Bultaco Alpina 250 Project
Here, I've begun the Alpina build. New parts above are the tank, seat, bars, fenders and front fender bracket. I started this project because I had most of the parts for a Model 115 engine and time, about two months. I also had the tank on hand, which was for a previous build that was purchased with the original fiberglass tank in stead. Major used parts purchased were the frame and the wheels and hubs. My plan was to build a bike for trail riding that was also good for trials. With that in mind, the engine which was being built as this photo was taken, will get a Sherpa T first gear set, so a large rear sprocket is not needed and top end is not sacrificed. The end result will be an Alpina that is practically like new with a completely rebuilt engine. Photos of the finished project coming soon, as well as testing results.
Unfortunately, the frame of this era was prone to twisting some at the neck when hit hard, making front and rear wheels misaligned. This one was "tweaked" but I was able to get it straightened.
The chain tensioner had lost its forward spring attachment hole, so this setup is experimental. I may move the attachment point to the swingarm if this one doesn't work well. After testing the spring has been moved to the swingarm.
Motor in place now, the exhaust is correct for this model, but I add a "stinger" to the end since it helped performance on another model previously. Since the fiberglass has long been "blown out" of the muffler, I cut out a front portion so it can be repacked. Self-tapping screws hold the cover plate in place so it can be repacked again when needed.
This motor has been built from spare parts from different models. The cases are Model 115 (250cc Alpina) as is the gearbox, with the exception of the first gear set, which is from a Model 92 Sherpa T (326cc). This keeps me from having to put on a large rear sprocket for trials riding. The primary flywheel and clutch basket are from the Model 98 Alpina, which is a 175cc. Flywheel weight is much reduced from the 250's double flywheel. This should make engine response quicker, while possibly losing some ability to get traction to the ground in the mud. This combination uses a duplex chain rather than a single row chain, which is what I'm waiting for here. This engine is like new with all new bearings and seals, including a new rod kit and a new piston in a fresh bore.
Of course, I'm using an OKO D slide, 26mm carburetor, in this case a black one. These carburetors are a vast improvement over the stock AMAL Concentrics, which were used since the late 1960's. Mikunis were the replacement for the day, though a correct needle for this Bultaco engine was impossible to find with them. OKOs also outperform the Mikuni as well as all other round slides. This is especially noticeable in the low and mid-ranges. In this photo I'm waiting for an Electrexworld electronic ignition to replace the points ignition. I've been using these for over ten years now and they are super reliable. They have auto advance which allows you to retard spark for smooth low rpm performance without losing quick throttle response and high rpm performance. Also waiting on sprockets, eleven tooth on the front and 50 tooth on the rear. This should allow me to creep through trials sections given the Sherpa T's first gear.
With the electronic ignition installed, photo shows the simplicity of the wiring. It uses the stock magneto flywheel. Two wires come off of the stator to the primary coil. One of these connects to the kill button which is grounded nearby. I've set idle timing at 2.3mms BTDC. Auto advance kicks in as the rpms rise.
Making good progress and appoaching the end. With the primary chain and clutch installed, the engine is complete. Waiting on a petcock since the tank came with a 13mm threaded hole instead of the stock 12mm. Also waiting on front and rear sprockets, 11 tooth and 50 tooth respectively. Shocks are new Betor Gas units, 360mms with 40 pound springs. These are a bit longer than stock, but I believe they help steering in sections a bit. Tires are IRCs, TR1 in the rear and TR011 in the front. I'm not going to paint the tank for now, and badges will be added last. Photos of the finished bike coming soon (I hope).
This is basically the finished bike. I was able to do some testing with an old tank and seat while waiting for the petcock. After testing I needed to move the spring for the chain tensioner onto the swing arm. It is working as expected now. Front portion of the rear fender needed some trimming to better clear the muffler. Other than that all went well. The engine is quiet and pulls strongly throughout the throttle range. First gear is fine, even for the tightest trials sections, with the gap to second gear not being too wide. The bike handles well, like a typical Alpina or Sherpa T of the day (early to mid-'70's). I'm very pleased with this project and will probably hang onto it for a while, if for no other reason than to compare it to my Model 92 Sherpa T that is my regular ride. Once the petcock is installed I will take several more photos of the finished bike and post them here.
Note: Click on the photos to see a larger image.
List of new parts: Cables: front brake and Clutch, Fenders: front and rear, Front fender bracket, Sammy Miller handlebars, IRC tires, Aluminum tank
w/new seat, Electrexworld electronic ignition, Betor gas shocks, Sprocket Specialists' front and rear sprockets, RK heavy duty 520 chain, Primary chain, OKO 26mm D slide carburetor, Shorty alloy levers, Domino 'slow' throttle, All engine bearings and seals, Ital rod kit, Tarabusi piston and rings in a fresh bore and all engine gaskets.
All used parts were refurbished.