Muscle Thumper: Yamaha SR400 Race Bike

Yamaha SR400 Race Bike

Prama Motorworks builds a race-winning custom…in just four weeks! 

Introduced back in 1978, the Yamaha SR400 would remain in production all the way until 2021, earning a cult following for its simplicity, ruggedness, and quick-revving character. The air-cooled thumper born from the iconic XT500 enduro, and many riders came to prefer the short-stroke SR400 over its longer stroke 500cc siblings.

“In ’78, the bike was a modern interpretation of a classic British 500cc single. Today, the SR400 is a classic interpretation of itself, so to speak.” –Cycle World

Enter our new friend Yosef Gumilar of Prama Motorworks, whose first bike was the Yamaha GT80 he got in 4th grade. Today, he runs his own workshop in Bandung, Indonesia.

Yamaha SR400 Race Bike

When the 2023 Motor Adu Classic Bike Race was announced — the first circuit racing to return to his area after the pandemic — Yosef jumped at the opportunity to race. The challenge? He had just one month to build a race bike!

“I love Japanese muscle bikes, and I wondered if I could build my SR400 in that style and with other adjustments and engine modifications to fit my riding style.”

Yamaha SR400 Race Bike

The modifications were incredibly extensive for such a short build time. On the suspension front, he swapped in the forks, wheels, and swingarm from a Yamaha XJR400, which required considerable modification, including shortening the swingarm and relocating rear sock brackets.

Yamaha SR400 Race Bike

He made the gas tank and fenders from 2mm and 3mm aluminum, respectively, and rebuilt the engine from the crank up to achieve 500cc displacement via longer piston stroke. A BRT Racing CDI allowed for custom mapping and a programmable rev limiter, while a Keihin 41 carburbetor feats the big 500cc single engine.

Yamaha SR400 Race Bike

Yosef finished the bike the night before leaving for the track! The next morning, he mounted the Triumph Scrambler oil cooler and trailered three hours to the circuit. The next day, he took pole position in two classes (FFA 500-750cc and FFA Japan 500cc up) and won both races!

Yamaha SR400 Race Bike

Congratulations, Yosef — that’s quite a feat! Appropriately enough, he named the bike “Sangkuriang” after the Sudanese legend of a man who must complete an impossible challenge of filling a whole valley with water and building a boat to sail it in one night. With the help of heavenly spirits, he dams a river to create Lake Bandung and the overturned boat becomes Mount Tangkuban Parahu, the great stratovolcano of West Java!

Yamaha SR400 Race Bike

Yosef is duly proud of what he accomplished with this build, and how it brings together the worlds of racing and custom bike building in his home country of Indonesia:

“It’s personal for me — in our country, racing and custom-built motorcycles are far apart, and I proved to everyone here that I can build a custom bike that can race on track. Thanks to all my team at the workshop and to all my friends who trusted in me that I could made it.”

Yamaha SR400 Race Bike

Below, we talk to Yosef for the full story on the creation of his SR400 race bike.

Yamaha SR400 Race Bike: Builder Interview

Yamaha SR400 Race Bike

• Please tell us a bit about yourself, your history with motorcycles, and your workshop.

My name is Yosef.  I’ve loved motorcycles since I was a kid. In 4th grade, I got my GT80, and then in 2013 I opened my own workshop named Prama Motorworks.

• What’s the make, model, and year of the donor bike?

I just know my donor bike is an SR400, but I didn’t know that year, because I just got it separated between engine and frame.

Yamaha SR400 Race Bike

• Why was this bike built?

I built this bike because I love racing. I was dreaming someday I could build my own old bike into a custom race bike and race at the circuit, and then on March 4-5, the classic bike race in my hometown returned after a long time not racing during pandemic.

So, after i heard the @motor_adu event announced, I started building this bike — I had just one month to build the bike.

Yamaha SR400 Race Bike

• What was the design concept and what influenced the build?

I love Japanese muscle bikes, and I wondered if I could build my SR400 in that style and with other adjustments and engine modifications to fit my riding style.

Yamaha SR400 Race Bike

• What custom work was done to the bike?

The swingarm I got is a donor from an XJR400. I had to cut and weld it because it was too long and wide for the SR400, and I move the bracket for the rear shocks back about 7cm.

Yamaha SR400 Race Bike

The gastank I made from 2mm alumunium and the front and rear fender made from 3mm aluminium.

Engine modification started with the crankshaft; I moved the crankshaft pin about 8mm from 76 stroke to 84 stroke. With a Honda Odyssey piston to reduce engine compression, I got 500cc and re-ground the camshaft for more lift and duration, but I forget the lift duration and intake / exhaust valve open and close degrees ????

Yamaha SR400 Race Bike

 I used a Keihin 41 carb and a Triumph Scramber oil cooler on the front of the bike. I used an XJR400 front end and wheels with Pirelli Diablo Supercorsa tires. Got an SC Project silencer and Coerce foot pegs, Brembo front brake, Protaper handlebar, and Domino clutch lever.

Yamaha SR400 Race Bike

For electrics, I use a local brand’s programmable CDI called BRT Racing, which allowed me to remap the CDI and put a rev limiter on it.

• Does the bike have a nickname?

Yes! I named it “Sangkuriang” after a myth about someone who builds mountain in one night.

Yamaha SR400 Race Bike

• Any idea of weight or power numbers?

No, I have no idea.

• Can you tell us what it’s like to ride? How did it do on the track?

I didn’t have much time before the race. I only finished the bike and had it ready to ride the night before I went to the track. I just rode it around my garage that night. The next morning I mounted the oil cooler, and that afternoon I brought it to the track, which is about three hours from my city. But on Saturday, I did QTT (Qualifying Time Trial) and the bike was great, good power and handling. I got pole in two classes (FFA 500-750 and FFA Japan 500 up) and finished 1st in both classes!

• Is there anything with this build that you’re especially proud of? And anyone you’d like to thank?

Of course, I’m really proud of my build. It’s personal for me — in our country, racing and custom-built motorcycles are far apart, and I proved to everyone here that I can build a custom bike that can race on track. Thanks to all my team at the workshop and to all my friends who trusted in me that I could made it.

Follow the Builder

Facebook: Yosef Gumilar
Instagram: @Pramamotorworks

Photographers: @gianggaw @arianstone_helltrust and @motostylerz

One Comment

  1. Elam Blacktree

    Nice!

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