Equivalece between needle clip position and needle

[georgesgiralt]

Hello !
There is only one needle available for our Bing carbs. The clips positions are widely spaced. So going from, say 3 to 4, is a big change in richness.
Suppose I want to go to 3.5 instead, I could do this by putting the needle at position four and reducing the needle jet size. But what amount ? What size should I select ? Do you know ?
It would be greatly appreciated ! Thanks in advance !

[Barry]

georges

You have hit on one of the more interesting problems with Bing CV carbs.

The steps between needle positions are much too big in my opinion. I believe one needle position equates to  2 or 3 changes in needle jet size. So changing the needle jet will make a smaller change to the mixture than moving the needle.

Where it gets very difficult is comparing different needle positions with different needle jets and putting them in order of mixture strength. For example:  A 2.66 jet with needle position 3 is obviously richer than a 2.64 jet with the same needle position but how do you compare a 2.66 position 3 with a 2.64 position 4 ?

I have seen it done mathematically on a spreadsheet. It involves a lot of work measuring the profile of the needle at small increments along the length of the  tapered section. Then you have to calculate the area of the needle and subtract that from the area of the needle jet for each increment.  Do that for each combination of needle and needle jet and you can make real comparisons.  As far as I know it hasn't been done for a BMW Bing CV but if you are interested I can dig out a document that describes it being done on a Bing carb in another application.


There is another way you can achieve position 3.5 which I have done for my carbs (except I was looking to run at 2.5) and that's by shimming the needle jet height There is the issue of this also changing the jet height relative to the fuel level in the bowl but it does work. You can follow a description of it on a previous thread here http://www.bmwr65.org/cgi-bin/yabb2/YaBB.pl?num=1254577036/0#0

[georgesgiralt]

Hello Barry,
You give valuable information by saying that one clip change equate about 3 needle jet change.
A I want to go from 2.64 and position 4 (too rich) to position 3, I've to get either a 2.66 or 2.68 needle jet. Lucky I am, I own both !
It would be very good to get the BMW part number for the 2.70 or above needle jets, if one has them !
Thanks for your help !

[Barry]

Georges

Finding a 2.7 needle jet shouldn't be a problem but I think it would be much too big. You would experience excessive richness at part throttle when the parallel part of the needle was still in the needle jet.

If you really want to try position 3.5 did you consider the shimming method in the last part of my previous post.

[georgesgiralt]

Yes, Barry I saw your clever method above !
But machining a washer is too time consuming to be made at night. I've to wait for the week end...
I think it should be enough to solve my mid throttle issue with stock jetting.
In the meantime, I've tried the #3 and 2.66 and this is not good. So I've switched for a 2.68 and see tomorrow how she behave...
It should make a difference from the 2.64 stock...
I'll keep you posted !

[georgesgiralt]

So I ran the bike under the heavy rain and in the traffic jam to go to work this morning.
Seems to appreciate the 2.68 needle jet combined to the position 3 on the needle.
So the 3 size of jet to a one clip position change seems to do the trick !
I'm envious to take her to a high speed ride on the highway instead of boring work...

[georgesgiralt]

Hello Barry,
Yesterday I've found nylon washer to play with. I had to augment the size of the hole to pass the needle jet. alas the washer are 0.9 ~ 1.0 mm thick.
So I wonder how to thin them and get a couple of equal thickness... to shim each carb equally.
As yet, I've not found a way... So any idea is a good one !

[Barry]

georges,

0.8-0.9 mm will give you half way between two needle positions. I found 0.8mm was the maximum thickness that can be used because there has to be some shoulder left on the needle jet to centralise it in the atomiser.  Shouldn't be hard to rub 0.1-0.2 mm off on wet and dry abrasive paper.

I originally used 0.5mm thick red fiber washers intending to replace them with brass but I have never needed to as the fibre washers have been fine.  They are very easy to  rub down to the required thickness.
  

[montmil]

Having once had to "thin" a couple washers, I used wet 100 grit wet or dry secured to a dead flat plate.

To hold those little washers, thin, double-faced tape on a wood block worked well.

Sand in a Figure-8 motion to avoid creating a tapered washer; which I have also once needed.

[georgesgiralt]

Hummm !
Seems to be an excellent advice !
Will try it some time this week !
(wonder what kind of sanding paper I've in  the garage... )

[tvrla]

Hobby stores here in the states carry different thicknesses of copper, brass, and aluminum sheet. I'd be tempted to make my own washers.

[georgesgiralt]

Well, I've thought of that, but cutting a 0.3 mm thick sheet to make a circle and an nice round hole in the centre is not that easy...

[Barry]

but cutting a 0.3 mm thick sheet to make a circle and an nice round hole in the centre is not that easy...  

We take for granted that mass produced items are accurate to very fine tolerances. Thin washers are hard things to make accurately to the same standards from sheet material. The only ones I ever did successfully were shims for the fork damper valves and that took a lot of time and ingenuity.  

Enlarging the existing holes in a washer is only a little less difficult when the washer is so small. I found it very hard to cut clean holes dead centre even in soft material like the red fibre washers.  The only way I could do it was to clamp the washer between two sheets of hardwood after first drilling a pilot hole to enable accurate alignment then drill through the sandwich. I still produced as many bad washers as good ones.  

[georgesgiralt]

Barry,
the Nylon washer where with a 5mm diameter hole and sold by the dozen.
So I stacked them and drilled the stack while retained between two 6 mm steel washer and a wise grip in the vice...
Now, I still have to grind them from 0.9 mm thick to something closer to 0.3 mm in order to be able to adjust precisely the resulting thickness...
A last question :
What do you think is the maximal outer diameter acceptable for these washer ?
I wonder if their presence won't affect air and gasoline passage and so affect the whole air/gasoline mix ?

[Barry]

Georges,

The diameter of my washer was slightly larger than the needle jet/emulsion tube assembly but only by approximately 0.2 mm. A larger diameter washer only intrudes into the annular space where the emulsion air enters so within reason I wouldn't have thought it's too critical as long as it doesn't interfere with the air flow. If it was several mm too big I think that might cause a problem.