Carb tuning

[Barry]

It’s a windy wet Saturday and my wife’s at work so it’s play time in the garage.

Anyone who has raised or lowered the needle in one of it’s 4 preset positions will know it makes a significant change to carburetion. Raising the needle means that the tapered portion of the needle comes into effect earlier which richens the mid range. The problem is that moving the needle one slot makes much too big a change to effect minor tuning adjustments.  My own bike with needles in stock position 2 has a mid range flat spot from cold which is still there to a slight extent when fully warm. Raising the needles to position 3 results in such a dramatic improvement from cold that it just has to be much too rich when hot. What I need is something in between. The preset needle positions are 1.8mm apart so maybe I need to raise the needle by some smaller amount.

I remember back in the 70’s I had a car with Stromberg carbs where the needle was raised or lowered in tiny increments by an adjusting screw.  SU carbs of that era had a similar infinitely variable adjustment in this case by  raising or lowering the needle jet instead of the needle.  In fact the cold start enrichment on an SU was done by lowering the jet how clever is that.

Most modern motorcycle carbs can also have the needle raised in small increments by fitting shims. Google it and you’ll discover it’s a common place tuning mod to richen the mid range. We can’t do this with our Bings because of the way the needle is secured so I set about looking at how to lower the jet in relation to the needle by a small amount

The pictures show my solution. I fitted a shim made from a 0.5 mm thick fibre washer between the needle jet and emulsion tube. If lowering the jet by 0.5mm is not enough I will devise a means of  making a shim 0.9mm thick to take it to the mid point between needle positions.  Pity I can’t test it until the rain stops.

[Rob Valdez 79 R65]

I can't wait to hear the outcome!

[nhmaf]

Sounds like a clever (and inexpensive) idea to try - do please let us know how it goes !

According to the Bing manual, the stock position on the 81-84 R65 bikes was #3, but they had also changed the needle slightly and the idle jet slightly at the same time, likely to compensate for the slightly too rich setting the #3 position might cause.

In any case, experimentation can be alot of fun!

[Rob Valdez 79 R65]

I wonder how my new '82 engine will like running with my old '79 flattop carburetors?

[Barry]

Well 0.5mm is less than 1/3rd of a needle position and you wouldn't have thought it would  have made much difference but it did. The Cold flat spot is all but gone and once warm its fine. [smiley=smiley.gif]

I'll ride it for a week before I try a thicker shim. I have to find something suitable anyway.

BTW the shim dimensions are 5.5mm ID X 7.8mm OD and they are made from fibre washers with the ID drilled out to size. I tested them with a soak in petrol and they seem fine. The washers are used to mount computer motherboards in the case. Any IT man will recognise them instantly.

I know changing to a 2.64 needle jet with the needle in position 3 or more likely 4  would have been an option but the way those old SU carbs worked got me thinking.

[Lucky_Lou]

I would be concerned that "fibre" washers wont handle vibration very well and may break up maybe better to try and find a metal alternative once you have decided what the best thickness is...shim steel or something like that
Lou

[Barry]

The washer is under compression from the main jet holder but yes your right for the long term I'll probably make something from a brass washer.

There was another option that didn't use washers. Put the needles in position 3 and machine the required amount off the shoulder of the needle jet for the same result. Harder to experiment though.

[Barry]

This weekend I fitted an additional 0.3mm shim between the needle jet and emulsion tube taking the total to 0.8mm giving me a setting close to the mid point between needle position 2 and needle position 3.  A test ride showed I had achieved a further improvement in mid range response eliminating the last trace of the flat spot even when cold. It still pulls a little more strongly when hot but I guess that is as it should be.

I’m very happy with this setting which is just as well because it would be impractical to fit a much thicker  shim as the shoulder on the needle jet is only 1.2mm deep and some of this is needed to centralize the jet in the emulsion tube.  Also I’ve increased the overall length of the jet assembly by 0.8 mm and anything much more than 1.0mm may not allow the O ring to fully engage and seal.

I think I’ve demonstrated a new method of fine tuning the mid range which overcomes the coarse adjustment provided by the 4 needle positions. I’m conscious of one failing though and that is by lowering the needle jet 0.8mm  I’ve effectively increased the fuel level by the same amount which must richen the mixture a little although I don’t know how to quantify this. All I know from experience is that a significant change of say 3mm in the fuel level has quite a noticeable effect while a change of 1mm is hard to detect.  I’d  welcome any views on this.

[Semper Gumby]

I did not know this was possible.  Interesting.  The BING Manual makes no suggestion of shimming the needle jet.

Very interesting.

[Barry]

I searched all over the net to see if someone had done this before and found nothing. Lots of info on shimming needles though and this amounts to the same thing except for the slight change with respect to the fuel level.