Twistgrip, Carburettor and Throttle Cable

When I first bought this bike. The throttle twist grip would hardly rotate. The cable was present and attached at the throttle end, although not attached to the carburettor. Investigation showed that the cable outer had become crushed and damaged, resulting a rusty amalgamation, and there was no way the cable could be rescued. New cables are very expensive. I therefore decided That eventually I will have to manufacture a replacement  Luckily throttle cable universal kits are around half the price of a new cable. It was not immediately obvious how to dismantle the twist grip. 

The grip itself consists of a rubber sleeve that slips over an inner tube. The tube has a spiral groove on the inside that engages with a slider in the handlebar and to which the cable is attached. Twisting the sleeve therefore pulls the cable to and fro in a groove in the handlebars. The sleeve itself has to be loose in order to rotate. There is a small grub screw that passes through a slot in the sleeve to prevent it from pulling off the bars. Finally there is a collar that slips into the seeve to ensure smooth motion. The grub screw also passes through this sleve to fix it in position. The screw  is normally covered by the twist grip rubber itself. To get at this screw, I closed the twist hard against its stop, and pushed it until the rubber started to slip/rotate on the sleeve. At this point I could pull the rubber outwards and gain access to the grub  screw, Rotating the twist grip against the stop. I.e. over closing it, prevents strain on the spiral groove inside, which operates the cable pull. In the picture you can see the twist grip partially pulled back, and the screw removed. The screw passes through the slot in the twist sleeve and a hole in twist collar.

Here the screw has been removed and the twistgrip collar slipped out from the sleeve. You can see the groove in the twistgrip sleeve though with the grub screw passes.

Here is the grub screw which should pass through the groove in the sleeve and a hole in the collar to screw into a threaded hole in the handlebars.

Once the screw is removed, the twist grip will slide off the handlebars. This reveals the throttle slide underneath. The  cable enters the twistgrip through the bars themselves. Its ferrule terminates in an anchor block that fits into a circular cutout in the handlebars. The nipple itself is held in a slider which can move forwards and backwards along a slot in the handlebars, the slider itself engages with a spiral groove inside the twist sleeve, such that rotating the twist grip, pulls the slider to-and-fro along the groove in the handlebars.

Here you see the cable nipple engaged in the slider. It's been raised to disengage it and the slider than lifts out.

And here, the slider has been removed leaving the cable anchor block in place. The nipple has been raised to disengage it from the block which will then come out easily. I could then pull the cable back out of the handlebars in order to replace it in due course. 

Here the cable has been pulled forwards in the bars so that the termination block can be slipped out.

There are 2 types of carburettor fitted to this bike. Mine is the later type and there is a shortage of parts for this version. It appears relatively complicated for a small carburettor but "overcomplicated" does seem to be a theme of this bike's design. There is a round float chamber, which is obvious. Air enters the top of the carb and airfuel mixture exits from the bottom. The throttle cable should enter via the top cap although in my case there were remnants of what I think was a choke cable here.

 Unscrewing, this cap allowed me to withdraw the remnants of the broken cable, the throttle slide the spring and the needle, it was clear that these internal components have been modified

The diagram shows a washer just below the cap, which presumably provides an airtight seal. It also shows a clip on the needle which retains the needle inside the throttle slide. Both of these have apparently been changed. It's not clear whether the original washer should be rubber or fibre, the item fitted in my carburettor was a badly fitting rubber ring and clearly not original. I decided to change this for a fibre washer.  There was no needle-retaining clip at all instead, a disc of rubber had been cut to perform this function. I ordered a new needle clip to fit when I reassemble the carburettor.

These are the components of the throttle slide assembly as found inside the carb on disassembly.

Here are some views of the full carburettor.

The float chamber is held on by three screws, two short and one longer. They should all have washers.

Once the screws were removed, the float cover will come off easily. Inside you can see the float and a Rubber o ring. In my case this appears flattened and hard. I doubt it will seal. This is described as a 40.5mm o ring but its thickness isn't given. I will need to experiment to find a replacement. It's also not clear whether this later version of the carb used a std O ring or a specially made seal... the seal is now unobtainable so I'll experiment with O rings to see If I can make something fit.

I removed the float and the needle valve then falls out. Cleaning everything thoroughly

There are several screws visible in the carburettor.These two accessible just below the top of the carb are the mixture screw (lower) and the throttle stop screw closest to the throttle cap.

View of carb, float chamber removed (note openings for jets at the bottom of the chamber.. tapered housings shown right).

On the other side of the carburettor are 2 more, screws, the aluminium one is the needle jet holder and the Brass is the slow running jet. Both of these were tightened fully home.

Before removing any screws, I screwed them in fully. So as to get an idea of their running positions once I had made a note of how far they were screwed from their full stops, I could remove them and refit them to this same distance. In this diagram, the screw nearest the throttle cap (throttle stop screw) is screwed out from its full stop position two turns

And the other screw (air screw) is one and three quarter turns from fully screwed home.

The aluminium screw appears to be a needle jet holder. Both of these screws are screwed fully tight.

Removing the aluminum screw shows the brass needle jet beneath. This should tap out, but as I don't (yet) think it needs replacing, and it's not blocked I left it alone.

The other screw (below) is the slow running jetwhich could be fully removed and cleaned.

Inside the float chamber were 2 more brass fittings.The left hand one appeared to have a screw head and is the main jet emulsifier tube. This comes in various sizes 55, 58, 60 annd 62. It should unscrew for cleaning but this had been so badly damaged that its removal was impossible. I may be able to remove it using an "easy-out" but this would inevitably destroy it so I'd need a replacement first. Unfortunately I cant tell what size is fitted here. I will size it with dental drills and then  Ive ordered a 50 and Ill try to match it with the dental drill bit that fitted the original.  The right hand one did not seem to be made for removal and is not indicated on the parts list. For now, I cleaned both of them in situ , using carburettor spray

I cleaned the entire carburettor in the sonicating bath and then reassembled pending the arrival of suitable replacement parts... if I am ever able to obtain any!