MR BRIDGES' VOL. V, PART II MOTORCYCLE DIARIES

IF YOU READ LAST MONTH’S MISSIVE, IT TURNED OUT I DID HAVE THE LURGY, THE DREADED LURGY, AND, LIKE A LOT OF OTHER PEOPLE, WHILE IT SEEMS TO HAVE CLEARED UP, THE FATIGUE IT BRINGS SEEMS TO HAVE SETTLED IN FOR A WHILE.

Iwasn’t, then, really feeling like doing anything that required much effort. I’ve still got to reassemble the brakes, and make the hoses, for the KLR (Fig.1), but I’ve sworn off brake articles for a while because practically every motorcycle that turns up at my workshop seems to have seized or, at least, partially seized brakes.

The other thing that motorcycles that’ve been sitting around tend to have wrong with them is cruddy carburettors. Exterior crud isn’t usually the issue – it’s the internal crud that tends to cause problems. The case in point is a GS500 twin that tends to foul the left pot; the usual amount of superficial twiddling hasn’t cured that and, at some point, someone’s tried using hotter spark plugs in it… which didn’t cure it either.

Since we know the thing was stood for a few years, a delve into the carburettors seemed like a good idea to see if there was anything obvious inside them. The first hurdle here is that clearly taking a carb apart, and putting it back together using all the same bits, isn’t necessarily going to be ideal, so a carb rebuild kit would seem useful. The question is, do you order the bits before you strip the carb and hope you get the right kit, or do

you strip the carb and identify what you need? Bear in mind that the carburettors on the motorcycle may not be the same year as the engine... As far as taking them apart’s concerned, if you’ve taken enough float bowls off to conclude that it can be a bit of a struggle, it’s probably time to invest in a set of JIS screwdrivers. Japanese screws are different from Phillips screws, they just look similar. You can identify a JIS screw by the dot mark on its head (Fig.2), and a set of JIS screwdrivers is around £10 on eBay, and you’ll thank yourself every time you have to undo a factory screw. Armed with a JIS screwdriver, I undid the two screws holding the tops on, and withdrew the slide to inspect the diaphragms (Fig.3). CV carbs work by dividing the top chamber of the carb into two sections – the lower section’s open to the atmosphere (Fig.4), and the upper section’s connected to the carburettor tract after the slide, where the engine’s sucking the air through, resulting in a lower pressure. The effect of all this is for the diaphragm to raise up like an airbed being inflated, taking the slide with it, until the pressure of the spring balances the differences in air pressure. That’s why you don’t want holes in the diaphragm, so the first step is to examine those against the light, and check for holes.

When I removed the tops, there were some small O-rings that seal the vacuum gauge take-offs located between the body and the lid, one of which fell out (Fig.5). On closer inspection it appeared to be quite perished, which is one of the reasons for buying the rebuild kit (Fig.6), and also a reminder that it’s sensible to do this kind of job on a clean, bright surface. As it happens that passageway isn’t the one that provides the vacuum above the diaphragm on these particular carburettors – that seems to be done through the body of the slide, and some holes next to the needle (Fig.7). Since that all seemed to be okay, I dropped the float bowl off,

and inspected that. While it had a light coating of rust-dust, there wasn’t a lot of it, and it all looked okay, too (Fig.8). Next step was to remove the main jet (Fig.9) which, again, looked good and I could see through the hole, but there was some crud accumulated underneath it. To remove the float assembly, I pried it off with a flat-bladed screwdriver (Fig.10) so I could inspect the float needle and its seat. The kit came with new float needles and seats, so I took the old ones out by gently gripping them with some mole grips, and twisting them out as they’re secured by the O-ring (Fig.11). This isn’t ideal if you’re going to reuse the seat as it’s too easy to damage it, but an alternative is to screw a bottoming tap into the seat and pull on that, rather than gripping the seat.

At this point I removed the pilot screws, and discovered that I had the wrong kit (Fig.12). Aside from the pilot screws being totally different, the needles were a different length, too. Turning the carb upside-down and screwing an M5 bolt into the main jet holder allowed me to tap the bolt, and drive the jet-holder out of the body (Fig.13).

This, too, was different from the one in the kit but, on the plus side, it didn’t have an accumulation of crud around it (Fig.14).

While I was there, I pried back the ears on the spring clips that hold the choke plungers in, and removed those (Fig.15). These have an O-ring in a recess in the business end, which can perish and not seat properly, but on this occasion looked okay. Again, though, they were different from the new ones in the kit. That brings me back to my point about whether to buy the kit first or strip the carb first. It took a week for the kit to arrive but, on the other hand, it was only £11, and I can use the float valves, their seats, and the float chamber gaskets, and there seem to be enough O-rings to replace the existing squashed ones, so not a waste of money by any means. I’ve spotted another kit, which is dearer, but looks right, and has

the diaphragms in it, on eBay for around £20, which is good value for a pair of diaphragms alone. What I’m probably going to do, then, is to order that, and wait for it to turn up. In the meantime I’ve given the carburettors a squirt down with carb cleaner, which is surprisingly effective at cleaning carburettors (Figs. 16 and 17), despite being new-fangled and not a thing I grew up with.

Since I’m delving into carburettors, one of the common problems with custom motorcycles is ‘it needs jetting’ after you fit pod filters. A good tip here is to take them off and try it – chances are it’ll run perfectly well which’d suggest that it doesn’t actually need jetting. To illustrate why this happens, I cut out a piece of card the size of the inlets on a pair of CX500 carburettors, and notched it for the passageway that allows air at atmospheric pressure to get under the slide (Fig.18). Placing that piece of card into the pod filters, it’s fairly clear that the passageway’s going to be obstructed by the rubber part of the filter (Fig.19). If the same style filters were fitted to the GS500 carburettors, they’d completely obstruct the passageway (Fig.20), and it’d run like a dog with no legs. This is why spacing the filters off with a piece of plastic plumbing pipe works, but you may find that carving out the rubber step to clear the passageway works just as well. Oh, and if you’re feeling I’ve abandoned you with your carb in pieces, remember, reassembly is the reverse of disassembly...