16012-354-004

Not Alan Whicker's jet set, but that from an 18mm Kehin carb. Where to start ? Induction issues with the CB Special are well documented on these pages, and as part of the earlier attempts to get the creature to run properly a matching pair of carb refurb kits was ordered from the great David Silver Spares. The refurb kit includes a bundle of gaskets, o rings, float jet and needle, pilot jet, emulsion tube, main jet, jet needle, mixture screws etc etc - what it doesn't include is a needle jet for the jet needle to run in. What I have discovered is that the OEM Needle Jet Set from Honda (part no 16012-354-004) comprises both, crucial, matching parts - tolerances here are down to 1/100th mm so size matters. Needless to say the kit pictured above appears to be the only OEM part in the world and I need a pair !! I can think of a bunch of goodies that come in pairs and that's great but in the case of carbs for my project I'd be happier to run a single. In the same way that I really need two eyes to fully appreciate distance, scale, perspective and allow my brain to perform lightening quick calculations of on-coming vehicle speeds, my little twin needs two very clever, minutely detailed, perfectly machined, finely balanced, infinitely variable fuel metering and delivery instruments to enable it to run properly. It's not. MORE JET SETS !

Fine Tuning

CB engine rebuilt, new cam chain, valve timing set, clearances at 2 thou, ignition timing spot on. Fuel on, tad of choke, ignition on, kick - fires first swing, marvelous. After initial warm up run of five minutes we cut the ignition and went over the engine looking for leaks and loose bits. Satisfied all's well I took the bike for a blast up the road - at 1/2 throttle it all went horribly wrong - EXACTLY the same misfire as before !?&@?!!$£?@%!. Back to the carbs then. These are 18mm round slide Kehins. Basically at tick over you've got a pilot jet and mixture control to adjust, from tick over to 1/4 throttle the mixture is determined by the shape/size of the cut-away at the bottom of the slide, from 1/4 up to 3/4 throttle the tapered jet needle and the needle jet it slides in control the mixture, from 3/4 to full throttle you're running on the main jet. All these stages are adjustable, plus float height adjustment is critical. Add to this you've more than one of these instruments to synch and you have a rough idea of what's required - PATIENCE. Lets forget about atmospheric humidity, temperature, altitude, pressure etc. The carb parts on the bench are: throttle stop screw, pilot mixture screw, float valve seat, float valve needle, pilot jet, assorted main jets #88,85 & 83, jet needle, needle jet, emulsion tube/jet holder. The mini pocket microscope (50 X mag) is handy for reading jet numbers and identifying wear. Unfortunately ALL of the above parts are subject to wear and, in the science of fuel/air mixing, this wear is ruinous. Off to stock up Yorkshire Gold supplies and try and find some decent Turkish cigarettes. MORE VARIABLES !