SU Piston Modification

[Randalla]

This is a first. I've probably rebuilt 60-80 Z SU carbs over the years, but have never seen this modification. These pistons were in a set of early, 4 screw carbs from a customer's 1970 240Z. As you can see in the pictures it appears someone drilled and inserted a short piece of steel rod into the bottom of each piston. The end protrudes below the bottom of the piston raising the piston perhaps 1/32" off the bridge. The net effect would richen the mixture significantly, similar to dropping the needle from the piston. The piece is small enough that I wouldn't think the extra weight would impact throttle tip in but I can't be sure. Anybody else seen this modification before? 

Edited February 12 by Randalla

[Captain Obvious]

2 hours ago, Randalla said:

The end protrudes below the bottom of the piston raising the piston perhaps 1/32" off the bridge. The net effect would richen the mixture significantly, similar to positioning the needle further into the piston.

Actually it would do just the opposite. It would lean out the mixture just like using the little lifter rod on the side of the carb body to push the piston up. Not sure how much with that amount of lift, but leaner none-the-less.

That aside... The concept is pretty much stock. I'm sure you are familiar with the little non-metallic chunk (presumably Phenolic) pressed into the bottom of the suction piston. That little button acted as a bumper stop in a crude attempt to prevent metal to metal clunk when the piston bottomed out on the inside of the carb body.

Are you sure that non-stock piece inserted is in fact metal? Is it magnetic? Hard to tell from the pics, but I wouldn't be surprised to find that it's a hard nylon or Delrin material. I'm thinking someone tried to replace there old smooshed out original bumper stop.

[Randalla]

Thanks for weighing in Captain Obvious. Not sure I understand your response. At idle, the effect of the higher sitting piston would pull the needle further out of the jet tube allowing more fuel to be sucked out of the tube and across the bridge, correct? I.e. the same as raising engine speed with the idle screw. The material is definitely steel, as I tried to carefully file one of inserts flush with the bottom of the piston. The non-metallic chunk you reference is in exactly the same spot as the steel inserts in the pistons I have. Seems to be someone experimenting/modifying the stock inserts with larger diameter steel ones???  

Edited February 13 by Randalla

[Captain Obvious]

13 hours ago, Randalla said:

At idle, the effect of the higher sitting piston would pull the needle further out of the jet tube allowing more fuel to be sucked out of the tube and across the bridge, correct? I.e. the same as raising engine speed with the idle screw.

You are correct that the higher sitting piston would pull the needle out of the jet tube further, but it's certainly not the same as raising the engine speed with the idle screw. Goes like this...

Raising the engine speed with the idle screw increases the airflow through the carb (because you have opened up the throttle butterfly). That increased air flow will cause the suction piston to rise a small amount to keep the air velocity across the venturi constant. That's the whole theory of these CV carbs.

In contrast, if you lift the piston using a protrusion on the underside of the piston (like the little nubbies you have discovered), you will actually DECREASE the air velocity across the venturi, and the result is a leaner mixture.

The part I really don't understand is why didn't they use a same diameter insert as the original non-metallic bumper. I mean, I can see someone thinking their bumpers are all smooshed out and ineffective and wanting to replace them, but the part I don't get is why they thought they needed to drill out the piston to accommodate the replacement. Why didn't they reduce the diameter of the insert to fit the piston. Why permanently modify the piston???

[Yarb]

@Captain Obvious One of those mysteries that we will never solve.

[Randalla]

Maybe I'll try running them in my Z to see if I notice a difference.

Edited February 14 by Randalla

[dutchzcarguy]

20 hours ago, Captain Obvious said:

but the part I don't get is why they thought they needed to drill out the piston to accommodate the replacement. Why didn't they reduce the diameter of the insert to fit the piston.

Maybe the insert is lead? To make the piston heavier? Now it will come up slower and drop faster? just a thought..

[Mark Maras]

Perhaps a mod to richen the mixture during acceleration? I would think a heavier spring would do the same thing.