Datsun Roadster Forum-311s.org

My slides (pistons) have a small button that protrudes from the bottom; this is what strikes against the carb body when the slide is all the way down. What is this part called and how far should it stick out? I haven't measured mine yet but one sticks out much more than the other.
Thanks

Update: maybe this is just a plug and it should be flush or below surface?? Mine are not which could explain my rich idle.

I believe you are referring to the piston lifter, used when tuning the carbs. They should be the same, though I do not have a measurement of how far they stick down.

Piston should not be stopped by these as they have little resistance. When properly tuned, you press it up from the bottom and it should disable the carb that you lifted it on (make it too lean) and engine should still run on the other carb. Do each carb individually. Really a test for proper mixture.

notoptoy wrote: ↑Mon Sep 16, 2019 5:00 pm I believe you are referring to the piston lifter, used when tuning the carbs. They should be the same, though I do not have a measurement of how far they stick down.

No. I referring to small plugs screwed/pressed into the bottom of the piston, close to where the needle. They are stopping my pistons from dropping all the way. I will shoot a picture.

Ahh, you mean the little nylon stopper/bumper?

notoptoy wrote: ↑Mon Sep 16, 2019 5:44 pm Ahh, you mean the little nylon stopper/bumper?

I think so. Not sure what it is made of. Does it screw in or pressed in? Should it be flush because mine are proud???

It should not be flush, it keeps the piston from closing off air flow completely. I believe they are like little tiny mushrooms. They also make it so the pistons go "thunk" when you drop them rather than "clank".

I checked the car parts manual, they do not list or show that part, but it is there. Perhaps Keith Williams or Dave Premo or other carb experts can chime in on what it is made of, how it is installed and how high it should sit.
https://www.carpartsmanual.com/datsun/s ... retor-(su)

notoptoy wrote: ↑Mon Sep 16, 2019 6:16 pm I checked the car parts manual, they do not list or show that part, but it is there. Perhaps Keith Williams or Dave Premo or other carb experts can chime in on what it is made of, how it is installed and how high it should sit.
https://www.carpartsmanual.com/datsun/s ... retor-(su)

Thank you very much. I think this is the cause of my rich idle problem. Hopefully someone can provide a measurement.

I found a nice picture online. Looks like it should be slightly proud.
https://thosbryant.wordpress.com/2014/0 ... ding-hs-6/

Ted, though having the slide up a little would raise the needle a tiny amount allowing more fuel in, it would also be allowing more air in, right? Do you not get response when adjusting the mixture knob?

I have the same issue w/my SU's. I have an old chilton book that covers old datsuns (1600-2000, among others, up to 1968). It is the only book I have seen that covers this area. It reads "Check the height of the piston stop in the bottom face near the needle- it should be no less than .016" ". I was going to try to replace them during overhaul, but I was afraid to damage the piston, I think they are glued or bonded in somehow. Mine are basically flush w/the face right now. Wonder if they are for easier starting, don't know what other use they would have. Need someone w/a new piston to measure the height of a new stopper, but no clue how to remove old ones.

Ted928 wrote: ↑Mon Sep 16, 2019 9:46 pm I found a nice picture online. Looks like it should be slightly proud.
https://thosbryant.wordpress.com/2014/0 ... ding-hs-6/

The particular photo you show is copied from my blog and is of a British SU. You should be aware that, AFAIK, there is not a single part that is common between a British SU and the Japanese version used on Datsuns. The particular bumper being discussed in this thread is made of plastic (possibly Nylon) and as Ted says, it should be slightly proud. Exactly how "proud" it is isn't of much importance, just as long as the two carbs are equal. At a normal idle, the piston should be lifted up a bit (by the venturi effect of the air flowing past the bridge) such that there is some clearance between the bridge and the bumper.

So, you see, the exact height is not important, but as I said, they should be equal so that, when using bent wires to measure piston movement, both pistons have the same "zero" point. See:
https://thosbryant.wordpress.com/2015/1 ... or-tuning/

In British SUs, the plastic bumper is simply pressed in; I assume the Japanese ones are the same design. They sometimes creep outward a bit, in which case you should just tap it back in with a hammer. You can apply a bit of Loctite 290 (Green, Wicking Grade) to help hold it in place. When I encounter a piston with the bumper entirely missing, I make a replacement out of a piece of Nylon, then tap it in and file it off to match its mate.

Tom

Hey Tom, Good to see you on this forum as I’ve read a lot of your SU rebuilds online. There is a wealth of knowledge there. I’ve tinkered quite a bit to extract a few more hidden horses out of these. A couple years ago I asked you if you were willing to take on Datsun H6 SU. I’ve since tried to mod the throttle shafts and profile some Needles. Thanks

Thanks for all the great replies.
Forever I have had bad fueling. Way to rich at idle so I compromised and ran rich at idle and lean under load. I tried new nozzles, needles, and shaft bearings but that didn't help.
Recently I bought some new needles from ZTherapy and they recommended RA needles for the 2000. They came with instructions not to use a straight edge to set the needle height. I had previously been using my Clymer manual which said to use a straight edge to set the needle shoulder "so its shoulder is on the same level with the underside of the suction piston small diameter". ZTherapy instead said to set the needle height so it's shoulder is flush with the nozzle. I paraphrase their instructions as (1) move the nozzles all the way up, (2) set the needles proud, held lightly by the needle set screw, (3) insert piston and push the piston down while holding the nozzle up - this will force the needle to move up so that the needle shoulder bottoms out on the nozzle, (4) remove piston and fully tighten the needle set screw.
When I did this I could finally get a proper idle mixture at 2 turns out. However, the rear carb still seemed a little off as I could not get it smooth. That is when I noticed just how far the rear needle protruded. That is when I noticed that button being much prouder than the front.

I will take them apart this weekend and measure the buttons. I'll tap it in or file them so they are equal.
I am optimistic that I will finally get my fueling problem solved. We shall see.
Thanks