HONDA CB750 K2
After having had a mountain to climb, Mark’s on the final push with the K2
Mr Haycock is almost done
The K2 is at last in working order: MOT passed, registered, number plate obtained and on the road. As you might expect, there are one or two teething difficulties, some small and others less so, and this time I shall have a look at one of the more significant items. One feature of Japanese bikes back in the 1960s was that the clutches were generally well designed and could be operated easily. The CB750 took this to the next level as for a very powerful bike (for the time) the clutch operation was incredibly light, and that is still the case in my experience. How was this achieved? The answer is that it is all to do with gearing. The clutch is normally mounted on the gearbox input shaft, and typically there is a large ratio step-down transmission system (gears or a chain) to slow down the rotational speed significantly as a first step on the journey between crankshaft and back wheel. By
reducing the speed, however, the necessary torque to transmit the power is increased and this makes it more likely that the clutch will slip, unless strong springs are used or the clutch is very large in terms of diameter or number of plates. Would it not then make more sense to mount the clutch on the crankshaft instead? In fact, some early Japanese bikes did use that layout, but in practice it was found that the action when engaging the clutch was rather sudden and thus it was a bit tricky to start off smoothly. It also increases what is known as the moment of inertia of the crankshaft, which reduces engine (and thus the bike’s) acceleration– in theory, at least. The CB750 adopts a compromise between these two plans, in that the clutch is in the conventional place, but the primary drive has a higher ratio, so the gearbox input is faster than normal. The extra reduction needed takes place within the gearbox so the rear sprocket sizes can remain normal. This sounds like a good plan and it works pretty well. The downside is that the gearbox is clunky and selection can be noisy, combined with rather fierce clutch engagement, but I can still quite literally pull my clutch lever in using my little finger. Unfortunately, there are a couple of points on my bike which are less than satisfactory. It can be quite a surprise to find that CB750 engines can be quite noisy when idling, even when they are apparently in good condition. On mine, when started for the first time it was particularly alarming. It sounded like there was a severe problem with the transmission, so I hoped that it was just the clutch showing normal behaviour, except maybe a bit more so. What is happening is that the clutch is rapidly changing its rotational speed and play within the mechanism makes a nasty rattle. This tendency is not helped by the high speed of rotation, which magnifies the constant acceleration and deceleration resulting from an engine with (almost) no flywheel. It is certainly reduced by making sure the engine is in a good state of tune, with consistent compressions, correctly adjusted ignition timing and carburettor vacuum settings, but still present. The other thing I found was that it was completely impossible to select neutral at a standstill when the engine was hot. It was even worse than my TX500, which we saw a while ago. The noise can be reduced further by a modification to the clutch, so let us see how this is done. I should be able to see why the clutch is dragging at the same time. We need to dismantle the clutch, but the great thing on the CB750 is that there is not too much preparation to do. If you put the bike on its side stand, you do not need to drain any oil, as it will accumulate on the opposite side. We start by removing the kick start, three screws and the chromeplated cover, followed by the clutch cable (Photo 1). The inner cover is held by screws all round and a few taps with a soft hammer will help it to detach (Photo 2), exposing the clutch itself (Photo 3). The square plate is held by four M6 screws, which should be undone evenly, and now we can see (Photo 4) the usual type of special nut used by Honda which holds the clutch centre in place. I can never quite grasp why an ordinary hexagonal nut would not have worked, and imagine it must have been because it was easier to engage the tabs of the locking washer with this one. After knocking the tab back you then smash the tab grooves on the nut using a hammer and cold chisel to undo it. Only joking! You are more sensible than countless thousands of Honda owners over the decades. Because the Honda tool is so expensive, you will laboriously make a special tool from a box spanner just like Haynes used to show in their Honda manuals. Actually, why bother? If you plan in advance, you can buy a very serviceable special tool for next to nothing directly from China (Photo 5), though naturally you will not get it quickly. Or quickly from the UK for rather more, or if you are nuts you can, it seems, get the same thing posted from the USA for over £100. Note that Honda used different sizes of that nut for a range of bikes, so make sure you get the right one, which is 24mm outside diameter. Now it is fairly easy to extract all the parts and again naturally you will make note of how they all went together. You can take all the plates off at once to save time (Photo 6). Having taken off the outer, you can just see through a little window to the outer primary chain (Photo 7), and I must say mine did seem to be a bit loose so I think that must have contributed to the rattle. Changing the primary chains is a major job, so… maybe later. Sooner than that (next month, I hope), we will see what the mod is and how to apply it.