Carb Sticking

[texasz]

I have a little knowledge about the SU carbs and have a question. My front carb seems to work properly the issue is my rear one. When I lift the piston up it does not feel the same as the front nor does it seem to drop quite the same, further when it is lifted all the way up it sticks and does not drop at all...I have to open the filler on the top of the dome and tap it before it drops.

I have already sprayed carb cleaner in through the air horn and that seemed to only help just ever so slightly.

Any thoughts, ideas, HELP!? Please....

[Arne]

Check for corrosion and/or dirt/carbon/etc inside the dome. The piston rises and falls from vacuum and so the clearances between the piston and the inside of the dome are very tight. Any crud inside or damage to the outside edge of the piston or inside of the dome will cause this.

[BD240Z]

I'm like you, I don't have much knowledge or experience with carbs. I just purchased and watched the "Just SU's" DVD from ZTherapy, there's a lot of good information regarding adjusting, cleaning and making sure they're setup properly. What they show is a little hard to explain here, consider getting a copy, I think it will answer your questions.

Good luck with your car.

[MikeW]

I thought I responded to this earlier today. Perhaps I forgot to hit send. In addition to Arne's advice there's a possibility that the needle is bent. The ZTherapy video mentioned above shows a clever trick for straightening needles. You chuck it in an electric drill and spin it while pressing it from the side with a piece of cloth.

[deedee]

I have the exact same problem with my 74 260z. Same carb and everything. weird ha! I decided to change to a Holley 390cfm w/intake and headers instead of dealing with carb rebuilt.

[beandip]

I have a little knowledge about the SU carbs and have a question. My front carb seems to work properly the issue is my rear one. When I lift the piston up it does not feel the same as the front nor does it seem to drop quite the same, further when it is lifted all the way up it sticks and does not drop at all...I have to open the filler on the top of the dome and tap it before it drops.

I have already sprayed carb cleaner in through the air horn and that seemed to only help just ever so slightly.

Any thoughts, ideas, HELP!? Please....

Are these 3 screw or 4 screw SUs ? On a 4 screw the mount for the nozzle can be adjusted for alignment . On the 3 screw the nozzle mount is fixed and there is no alignment adjustment. The adjustment or alignment is in the dome . So if the needles prove to be straight and still are sticking look to the alignment issue . Just be sure there is no grit on the vacuum pistons and or the cylinders they slide in. Some times when you spray cleaner it the throat of the carb , you will dislodge crud that will then bind the piston . DO NOT MIX the parts from on carb to the other they are fitted at the factory. Gary:rambo:

[texasz]

Beandip, they are 4 screw. Do you have more information on this, I really know very little about the carbs. Also, it was sticking prior to my hitting it with carb cleaner.

Before removing the dome is there anything I should know or have on hand (i.e. drain anything, new gaskets, etc)?

[Arne]

You can remove the domes without any special parts on hand. Just make certain to keep the parts matched - I normally only remove one at a time to ensure that I don't mix anything up.

Lots of SU info available on the 'net, and the video from ZTherapy is very good as well.

[Bruce Palmer]

There are no gaskets to fuss with. Merely remove the screws holding the domes. Maybe use a small pair of vice grips to break them loose as the screw heads tend to deform with a screw driver. We believe Hitachi had Magilla Gorilla installing the domes at the factory.

Remove the domes and pistons (don't bend the needles, lift the piston carefully straight up) and clean 'em up internally. Put 'em back on and while retightening the dome screws keep raising and dropping the dome to make sure they aren't binding.

[ezzzzzzz]

Try this... remove the damper from the dome. Loosen the four screws at the base of the dome. Gently wiggle until the piston drops fully. If it won't drop fully then you probably have a bent needle or misaligned nozzle. If it does drop then slowly start to tighten the four screws continuing to test piston drop. If all is well, you should be able to tighten the dome screws with proper piston movement. If not, remove the dome and clean the interior with brake cleaner and a soft cloth. Blow it out if you can. Remove the piston careful not to damage the needle and clean the fine grooves that fit into the dome. Reinsert piston and install dome as suggested above. If it still won't drop you can test for needle binding by pulling the nozzle downward (as if choke were on) to see if the piston will drop accordingly. If so, loosen the nozzle locking nut (not the mixture screw) and gently wiggle the nozzle until the piston drops then carefully tighten the lock nut again. Test for piston drop again. If nothing is bent then you'll get it to work properly. Worse case will be replacing the needle(s)/nozzle(s) or entire carb(s). It is a bit time consuming but not difficult. The Ztherapy video is a big help for the visual assistance alone.