I did a bit of work in the garage this weekend, all of which will end up on the blog in the coming week. The first scooter I worked on was a '59 Vespa 150. The '58-59 Vespa 150's were transition scooters. These were the first of the rotary valve models. In fact, the very early VBA's used a "hybrid" piston ported motor in the new narrower frame. These didn't last very long, and I've only seen a few of them over the years.
On thing about living in Berkeley California is that there are a lot of scooters around, and no real shops in town. There are two great shops in San Francisco, but many people just can't or don't want to make the trek over the Bay Bridge with their scooter for service. I end up working on quite a few scooters owned by people in my neighborhood. This VBA was one such scoot.
It is an interesing scooter from the perspective of its status as a short time transition model. This motor is one of the first rotary valve versions. It has some interesting features which are only visible when you open up the carb airbox. I took a few photos to document it.
This scooter had several problems which together manifested themselves in hard starting and non-reliable running. The owner had taken the scooter to the a new Vespa dealer, who did some work on it. By work, I mean they took $500 of the owner's money and came nowhere near fixing the problem. Warning to all, be cautious before taking your vintage scooter to a new Vespa shop!
After a bit of checking, I think I found the sources of the problem. First, upon checking the spark with my trusty spark check tool, it appeared to be weak and somewhat intermittent. Looking at the ignition coil, it looked like it was the original HT coil fitted to the scooter at the factory 50 years ago. I sourced a new coil, and will post about the replacement when it arrives.
The other problem that I noticed was that the scooter didn't appear to be getting fuel from the carb. I checked the fuel supply to the carb from the gas tank, and that was working ok. So, it appeared that a carb clean/rebuild was in order.
I took the carb off the scooter. This carb is somewhat interesting in that it is the first series of these Del'Orto SI carbs which were used on all the later Vespas. These first SI's are different from the later ones in a couple of significant ways. The first, and most obvious is that they do not have a enrichener choke circuit. As you can see from the photos below, these early scooters used an air choke, rather than a fuel enrichener choke. I guess this system didn't work too well, because they changed to the fuel enrichener choke on the motors just after this one.
On the inside of the carb, there were also some differences. The main one is that the fuel filter bowl above the float is significantly smaller than on the later carbs. As an aside, when I took this particular carb apart, I found that the needle was toast, and the float was completely stuck. So fixing that should help the running.
Given that this carb looked a bit worn, and the owner is not a home hobbyist mechanic, I would normally simply replace the float or the entire carb to ensure reliable running. However, since this scooter is so early, and has the unique carb setup, I could not do that. After thoroughly cleaning the carb, I just replaced the needle and float, and put in some new gaskets.
Above, you see a close up of the carb and airbox interior. Very sparse and unfussy in here compared to the later scooters. Note the carb stamp "SI 20/17". Also note that the fuel line hole on the airbox is small, with no rubber grommet as on the later airboxes.
Here is a detail shot on the choke mechanism. The choke cable attaches to that little arm on the right side of the picture. The arm actuates a small flap, which you can see at the top of the picture. That flap closes off the air flow to the carb, thus richening the mixture. I'm not sure how well this system works. I guess I'll find out once I get the scooter all back together!
