ALLEN MILLYARD
It was a chance meeting with Guy Martin and a real Honda Six that led to the creation of this beauty. Allen ends the series on his amazing RC374
One last look at Allen’s RC374
My RC374 project totally absorbed me from day one. There was a lot to think about and details to research online, in books and magazines, and from talking to people that worked on the original Honda sixes. I then had to work out how I could make all the parts in my workshop at home. Looking back, it was an exciting time and I often found it hard to switch off, sometimes finding myself going over problems in my mind as I fell asleep, and then waking at two in the morning with solutions. As I’ve said before, I rarely do any drawings or plans on paper, preferring to visualise parts and how they will fit together, then I work freehand to a vision. After eight months my RC374 was complete, running and painted, but I wondered whether it was really finished? I guess it was if I only wanted to display it at shows and start it up occasionally, but probably not if I intended to ride it on the track. A bike like this is never really finished because there will be always something that needs attention or refining; it will be an evolution over time. I had made a bike that was based on a 55-yearold, state-of-the-art factory race bike and I think it will be as much a challenge to get it running reliably as it was making the bike in the first place. One initial problem was the noise it makes. Although I think it’s one of the best-sounding motorcycles ever made, my neighbours probably won’t have the same opinion. Since completion, I have run the engine for probably an hour or so in total, but most of it was in neutral with the exception of a short ride around the grounds of Blenheim Palace during Salon Privé. My first real outing on the RC374 will be at Prescott Hill Climb in June, where I hope to ride it up the hill several times. Before Prescott, however, I need to ride it to check that the engine is running ok under load.
I decided to make a set of absorption baffles that fit nicely into the end of each of the six megaphone pipes, so I can run the engine at home and also have more chance of test riding the bike on a track. I wanted to make them look like the outlets of the CBX1000 exhaust system. This was quite easy to do. Firstly I made the end caps by cutting out six discs from 1.5mm steel sheet with my tin snips and mounted them in my lathe, all six sandwiched together between pressure pads. I then machined the diameter of all six at the same time until they were about 6mm bigger than the exhaust outlet. I then formed them over a steel former that I had machined to the exhaust outlet diameter minus 3mm (twice the thickness of the steel sheet.) The next job was to place them back in the lathe one at a time and bore the central outlet hole. I then made the six, smaller diameter inner baffle plates in a similar way. I ordered a length of perforated tube for the core of the baffle. This was welded to the end cap and inner baffle plate and wrapped with a layer of glass fibre silencer matting, secured in place with wire. Lastly, I drilled a securing hole in the baffle and silencer for a M5 screw and nut and sprayed the six baffles satin black. I fitted the baffles and started the engine. What a difference! I could hear the engine whirring away and the exhaust sounded like a higher pitched Honda CBX. I am aware that baffling the exhaust will affect how the engine runs, but surprisingly it seemed to run nicer with the baffles in than it did on open pipes. This reminded me of the moped I had when I was 16. Like many, I took the baffles out; it sounded faster but in reality it was slower! For me
it’s a compromise. I’m not really intending to race the RC374, I simply want it to ride well and sound the part, so this was a way forward to achieving my goal. With the baffles fitted I noticed it was gasping if I opened the throttle past half. I hadn’t noticed this before because the engine picks up really quick on minimal throttle and was so loud I immediately shut it down! I tried several different jet sizes and needle positions, but it still seemed to gasp and stall above half throttle. I started to think about the carbs themselves. They did look a bit big on the engine, although they were the same bore as the standard FZR250 28mm carbs. They were flat-slide carbs rather than CV carbs. I had a look on ebay and found some 26mm round-slide Mikunis. Tthese were copies, but looked to be good quality and worth a try. The 26mm carb bodies were much smaller and allowed me to make new inlet manifolds that inclined the carbs at a steeper angle, more like the original RC174. I fitted standard-size FZR250 jets, made up a new throttle cable and fuel distribution manifold, then started the engine. I couldn’t believe the difference! After adjusting the idle screws, it would idle at 2000rpm and revved up nicely with full throttle blips. Looking back, I should have realised the carbs were too big because they were listed as suitable for 125-160cc fourstroke engines. My six is only 62cc a pot, whereas the 26mm Mikuni copies were listed as suitable for 50-70cc four-stroke engines. I then made a new set of polished bell-mouths to complete the look. Winter was approaching fast so I haven’t carried out any more testing, but I will be testing the new carbs properly in the Spring ready for Prescott Hill Climb. If all goes well I’ve been invited to take it over to the Isle of Man in August to ride it at the festival of Jurby. With the RC374 parked up for Winter, I started thinking about my next project. People often ask me what my next project will be, and I tend to give an evasive answer, as I like to enjoy what I have just made first. Although there are normally a few ideas buzzing around in my head, I also need to feel inspired before I start the next thing. Check out next month to see where my mind is headed this time.