4-screw roundtop piston sticking

[Jeff Berk]

I rebuilt my pair of carb's using Z-therapy's kit. My rear carb seems to function correctly but the piston sticks and does not fall to the bottom in the front carb. I tried moving the orifice around to better center the needle but then out of desperation, removed the needle and it still is getting stuck so it's not the orifice or the needle causing the binding. I also tried switching the dashpot domes front and back and the front one still sticks. The piston moves freely only with the dome off.

I've read where people say never to remove metal (e.g. sand) the dashpot dome and elsewhere where it is offered as a suggestion. 

Does anyone have an idea how I should resolve this?

I was thinking of rubbing something on the internal surfaces and then seeing where it rubs off after forcing the piston up and down to figure out where it's binding.

thanks

Jeff

[Mark Maras]

Did they slide freely before the rebuild? Use a Sharpie pen on the piston. You might also try leaving the 4 screws slightly loose and gradually tighten them as the piston is being slid up and down. Did you try switching the pistons too?

[240260280]

Remove needle and plunger

Install dome but with all 4 screws very loose and not touching dome

Rotate  Dome CCW and CW while lifting and dropping piston to explore where it binds and does not bind

Put dome in middle of its free rotation limits

Turn down 4 screws so they just kiss the metal dome

Slowly and evenly tighten screws in a cross pattern  (like lug nuts) in quarter turns while lifting and dropping piston

Repeat if necessary

If you can get it to drop w/o binding then install needle and repeat drop test.

Install plunger and repeat drop test

[Captain Obvious]

Don't remove any metal.

If the piston is sticking even without the needle installed, then it should stick even without the dome installed on the carb body. In theory, the only part of the piston that should contact the carb body at all is only at the bottom face when it's all the way down. So unless the suction piston is bottomed out all the way down, the only part of the piston that should EVER contact ANYTHING other than the tube at the top is the little anti-rotation slot. And that should be a sloppy non-binding fit.

With that in mind... Did you try mixing and matching all the domes, pistons, and carb bodies? Maybe something got mixed up during the rebuild?

Did you accidently drop a piston or a dome? The clearances along the "sealing" face along the outer rim of the piston are very small and it doesn't take a lot of distortion to make things not work properly. Maybe look for anomalies on the precision machined surfaces under magnification? Like a bump or burr kicked up by hitting something by accident?

[Jeff Berk]

Response to Mark:

Did they slide freely before the rebuild?<- Yes,

Use a Sharpie pen on the piston. <-I'm assuming you mean to look for scuffs when I move the piston up/down?

You might also try leaving the 4 screws slightly loose and gradually tighten them as the piston is being slid up and down. <- I need to hold the dome down or it will lift when I move the piston

Did you try switching the pistons too? <-I'll try that maybe tonight.

Response to 240260280 

Remove needle and plunger, Install dome but with all 4 screws very loose and not touching dome, Rotate  Dome CCW and CW while lifting and dropping piston to explore where it binds and does not bind.  Put dome in middle of its free rotation limits. Turn down 4 screws so they just kiss the metal dome <- this is where I don't understand. The dome screw holes will only permit two orientations. Neither resolves the problem, even with the needle removed.

Response to the Great CO:

If the piston is sticking even without the needle installed, then it should stick even without the dome installed on the carb body. <- No, it freely moves without the dome but sticks with the dome.

With that in mind... Did you try mixing and matching all the domes, pistons, and carb bodies? Maybe something got mixed up during the rebuild? <-That is a strong possibility. I've only tried switching the domes but not the pistons. I'll try that tonight.

Did you accidentally drop a piston or a dome? <-not that I recall but if I can figure out how to mount the piston and dome on a rotating jig, I can use a precision micrometer to look for deformation.

My next steps:

1) Switch the pistons and domes around in all combinations to try to isolate the problem.

 2) Mark the piston with something that easily rubs off and figure out what is rubbing against what.

[Mark Maras]

If there was no damage during the rebuild, it should be just a matter of trying all the piston-dome combinations. 

 

[Captain Obvious]

Jeff,

When I said "if the piston is sticking even without the needle installed, then it should stick even without the dome installed on the carb body" what I meant was...

Since the only point of contact between the piston and the carb body should be at the bottom face of the piston (and that's only when it's all the way down), then the binding you're experiencing must be caused by contact between the piston and somewhere inside the dome.

And if that's the case, then you should experience the same binding just by slipping the piston up into the dome and trying to move it up and down. Even before screwing the piston and dome onto the carb body.

I was looking for confirmation that the binding occurs with the piston up in the dome, but without the two of them even being attached to the rest of the carb body.

I'm in agreement with the above... If there wasn't anything dropped or banged around during the rebuild and they didn't stick before the rebuild, then it's most likely just a mix-n-match issue between parts from the two carbs.

And I know it's probably not the issue, but I gotta ask... Everything is spotless clean, right? You don't feel grit/sand/dirt grinding away when you slide things together?

[Jeff Berk]

I think I got it now. I ended up tightening each of the four screws on the dome a little at a time, checking every 1/2 turn to make sure the piston moved freely until all screws seemed snug. Now time to try and tune it.

[siteunseen]

This is a life saver for me. I knew zero on SUs and with the help of this forum and the quick and dirty method mine "tuned up" nicely.

http://www.classiczcars.com/forums/topic/39221-quick-and-dirty-su-tuning/

[Captain Obvious]

Excellent. Did you have to swap parts around, or was careful even tightening all you had to do?

[Jeff Berk]

CO: Yes, I had to swap parts around but the best I could do is was to get one moving freely and the other binding a little. Using the four screws to adjust the angle of the dome seemed to be what I needed. 

On a related topic, I find that I'm not able to distinguish rpm by ear as well as when I was younger. I have an old tachometer but it doesn't have a high range. Am I better off buying an analog (off of e-bay) or digital?

Siteunseen:

Thanks for the link. I'll read through it carfully ; )

Jeff 

[240260280]

https://www.innova.com/en-US/Product/Detail/3568