So I had reason to read a book on SU tips and tricks a couple of weeks ago, and came across an interesting tidbit that I need to share.

It talked about piston movement and how venturi vacuum makes it move according to load. Great. Then they mention the special case where the piston is all the way down, like at idle, or at startup when the flow rate is so low that the DeltaP over the bridge hasn't gotten high enough so that the pressure diff can't move it yet. The piston has to be held mechanically slightly off the bridge to allow SOME air to get through. How is this accomplished? WIth this apparently long forgotten item on the bottom of every piston.

 

 

 

 

That little round plastic thing is supposed to stick up ~.010 below the piston face to keep it off the bridge.

So far I've checked two sets of darn near fresh SU's, and found both to have that little spacer guy mostly flush with the surface on both pistons. I have another car with a fresh set and a couple of local cars with more fresh sets to check yet.

Here is it in (crappy) close up. Its a little plastic tube with a ball bearing pressed into it to slightly expand it. It needs to be pressed into the piston, and left proud 0.010 or so. I suppose it gets pressed in a bit everytime the piston drops if its old and doesn't have enough friction to stay put.

Here is pictures of how I found them in my Z, looking from a sharp angle you can barely see the black line that just proud of the surface (like .001)

I have added a blob of epoxy on top of the button thing on the SU pistons in the 71, then shaved them down to 0.01. This will provide a more stable and lasting bridge spacer. The car starts better, not that it was poor before, and idles at a more stable rate.

I'm still in the process to doing this mod to my 73, so I don't have any comment on improvements, but that dang this is not easy to start and idles very poorly. Big cam, crappy vacuum (not that engine vacuum is what actuates the piston), blah blah blah

So, I'm not out here saying this will fix all your SU related ills, more like it seems to be a forgotten rebuild setup item that should improve low flow rate operating conditions.  @BrucePalmer ? Steve? You out there? 

 

 

 

 

 

There was some discussion between Blue and I about that little nubbin in this thread. About halfway down:
http://www.classiczcars.com/forums/topic/40640-su-mixture-question/

I ran some tests on my carbed Z a number of years ago (so the details are fuzzy) where I actually measured the height of the piston lift at idle, and (in my car at least) the pistons were NOT resting on those little stops even at idle. In other words, even at idle, there was some piston lift and the protrusion height of those phenolic stops was a non-sequitur. I thought I had documented the results of those tests somewhere on here, but darned if I could find it...

Fading memory says my nubbin stops resulted in a .005 inch gap between the bottom of the piston and the bridge. IIRC, I had .008 inches gap at idle meaning that my pistons had lifted .003 off the stops.

I wonder if those nubbins were characterized and shaved at the factory, or if proper operation includes being lifted off the stops, even at idle.

Maybe you're a little rich at idle and by increasing that bumper height, you have leaned it out a little and the engine likes it?

Forgot something... This part about "The piston has to be held mechanically slightly off the bridge to allow SOME air to get through." I'm no SU expert, but I do not believe that to be true.

I would propose that's what the milled slot down the center of the piston does. It provides a passageway across the bridge right at the needle even when the piston is all the way down.

That slot down the center is in fact the key to all this. Thanks CO.

The book I was reading, when making reference to this plastic stop, shows a piston without the slot we have in our version of the SU. I have set of SU's (not the ones above) that have this piston style as well, which just served to confuse me more.

 

This style of piston clear depends on that stop to provide at least startup air access. Ours do not have this issue. The plastic stop likely should be flush or just slightly off to do nothing more than provide a soft landing for the piston.

Thanks for the clarifications.

My only experience is with the Hitachi licensed varieties so I've never seen those others without the milled slot. Thanks for that pic of the others!

So you really think yours are resting on those stops at idle? Are you sure you don't have any vacuum leaks? At idle, it's especially important to have every molecule of air going through the venturis instead of around them.

An easy way to tell if you're on the stops is to remove the damper plungers and use a small screwdriver to push the pistons down while the engine is running. See if there is a change when you force the piston all the way to the bottom?