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What I ended up doing was removing the carbs, but left the cables attached.
I inverted the carb and reinstalled the stack, checking for the correct position of the atomiser. The top was left on, so the needle allowed some guidance, but the atomiser still needed some careful coaxing to seat correctly.

So now I know the bottom part of the system is clean and working well. The bike is running better.

When I have some more time to fiddle and play with it, I'll take the tops off and change over the needles and check the diaphragms. The needles were visibly worn, with an obvious colour change along the shaft.

So now I have MJ 160, NJ 2.68, clean atomiser and worn needles at the 2nd position!
In my box of carb bits I also have MJ 165 & 170, NJ 2.66 and new needles. I plan to wait for a pleasant day when I'll change the needles over, maybe even put new cables on (current ones look ok though) and take the bike for a ride up thru the local hills http://www.visitmelbourne.com/Regions/Y ... anges.aspx. I can stop at a winery or two, tweak the carbs before shutting them off and try new wines. If I struggle to get it smooth, then I'll need to restrict the procedure to three wineries!

Just back from a rollicking good ride over Mt. Hotham, about 900ks all up this weekend. Weather was magnificent. There was just the occasional patch of snow left up there.

netpic

Mt Hotham's summit rises to an altitude of 1,861 metres (6,106 ft) above sea level. Hotham village stands at a height of 1,750 metres (5,740 ft) , so not too tall, but it did seem that the bike ran very rich up there. It seemed to struggle for a smooth acceleration in the higher rev range when belting it going up the hill. I did expect it to run rich until I put the new needles in to match the needle jets. It didn't need any choke to start either.

Over one stretch of 135miles/216kms I used 21.5 litres of fuel. That included the tight and challenging 40mile Eildon/Jamieson Road where the rpm stayed mostly high.

montmil wrote:Prior to removing the needles, take an accurate measurement as to how far the needles protrude out of the slide. Replacing the needles in the correct clip notch can occasionally be frustrating. Your original measurements will confirm the correct positioning.

The measurements at each notch would be handy info.

Steve,

Maybe the bike is running slightly rich at sea level, I would not really expect to fell the effects of altitude until at least 8000 ft and would not expect to feel anything crossing a 2000m pass. I find that during twisty mountain roads my fuel consumption drops rather than rises

chasbmw wrote:Steve,

Maybe the bike is running slightly rich at sea level, I would not really expect to fell the effects of altitude until at least 8000 ft and would not expect to feel anything crossing a 2000m pass. I find that during twisty mountain roads my fuel consumption drops rather than rises

I expect it was a little rich as the 2.68 needle jets were one size up and the worn needles were still in, so the orifice would be wider.
Going up this hill is just great fun as the road is damn good and it's almost wot all the way. Consumption always plummets with that behaviour! Fun though.

New problem...I tried to change the needles today. I couldn't get one of the carb top screws out. They're ss allen key screws, with adequate antiseize. 3 out easy enough, the 4th proved difficult. I took that carb off and swapped it for a spare I have.
I had a crack at drilling the screw out, but wimped out before it got close to the threads.

On the other carb (the first one I opened) I did change the needle, but despite due care the retaining spring and it's small washer fell out. The spring appears to be broken as there isn't complete continuity. I shoved in back in anyhow and positioned the needle to second notch in. Second notch is correct, but there appears to be four options so is it second from the top in or second from the bottom? That's where the measured distances would be very handy.

Anyhow, it's a work in progress.

Position 2 is 2nd from the top. Position 1 = weakest 4 =richest.

These are the exposed needle lengths for my 79 flat tops. Curiously the gaps are not exatly equal even when measured reasonably accurately with a vernier but approx. 1.8mm between positions.

Position.............Needle Length mm
1 ......................42.37
2 ......................40.57
3 ......................38.79
4 ......................37.10

barryh wrote:Position 2 is 2nd from the top. Position 1 = weakest 4 =richest.

Thanks Barry. That's where I put 'em in to the second notch from the top, ie from the non pointy end. Cheers.

Currently I am running two 40mm BINGS but one is from an earlier S and the other is the correct one for the bike.
Needles and jets are the same for both of course, but the older one has a different top. It's missing the small protuberance for the location of the choke spring. Where does the choke spring go on the earlier 40mm carbs? Did they have one?

How early S ?

Carbs which had the stiff wire choke cables and operating lever on the air filter housing would not have had a choke spring.

My spare set of carbs are 94/40/103-104, or R100S carbs Barry. I was ferreting around looking for Koni seals when I found the older choke lever and wires. I didn't use them, but did put the correct carbs back on and tried syncing them again. Ran like a dog for a couple of days. Each time I rode to or from work it had a tuning attempt made. Last night I found a small spring on the bench, hidden amongst some stuff. Off came the mixture screws and sure enough the spring was missing. I coulda swore that it was assembled right...I coulda swore and I did!

What BING carbs use this piece above the needle to keep it in position? Mine sure don't.

Hello !
Only the late type of carbs have switched from a needle hold by a spring to a needle hold by a big screw. The pivot date was, IRC 1983 or later.