ALLEN MILLYARD
Allen sorts the carbs and sparks for his amazing six-cylinder build and finally fires her up!
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Sorting six carbs? AND chokes?
From the day I started making my RC374 engine I’ve been looking forward to the time when I could push the starter button to bring it to life and hear the howl of my own small capacity six-cylinder engine. This day was drawing closer, but I still had a few jobs left to do. The first one was to tackle the carburation. I was reluctant to simply fit the standard 26mm FZR carbs because they were CV type and looked too modern. I really wanted to fit a set of six direct lift carbs, so I had a look online and found that I could buy new 26mm OKO carbs for around £35 each. I needed six but thought it would be a good idea to buy one first to check out the quality and to make sure they were suitable for my engine. The carb arrived a few days later and looked perfect for the job. The first thing I did was to measure its width and multiply by six to see if six carbs would fit across my engine. The total dimension was about 15mm too wide, but if I machined down the long protruding idle screws and relocated the overflow pipes from the side of the float chamber to the back and filed off a few protrusions, then a set of six carbs would fit across the engine and remain within the spacing of the six inlet ports. I ordered the remaining five carbs and when they arrived I stripped them all down to carry out the width reduction modifications to the bodies, float bowls and idle screws. The FZR250 engine is fitted with individual inlet rubbers that are secured to the head with two screws, these would orientate the carbs at a steep angle, but I wanted a shallower angle so that the carbs would point rearwards and provide room for aluminium bell-mouths. To resolve the problem I fabricated a pair of triple stub inlet manifolds from aluminium tube and flat strip, cut and welded together and when finished I roughed up the surface with rotary burrs to produce a cast looking finish. The OKO carbs are supplied with rubber mounting stubs, which I used to connect the carbs to my new inlet manifolds. I assembled the carb bodies without slides or tops then bolted the new inlet manifolds to the engine and pushed on the six carbs. They fitted nicely, looking evenly spaced in two banks of three. Looking at the carbs I wondered if I could change the choke plungers from individual cable operation to a link rod that operates all chokes at the same time, with a lever on the left of the bank of six carbs. I had a box of old Kawasaki Z650 carbs in the shed and was wondering if I could fit the linked choke system from a Z650 onto my bank of OKO carbs. I looked at the parts in front of me and could see that with a bit of machining to change the shape of the plungers slightly and if I made up a few brackets and extended the linking rod, the original Kawasaki parts would fit perfectly. At this point I also needed to convert the OKO carb throttle slides from cable operation by adding a link-rod and pulleys to lift all six carb slides with one central throttle cable. The easiest way to join the
carbs together was to discard the individual top covers and make a new one that would span all six carbs and that was secured to each carb body with its original pair of M4 screws. I made a cardboard template and used this to produce the new one-piece top from 6mm thick aluminium alloy. I then made brass bearing blocks to hold the link-rod and six pulleys that lift the carb slides via short lengths of cable. I also made a central pulley to connect to the main throttle control cable. The pulleys were locked in place with M4 grub screws that locate in spot drilled dimples on the link rod. After several trial assemblies and a fair bit of filing and fettling, the six carbs could be operated as one from a single throttle cable. When I was making the pulleys I had experimented with different diameters to achieve the best throttle action and I was pleased that I could operate the throttle easily with a 90° twist to full throttle. I set the jetting to the original specification for a FZR250 so it will hopefully be close enough for the initial running tests.
The throttle slides were then synchronised using a 4mm drill as a gauge to set the gap beneath the throttle slide. After a quick succession of open and snap-shuts, the settings were rechecked and found to be perfect. Brilliant, I thought, that’s the carbs sorted for now. Next up was the ignition system. I couldn’t use the standard ignition system because it was from a four-cylinder engine, so a new ignition system had to be fitted. I decided to fit an electronic system using three Hall-effect sensors and a single point trigger on the right-hand side of the engine. I ordered a system from Boyer ignitions and machined up a casing to install the Hall sensors and then made a crankshaft trigger. The system arrived and I found that fitting it was quite easy following the instructions provided. I arranged the three Hall sensors at 120º in the housing so that I could use the wasted spark principle of ignition. Wasted spark is a term used to describe an ignition system on an engine where two pistons rise and fall together and both spark plugs fire simultaneously, once every revolution. One cylinder will be on the compression stroke and the other on the exhaust stroke, hence the term wasted spark. The timing was then set at 2mm Before Top Dead Centre on cylinder number 1 and 6, 3 and 4 and finally 2 and 5 using a dial gauge. The electronic ignition boxes and battery were then mounted onto my test stand along with a temporary twist grip throttle, radiator, rev-counter and starter button. There was one last thing I needed to do before attempting the first start and this was to make a set of header pipes and clamps. Then I could easily see if the engine was running evenly on all six cylinders by checking the header temperatures with my optical thermometer. I ordered six pre-bent 28mm steel pipe bends and made a set of flanges and clamps. The flanges were welded to one end of the bend and then clamped in place on the exhaust ports. The day was finally here when I could see if it would run. The engine was filled with oil and coolant, and the carbs filled with petrol from a plastic bottle. I stood back a bit, switched on the ignition and pressed the starter button. The engine turned over rapidly for several seconds then started to fire on a couple of cylinders, and then suddenly burst into life on all cylinders almost blowing out my ear drums in the process! It revved from around 2500rpm to 11,500rpm in a split second! Over several runs on the test bed it was starting and running well. I will be fine tuning the carbs later on, once I’ve made the six exhaust megaphones. With the engine running, I was now ready to start making the bike for it to fit in.
As you may well know, this series is behind the completion of the bike which was at April’s Stafford Show! In addition to other videos, I have uploaded a video of the first start to Youtube, Search: Millyard RC374.