Carburettor question

[mach2andy]

My carbs were completely overhauled by Southern Carburettors so I have no reason to doubt that they are anything other than spot on. However, I notice that 1 of the dashpots offers slightly more resistance to being lifted than the other 2. They all have the correct amount of the same SAE 20 damper oil.

Should I be worried?

In the meantime, I'm going to remove all the dashpots and carry out the drop test.

[mgcjag]

Hi Andy.....unscrew each cap and remove the plungers...then lift each piston...could be one piston slightly rubbing in the dashpot....or could just be oil between piston and dashpot from a bit of overfill....then check each of the plungers..you dont want to be swapping around the dashpots they should be set up with their corresponding piston.....Cant remember exact details but the dashpots are vented through a hole in the angled web just below the plastec cap....check for blockage.......Steve

[abowie]

Check that the jets are centred.

[malcolm]

As well as not swapping the pots between carbs, I was always told to mark the dashpots so that you put them back with the same orientation, rather than them rotating to a new position. Don't know if that is correct, but as I say, it's what I was told.

[mach2andy]

Thanks all. As a matter of interest, the asymmetric arrangement of the dashpot attachment lugs means that that they can only be fitted in one position. I've spoken with Southern Carburettors, who have been in business a long time, and they assure me that it is quite normal for there to be a slight difference in the force required to lift the dashpots. Their SU guy has been overhauling these carbs for 40 + years. They say that once the carbs are subjected to engine vacuum, they will perform correctly.

Another question? When starting to tune these carbs starting at basic settings, generally speaking, is idle mixture more like to need enriching or to be leaned off? Or is this too variable a feast to comment on?

[mgcjag]

Why is it normal to have a slight difference.....depends what slight actually means.....but it you could notice it then there is a difference....wether it will make any difference to their operation is debatable.....but if there is a defferance then something must be causing it......as re tuning there are too many other factors involved....Steve

[christopher storey]

For tuning, the best tool to start with is a depth gauge, together with a CO meter and optionally a colourtune or three!
The method I use is : -

1. make sure ignition timing is correct

2. After synchronising the throttles, set the idle using the big idle screws . Then use your depth gauge to set the jets about 65 thou down from the bridge. Then with the engine fully warmed set the Co to something in the region 5 to 5.5 % . You can check individual cylinders with colourtune if you wish, and experience shows that these engines need to be rather richer than the ideal, with a mixed yellow/blue flame. Then remeasure the depths, take the average of all three and reset the depth to the average