Vacuum gauges on 73 Commando

Back in the day, on old BL A Series with twin SUs, we used to use a length of eight bore plastic tube to listen to the hiss of air passing the butterfly's to match the idling positions.  On an Amal, one could do the same with the slides for the idle stops, then match the cable adjustment just lifting them. Again by the hiss of airflow.

Nowadays you probably need a computer and/or an app to do this, at great cost!

I understand. Is there something amiss or are you just curious?

You only need some rubber tubing to link each gauge to your balance tube connection on each manifold and you tune accordingly. What broke, the gauges?  I guess they  were perished inside.  You are only measuring depression in the manifold.  You can affect this by cable, slide, jets and basic engine gas flow issues as described in your link.

Ensure your carbs are built to same spec, note your current settings; Idle, mix etc) adjust lift of slide to be equal with drill bit balance etc.  Connect your gauges with equal tube lengths and  diameters (no leaks). Start her up and observe the depression. Note your running state and determine your cause. 

Tune her in Carb by Carb initially (one plug start up), then run with the pair and make them balance. If internals are well they should not be too far off.

I cant work with the "hiss" method, its too noisy to acclimatise the ear for me...

Good  Luck

J

Peter,

I use a pair of Davida gauges for checking the balance of my BMW R1200S, which will create more of an intake depression than the Commando. There is noticeable but unsurprising needle fluctuation at Idle, which quickly settles as the revs rise.  As mentioned already, if your gauges were perhaps old, they may have suffered internally and just given up?

Andy

Unless I missed something, if you connect vacuum gauges to the commando without inline restrictors, then you will most likely damage the gauges. Assuming you are using 6mm tube, a brass restrictor, say with a 0.5mm or less hole through the centre might work.

The vacuum pulses will be quite large so you may need to construct a accumulator fitted to the gauge side of the restrictor. This could be a meter (maybe longer) length of tube blanked off at the far end.

Even with the above in place, I would think the best way to set the carbs is using the old methods, e.g testing both slides open together and the old tweeking by ear and noting RPM changes.

Steve