The carburettor is a Bing 16mm (type 22 no 1/16/61) that is attached to the bike via rubber mountings; a mushroom shaped one connects to the air filter behind and a cylindrical hose connects to the intake manifold tube in front. Both held by wire clips. On my bike the intake manifold is missing so the carb is removed by simply unscrewing the large wire clip binding it to the air filter mushroom. The screw and nut that hold the wire clip are removed and the clip can be bent out of the way. Note fuel intake isnt visible since fuel enters behind the carb. Note also that the float chamber is at the base.
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| Slipping the wire clip |
The rubber mushroom is a tight fit but levers off the rear of the carb.
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| And levering off the rubber mushroom |
The carb is then held by the throttle cable passing through the carburettor cap. Note adjuster on the cable, the choke slide and the two attachment screws.
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| Top of carburettor |
The screws are loosened but remain captive in the cap. The cap comes off under tension of the throttle slide spring below.
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| Withdrawing the throttle slide and choke |
As the cap comes up you can see that the choke slide is hooked onto both the choke actuating pin and throttle slide as in the NSU Quickly. The throttle cable passes through the spring and is retained in the base of the throttle slide.
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| Withdrawal continued |
The needle is in the base of the slide and visible as the slide pulls free.The following shows the view inside the carb body
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| Looking inside carb body- needle valve at the base. |
Whilst the next shows the throttle and choke slides as they are assembled onto the choke slider rod and throttle cable.
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| Articulation of throttle slide and choke slide |
The cable is held by its nipple passing through the base of the slide. Compress the spring slightly to allow the cable nipple to slip out of the slide and it can then by switched to the pass through hole and withdrawn from the top of the slide. Here the pass-through and retention holes are both visible after the cable has been removed.
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| Pass through and nipple retention holes in base of slide. |
Inside the slide the spring base equalisation washer and needle clip are visible. The different widths of the pass though (wider) and retention (narrower) cable holes are evident.
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| Needle clip and spring base washer. |
There was no gasket under the carburettor cap and I feel sure that there should have been one here. This was the first clue that the carb has been disassembled previously. I later found it was very clean internally. I'd expected it to be quite choked.
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| Inside cap-choke plunger on left, cross wire locates in raised and lowered positions using "figure of 8" clip |
Choke pin jammed with some sort of debris which needed to be removed.
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| No gasket below cap |
The choke sliding pin was however jammed solid and needed to be freed with carb cleaner and wd40 until it could slide smoothly between the two locking positions where grooves on the pin engage with a locking cross wire and clip fixed in the underside of the cap. Above- pin now free to slide.
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| Idle speed screw (round head) |
Idle speed screw and spring removed. It had been fitted at 1/2 turn from fully screwed in but I suspect this is a meaningless setting that probably postdates the engines failure.
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| Idle speed screw and spring removed |
I could then turn the carb over and address the float chamber. The float chamber unscrews with a 14mm spanner. The whole bowl unscrews from the carb body revealing the ring shaped float inside.
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| Removing float bowl |
Looking inside the float chamber its clear that fuel should pass through the two small holes each side of the central column. An additional hole halfway up the column also admits fuel. The column is screwed onto the main jet housing so fuel is actually extracted from this small chamber at the base. Its not clear whether there should be a filter in this small chamber although there was none on disassembly. There was a little sediment at the base of this small chamber which I cleaned out before checking that the entry holes were clear.
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| Inside float bowl, note two base entry holes top/bottom of Central bore. Some sediment at base. |
Before removing the jet it was safest to remove the float . This was held by a short pivot pin which was carefully tapped through. The following show a few general views of the float in its assembled position.
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| Float in situ |
Note that the main jet penetrates through the column up the centre of the ring float, and the float chamber screws onto the outside of the jet-surrounding column.
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| Float in situ |
The float is retained by a sliding pin through two lugs in the carb body.
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| Float pivot pin |
This needs to be carefully tapped down using a drift. Strange witness marks on the float body so I'll check it for leakage.
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| Tapping pivot pin down- complete process with very fine drift. |
The process was completed using half of a split pin that could pass through the lugs without enlarging the holes. Once the pin was through far enough the float could be lifted off.
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| Needle tapped out and float removed |
... and the needle removed from the fuel entry needle valve. This was a solid brass needle and has a flat base. In some carbs the needle is hung downwards from the float and fuel enters from below but in this carb the needle sits above the float and fuel enters the float chamber from above. Ideally I'd replace with a viton tipped needle, but this only seems available for the hanging needle type.
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| Removing the needle |
Once the float and needle have been removed the main jet can be unscrewed. It needs a carefully chosen screwdriver and I found an angled one fitted perfectly. I unscrewed the jet which is a small threaded plug screwed into the long brass holder. I unscrewed the jet and the holder came with it.
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| Unscrewing the main jet |
The main jet and holder can then be removed and cleaned by blasting through with carb cleaner. If necessary the jet itself can be removed from the holder by holding the hex section of the holder in a vice or wrench. I found no real debris in the carb although the main jet was blocked.
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| Withdrawing jet and jet holder as a unit |
The individual carb components were then thoroughly cleaned and all the passages cleaned out with carb cleaner and put in my ultrasonic cleaner. The main jet was cleaned and re screwed into the carb body. the Idle speed adjuster returned with its spring. I slipped the needle valve into the valve body and then refitted the float to keep it in place. Easier now the pivot pin has been polished (below). I checked that the needle sealed by blowing through the fuel inlet whilst inverting and righting the carb body. When the float pressed on the needle, passage of air was blocked. This is a good sign but it remains to be seen whether the float will successfully regulate the flow of fuel.
On reassembly I noticed that both the choke pin and float pivot pin were very tight in the body. In time this would damage the float lugs as the pin is fitted and removed so I polished both in Emery cloth by rotating them in a Dremell until they were a smooth sliding fit.
I am concerned that I will be dismantling this carb again fairly soon as I am using a substitute float bowl seal (see on) and I have changed neither needle nor the needle valve seat. I noted a mark in the valve seat and so it's possible this is going to leak. The pic below shows the view inside the needle seat and the scratch is visible inside the valve seat at around the 6 oclock position. Note cleaned O ring grooves.
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| Looking inside needle valve (right). Note scratch or mark in base around 6 o clock position... zoom in. |
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| Looking down fuel entry hole...needle valve body visible in base but there is no direct line to tap the valve body out. |
Note that there is a small basket filter inside the fuel pipe connector which also needs to be removed and cleaned. I replaced the fuel inlet using a new fibre washer.
I also found that the cable adjuster would not screw fully into the carb cap. This could be deliberate, perhaps to prevent air leaks, but since the tapping passes right through, the tight point being in the middle, it seems likely to be simply a damaged thread rather than a deliberately truncated one. The thread is M6 fine (0.75 putch) so I retapped the carb cap allowing the adjuster to screw along all of its length.
Having finished cleaning I started reassembly. The float chamber requires a square or flat O ring 41x45x1.5mm in size. These are surprisingly hard to come by so I used a round 41x45 nitrile O ring. This seems to seal but I suspect will need replacing each time the bowl is removed.
I cut a paper gasket to go beneath the carburettor cap- very fiddly but it can be done. I sprayed some paper with a light coat of black paint, inverted the carb into the wet paint and then stamped out a few gasket outlines. I stamped the holes with a punch and then carefully cut the fiddly bits with a scalpel.
I then reassembled the carb- as follows; fit the gasket to the cap, place the needle and spring seat washer into the throttle slide, aligning the grooves with the holes. Put the spring inside. Feed the throttle cable through the cable adjuster in the cap and insert it through the spring into the larger hole in the throttle slide base. Flick it to the smaller hole to retain. Finally fit the preassembled carb body onto the cap ensuring the gasket is still in position. To do this fit the choke slide onto the choke pin and engage its lug into the slot in the throttle slide. Carefully feed both into the carb body ensuring that the throttle slide engages with its location pin in the carb body and that the needle enters the orifice in the carb base. Slide home and tighten the 2 cap screws. Finally tighten the mushroom clamp.
Cleaned carburettor refitted.
