This is for cheapskates (and the hard of hearing).

My attempts to balance the air flow in the twin SU’s of my car have always ended in utter frustration. I can see a difference in the height of the carb pistons before I can detect any difference in the hissing sound. It appears my ears need tuning more than my carbs. I’ve even tried to amplify the sound and shield against background noise by fitting a funnel to the end of the listening tube and holding my ear in the wide end of the funnel (made me look like the doggie in the ‘His Masters Voice’ logo). That didn’t work either.

And I wasn’t going to spend money on an instrument to do what others claim to be able to do by just listening.

So, I played around with some ideas to make my own air flow indicator – purely for fun and not expecting too much to come of it. It turned out, however, that the very first one I put together worked so well that it has now become part of my toolbox. Very sensitive, and adjustable to cater for different engine speeds.

Here are some pictures:









Key components are the tube of a dropper (cost about $1) and a little 7mm ball which I carved from a wine cork. The dropper tube is of the plastic type. These are slightly tapered (to facilitate de-moulding during the manufacturing process, I think) – an essential feature for this kind of flow indicator. The rest basically consists of some PVC tubing off-cuts and other odds and ends found around the house.

Simplesimon

Nice!! Very slick DIY setup.

I might remember this for balancing my motorcycle's carbs when and if I get around to it.

Happy Bricking :D

Wow, very neat piece of work. I think a lot more effort went into it than you are willing to admit to!

That is a pretty slick instrument.
I usually just adjust the linkage to where both throttles are completely closed and both start to open at exactly the same time. Then when I adjust the idle I turn both idle adjusting screws exactly the same amount. That seems to get them close enough that the balance tube in the manifold does the rest.

True, but doesn’t the balance tube _always_ "do the rest", the whole idea of balancing the air flow in the carbs being to get to a point where there is no flow in the balance tube?

Simplesimon

I guess so but if you carried it to that extent, there would be no sense in even having
a balance tube.
I strongly suspect that my method produces results sufficiently close to the optimum that
you would not be able to detect a performance difference.
That should be "good enough".
I only measure cylinder bores to a tenth of a thousandth but the manual says they can
be up to 0.005" oversize before serious attention is needed, for example.
At age 72 I like to spend my time on things that will indeed make a difference.
--
George Downs Bartlesville, Oklahoma

Nicely done!

Nicely done is right, measured drawings please so I can patent the design. I mean build one myself! Oh and don't forget a set of carefuly crafted instructions-actually a video would be best and you can put it on you tube.

First thing to do is to find a suitable plastic dropper tube. The one I used has an OD of 9 to 10mm and length of 88mm. Next you must find a ball of reasonably light-weight material that will fit in the tube with a little clearance. I ended up carving my own from a wine cork.

The rest is really a matter of taste and availability of materials. I am sure many other and better designs are possible.

The main tube I used has an OD of 40mm. The SU’s (HS6) happen to have a throat diameter slightly less than this, so I bevelled the front end of the main tube and stretched a short length of bicycle inner tube over the end. The rubber of the inner tube then forms a nice seal with the throat of the carb. At the back of the main tube there is sliding damper formed from a short length of the same material as the main tube. This is slit length-wise so that it can be opened up a bit to fit on the outside of the main tube. Two holes are drilled through the damper and the main tube, in the 10 o'clock and 2 o’clock positions. Adjustment takes place by rotating the damper. The rear end of the main tube is closed.

I have two connecting tubes (the black ones) going from the main tube to the cross-over tube (the small-diameter white one). The inner ends of the connecting tubes protrude into the main tube and are cut at an angle so that the openings will face the carb. There is no particular reason for there being two. A single one may work just as well or better.

All the plastic parts are simply press-fit together. No need for glue.

Simplesimon