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I tried all sorts but repeatedly came back to the same idea - I wanted slide carbs. The original CV KeiHins were fine, although the chrome was long since gone the way of all Japanese chrome of that era. Other than that, they were in great shape. I cleaned them out, repainted the tops with silver acrylic and laquered them. I then took all the brackets off and plated them, reassembling with stainless cap heads. Before this, I had tried to adapt newer and bigger carbs from other Honda's but nothing seemed to work, mostly due to linkage issues. I still thought the originals would be restrictive as they had small 26mm venturis and 30mm would be my optimum size. In the absence of anything else, I fitted the originals and when fired up, I was surprised at how responsive and clean the carburation was. Then, I came across a scrap set of CB750 slide carbs which looked like they would make two out of the four but even these were lacking some parts. I already had one 928 Concentric which had been bored to 30mm and eventually I managed to find another one the same on Ebay. I bought it, bored it to 30mm and bought two overhaul kits which effectively made them like new. I made stub adapters to suit by cutting out the flanges from steel plate and filing to shape. I bored them 34mm in the four jaw to suit some tube with a 28mm ID/34mm OD then silver soldered the flanges to the tube. The tubes were then mounted in the three jaw and bored to 30mm. A groove was machined in the tubes to suit the original carb rubbers and then they were plated. Some may wonder why I didn't weld the tubes to the flanges, but I consider silver solder perfectly adequate for the job, it is, after all, brazing rather than soldering, and the amount of heat required to braze negated any risk of distortion which would have been unavoidable if arc welded. I have yet to clean up the carburation on these - waiting for the neighbours to go out for the day, but they look good, and I firmly believe that if they look right, they will be right.
The picture shows rather long stubs which seem excessive. Since fitting these carbs, I have shortened the stubs and brought them to a rough state of tune. Final tuning will be done at the track on full throttle and under load. For information, I started with a needle jet of 105 and a main jet of 200, and a throttle cutaway number 3 with the needle in the central groove. This should be close to the optimum although the main jets might be a little large. A plug chop will confirm that one way or the other. The original new CB250 throttle cables were modified by unsoldering the end nipples, shortening the inner cable, and re-soldering to fit. The original carb rubbers were re-used as these were already 30mm ID. |
