SU Round Top Carb Problem

[srezzo]

Hello,

I am in the process of converting from flat tops to round tops on my 74 260Z. I bought round tops from ebay and a rebuild kit from Z Therapy. While installing the rebuild kit from Z Therapy, I conducted a drop test per the DVD. There was one piston that was sticking on the way down. This piston had the same behavior using both domes. I used 400 grit sandpaper to remove some of the aluminum per the DVD. While doing so, I noticed an imperfection in the piston. I've attached photos to help illustrate. At this point I am wonering what my options are.

1) Is there any way to repair this? Should I just keep removing material using the 400 grit sandpaper (does it even need to be repaired)?

2) Is my piston junk? If so can I replace just the piston or am I better off just buying another carb from somewhere (ebay, online, etc)?

Any adivce you can give would be greatly appreciated.

Thanks,

Sari

[tlorber]

Have you checked the alignment of the nozzles with respect to the needles? This is where these things normally hang up. It is easier to do with the carbs out of the car, but you need to back off the large jam nut that holds the nozzles in place and keep realiging and barely snugging the nut down until it the piston drops all the way without hanging up.

I used a blue sharpie on the needle to see if there were any contact points with the nozzle.

I don't think the outer part of shaft that is blemished actually contacts the dome. The tolerance is on the inner bore of that shaft with the dashpot plunger, so unless that shaft is bent, I doubt your problem is there.

[beandip]

Personally I would carefully use some 600 wet/dry with oil and Polish more than just sand the tube. But this needs to be smooth.

Gary

[ChrisZ]

This is what you should do, take off dome and piston, block off the holes in the bottom of piston, put the piston inside the dome, without spring, now support the piston with a finger, turn the dome so the piston will face down, put your other hand under the piston to catch and let go of the piston while counting how long it will take to come out, if it stays up there, you have a clearance problem, do the same to the other piston, if one piston comes out faster than the other, try to swap, the time it takes to come out should be almost the same, if none of the pistons are sticking, tlober is right, the problem is in your needle and nozzles.

Hope this make sense

Chris