Building a Bultaco Alpina Trail/Trials Bike
I bought my first Bultaco in 1972 while in Barcelona, Spain courtesy of the US Navy. A factory rep came onto the ship asking who was interested in purchasing a Bultaco. The 250cc Alpina was getting good reviews as the new, hot Enduro bike, light weight with trials bike heritage. So of course, I had to have one so I could try enduros and possibly trials, since trials had a trailbike class at that time. Over the years I appreciated how well Bultacos were built and found ways to improve their performance. Since those early days I have been building or restoring these bikes and so I accumulated quite a few spares. But now, after years of building, I found myself short on spares and not having enough parts to begin a build. This is where this story begins.
While looking through Ebay Bultaco parts, I found a 350cc, Model 116 Alpina engine at a very reasonable price. Initially I was not impressed with the 326cc engine as the power was very soft due to the exhaust system, but with a free-flowing intake and exhaust, this engine has great lugging power that puts that power to the ground nicely. I change the ignition to an electronic unit with auto-advance and this allows me to retard spark for smooth low rpm running without losing acceleration. The addition of a 26mm OKO D slide carburetor increases power and throttle response, making this a great trials engine.
So with this purchase I began casually looking for other parts. If none were found, this would be my back-up engine for my Model 92 Sherpa T engine, as the only differences were flywheel weight and the gear box. With an eleven tooth counter shaft sprocket and a 51 tooth rear sprocket I had on hand, it was close to trials gearing in first gear. The engine turned out to only need seals. It had recently been rebored with a new piston and the bearings throughout were in good shape. The only problem was a stripped hole that attached the head to the cylinder, so I drilled and tapped it for a 10mm bolt over the 8mm.
What happened next propelled me into this build as a friend was looking for an earlier Alpina gas tank, which I had. Turned out he had the right gas tank for this model so we did a swap. He also had a Model 116 frame, and so we traded that for engine labor I would do for him. With an engine, frame and tank (as well as new side panels) I could not resist starting a new build. As a landscaper for the last thirty-five years, I have no work in the winter and have time to "play" with motorcycle builds and restorations.
This Model 116 engine was in very good condition and had just been bored for a fresh piston. Ignition was changed to Electrexworld electronic, giving auto advance which means timing can be retarded for great slow running without impeding throttle response and high end running. New chrome header pipe connects to an early 250cc silencer. This system breathes very well and the silencer is easy to repack.
Making good progress. This build is just about done. I'm waiting on a chain and inner tubes to finish up. Lots of new parts and refurbished used parts were used in this build, making it almost like a new bike. Photos and report on performance coming soon.
Just finished this build and took it out for photos. The next step is to fire it up, dial in the carburetor and timing and test ride it to see if all is working as it should.
This build used a combination of new and used parts. Used parts were carefully inspected, cleaned, painted, etc... to make as good as new.
New parts include but are not limited to: Front and rear aluminum fenders, front fender brackets, Renthal 6" rise trials bars, Domino throttle, 26mm OKO D slide carburetor, Electrexworld auto-advance electronic ignition, piston on a fresh bore (first over), all crank seals, exhaust header pipe, all cables, brake and clutch levers, Renthal grips, rear brake pedal, front and rear IRC TR-011 trials tires, sprockets and chain, rear shocks, side panels and complete seat.