re already sub 400lbs from the factory. Even with the two turbo’s and intercooler (intercooler only weighs 3lbs), I’d bet with the 4.2 gallon tank full, it’s not even 350lbs.
• Was there anything done during this build that you are particularly proud of?
I’m particularly proud of the sticker bombed tank — the idea didn’t come to me until the day before filming the final shots of the build video. I’ve always thought sticker bombing was cool, but thought that it looked like there was too much going on when people do the whole tank. So I had the idea of using vinyl cutting tape to cut it into a shape; to my surprise, it actually worked! It was one of those rare moments where it worked out how I thought it would in my head.
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