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I've been trying to fix a slightly rough idle and thought I'd pull the carbs and double check that both the small idle holes were unblocked on both carbs and everything else seemed good.

I found that my slide needles were in the 2nd position, the manual says they should be in the 1st posision. (R90/6)

I know this makes it richer running in all throttle posiitons that are affected by the needle (maybe not at idle at all? Or can it have some affect?)

Why would I want to keep them in the 2nd position (the jet sizes were stock, needle jet and main jet) BTW.

I haven't ridden it yet, but found a GSPD post saying if it's running lean it will surge when maintaining a constant partial throttle while riding, so I'll look for that.

Just curious, what makes the R90/6 the only bike (in my manual at least) using position 1? 2 and 3 seem to be the commong ones.

Thanks for any thoughts

I just saw series of posts on the Airlist on this exact subject. I don't think the 1st clip position is right at all. The discussion was from Oak who said that it was the 2nd position. Then Tom Cutter chimed in and said that the 3rd position was best. I think you should use the 2nd position, see how the bike runs, look at plugs (for as much as they tell you these days), and experiment with the 3rd position.

Kurt in S.A.

Did you just get this bike? or is it a bike you've ridden for a while that developed a new problem?
Is your only issue a rough idle?

To see if the idle circuit is working, turn the mixture screw all the way in and see what happens.
If it doesn't stall you need to check the idle passages, particularly that small hole in the carb mouth.
If it does stall when turning in the mixture screw, that hole is not blocked and the idle circuit is working properly..

The needle clip position does slightly affect the mixture at idle, but can this can usually be compensated for with the mixture screw. If the bike runs well and shows no sign of hesitation or surging, it's not too lean.

Before attacking the carbs to correct a rough idle, check the compression, valve clearances, ignition timing, spark plugs and HT wires.

My 90/6 runs nicely with the needles in the second position, especially after I replaced the main jets with new ones of the same size(!). I'll add to GS's worthy advice that ignition timing for each side may be out. In my case, with stock points, the timing was about 30 degrees different at the crank. This showed up when dynamic timing with a strobe - as I revved I noticed a second S mark in a very odd spot, nowhere near the first. The camshaft nose was bent and somebody had butchered the points cam. I butchered it further and got it all working reasonable smoothly. I mention this just in case.

Not all carbs came with the spring above the pistons, and the settings are a bit different between those that had them and those that dont

I appreciate the thoughts. I've had the bike a while but it has a continueing process, both in the bike running better and me learning more.

And as I learn more and do more I am expecting more from the bike.

This idle issue is a minor thing. It idles fine, then every 3 seconds you can hear something that isn't right, light a single miss.

The neddle jet possition is that the plugs seem like the bike is running rich and that both my books say poistion one and it was on #2.

And I was also curious that most other bikes say a US setting and Euro position. (US being 1 leaner).

I've had the same thing with my points airbear. Ended up getting a new advance (after straightening the cam tip made the second timing mark even worse than it was).

Roy - no spring on top of the pistons on my carbs, what is there advantage??

The spring is supposed to reduce flutter by the carb slide.

In most bikes of the period, the European carbs had different needles, so different clip positions would be understandable. U.S was way ahead of Europe on emissions regulations back then, but not anymore.

The spring slows down the movement of the piston when the butterfly is opened, and this gives a temporary enriching of the mixture, like the accelerator pump on other carbs.
It means that you can use a leaner needle and get the benefits of both good mileage and good throttle response.
IMHO , and I used to be almost on my own on this, selection of the correct spring is one of the secrets to tuning CV carbs.
Bing doesn't have much choice, but SU springs fit and you have a choice of eight of them.
Bing have a choice of two - the part for the latter bikes has a different number and is slightly stiffer, and tests by the UK owners club found it worked better on most bikes.

I don't recall any Bing carbs that came with the needle in the 1st position. They were usually the 2nd position.

What is the recommended distance b/w needle/seat replacement?