Introduced in 1973, the Yamaha TZ350 would revolutionize the 350cc GP class, a production road racer offering a 60 hp liquid-cooled twin-cylinder engine and lightweight chassis. Available to privateer racers, the bike quickly made a name for itself on race tracks around the world:
“An almost unbeatable weapon in the hands of the right rider.” –TZ350.net
Enter Isaac Siegl of Seattle’s Speedy Siegl Racing, who cut his teeth at Twinline Motorcycles before opening his own lightweight parts business in 2015. Previously, we’ve featured a pair of his custom supermotos, including a Husky 610 retromotard and his own personal 100,000-mile (!) DR-Z. Now he’s back with a ’75 Yamaha RD350 built with a clear vision in mind:
“I built this bike to fulfill a personal dream of my 20 year-old self: to ride a vintage race bike on the street.”
Now that’s our kind of dream! Isaac’s inspiration, not surprisingly, was the first of the TZ line, the TZ350A. The result is a bike that looks just like a street-legal TZ350, and has the performance to boot:
“I got it out on the track and got to see just how much fun it can be at the limit! It handles fantastic, very nimble with excellent control at all lean angles.”
Isaac is most proud of the engine, which has survived years of abuse on the street and track without a hitch:
“It has reliable, docile power below the powerband, and above the power and it becomes a true superbike!”
Below, we get the full story on this TZ-inspired street racer, along with some killer track photos from Dane Añar (@whoisdane).
Yamaha RD350: In the Builder’s Words
I got the bike in 2008. I traded my first race bike – a 1974 Honda CB450 straight across for it. It was a barely running, crusty, halfway chopped up cafe bike that needed a lot of love.
For the first time, I had the money, time, and effort to turn this bike into one that I had always listed after: a TZ350A replica. I bought a fuel tank, seat, and Daytona camber fairing from Airtech, some DG pipes, and got to work!
I stripped the bike down completely and painted the frame, installed new bearings and bushings, rebuilt the forks and brakes, and drilled everything I could along the way!
I was too excited to put the fairing on, so I rode it as a cafe racer with the long tank for a few months before I took some time to fabricate the fairing brackets. During that time, I rode the bike a lot! I did a big ride from Seattle to Leavenworth for a weekend, commuted to work and back, I thought it was the coolest thing to hit the streets of Seattle in 32 years! One day I broke a clutch cable and decided to take a look at my pistons and make sure everything was ok while it was on the bench. One piston was missing most of the crown above the top ring, and the other had a wrist pin clip fail and the pin wore into the cylinder over 2mm!
Needless to say, I tore the bike down again and built the motor. I ported the cylinders and reed cages, installed yz125 reeds and RZ350 crossover intake manifolds, cut 0.025” off the cylinder heads, installed pro-x banshee pistons, lightened the clutch basket and pressure plate, and cleaned/inspected everything else. I also installed a vintage smoke electronic ignition.
After installing the motor I fabricated all the necessary mounts for the fairing and had all the fiberglass painted in the classic TZ livery. The bike was beautiful, and made its first show appearance at Backfire motorcycle night in Seattle. It also went to The 1 show in Portland that winter of 2010.
From that point on, I just wanted to ride the bike! I rode it to lots of events in the Seattle area and continued to commute with it. I started to crave more power. Then I saw the beautiful Jim Lomas exhausts and knew that was my ticket! I did back to back dyno runs from the DG pipes to the JL’s and found an additional 10hp, a 22% gain!
After completing the build, I grew bored riding it around town, so I got it out on the track and got to see just how much fun it can be at the limit! It handles fantastic, very nimble with excellent control at all lean angles. The power is much smoother than the reputation may lead you to believe. Granted, this isn’t a full race build, but it runs very well below the powerband. When the tach needle sweeps past 6k, the exhaust note changes and the bike surges forward forcing you to tense up and prepare for whatever you are pointing at! The brakes are confidence inspiring, and the rear drum has a lot more stopping power than expected. The suspension (and riding posture) remind you of the bike’s age. You feel everything on the road!
Unfortunately, I don’t ride this bike as much as I used to, and instead of pushing the build further I have decided that it is time for someone else to enjoy it. I learned so much from this bike, and I doubt anything will ever compare to it.
Yamaha RD350: Builder Interview
• Please tell us a bit about yourself, your history with motorcycles, and your workshop.
My name is Isaac Siegl, and my shop is Speedy Siegl Racing. I have been building motorcycles for 15 years, and gained much of my experience at Twinline Motorcycles in Seattle. I started Speedy Siegl Racing as a lightweight parts business in 2015, and slowly started building and maintaining motorcycles again.
• What’s the make, model, and year of the bike?
This is a 1975 Yamaha RD350.
• Why was this bike built?
I built this bike to fulfill a personal dream of my 20 year old self: to ride a vintage race bike on the street.
• What was the design concept and what influenced the build?
I wanted to build a street replica of a 1974 TZ350A.
• What custom work was done to the bike?
“Stage 2” cylinder porting, shaved cylinder heads 0.025”, YZ125 reeds, RZ350 intake manifolds, lightened clutch basket, electronic ignition, lightened brake rotor, lightened rear brake shoes, lightened rear brake plate, JL stainless exhausts, airtech streamlining bodywork.
• How would you classify this bike?
I would call it a vintage race bike for the street.
• Was there anything done during this build that you are particularly proud of?
I am most proud of the motor. It has held up to all my abuse for years without a fuss! It has reliable, docile power below the powerband, and above the power and it becomes a true superbike!