My '72 240Z is running pretty good after I have rebuilt the carbs. However, when I come to a stop, the idle stays around 1000 rpm unless I apply a little rev to the engine and then it falls down to 750-800 rpm. Also, the rpm has a little flucuation at idle. Anyone have any ideas?

I had this same problem. Try lubricating the throttle linkage with some white lithium grease.

Check the condition of the return springs and make sure both are hooked up. As well as applying the grease as Si|v3r72 says, check for binding in any of the links.

Also check the level of damper oil in your carbs - that may play into the fluctuation.

Is it worth changing the carb piston springs? I did not change these out when I rebuilt the carbs.

Those springs are so flimsy and loose (sounds like my ex-wife) that I don't think a new set would make a difference - but you should probably check with the Z-Therapy guys for the final on that one.

How much of a fluctuation do you notice? Can you describe it a bit? Does it act like a drop or a surge?

I'm sure that you used a synchronizer to get them both in-sync - and not off by 98 degrees (sorry, more bad humor here - as usual).

Also check that they are both set with the correct idle mixture. Maybe one carb is getting a bit more/less fuel at idle, thus causing it to surge or drop off until it gets in check again with the other one. This probably is where the cause is. Also, check that the floats are adjusted correctly - again I'm assuming a lot of this was done since you just rebuilt them - just going over some of the possible causes. Also, make sure that the needles are seated at the same height.

Let me know what you find...

Rick

hmmm, my memory of SUs maybe a little off here... but don't SUs have a butterfly valve as well as the 'piston' which is vacuum operated ? If that is the case then putting stronger springs in the pistons would be a no-no as the vacuum generated under accleration would not lift the pistons as high as required. The throttle controls only the butterfly (correct me if I'm wrong... Alan?) and its that mechanism you should be checking.

In a 'slide' carburettor, where the throttle action actually lifts the slides (there is no butterfly), stronger springs would be warranted. I had this very problem on my bike which had 40mm DelOrto carbs and had the alarming habit of maintaining high revs when you were hoping to back off for a corner:surprised (although this did improve my corneringLOL ).

This is also caused by worn throttle shaft bushings. When you let off the gas they don't always return to the same place due to slop in the shaft. There's really nothing you can due about it besides getting newer carb bodies. If you apply a spray on grease at the throttle shaft ends and the problem temporarily disappears you've found your problem.

My thoughts on linkage lubrication. The teflon ends on the linkage do not need any lubrication. I find mine sticks the least if I take the linkage apart and clean the joints with break parts cleaner. The only place I use grease is at the carb body and where the linkage shaft rotates in the balance tube mount.

SoM