How to Center the Jets on Su Carburetors

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If you have an old British sports car or a Z type car from the 60s or 70’s, chances are it is equipped with su Carbs. Su carbs are side draft carbs and are a marvel of simplicity, but can be a headache if not set up correctly. If you are thinking of rebuilding your carbs or changing needles, one of your essential chores will be centering the needle in the jet. If you neglect this most essential chore, your carburetors will never work correctly and will accelerate needle and jet wear. The needle must rise and fall in the jet without touching the sides. The test for that is the noticeable ‘click’ you will hear when you reach into the throat of the carb, lift the piston and let it free fall back. No click means that something is interfering with the free and easy rise and fall of the needle in the jet. Everything on your carb affects this: the jet, the piston, the piston cap.

Steps

1. 1
   Loosen the 13/16" locking nut on the bottom of the jet assembly with a deep well spark plug socket wrench.

   
   
2. 2
   Get out your 5/8” socket and unscrew the mixture nut just below it.
3. 3
   Pull out the jet and remove the mixture tension spring.
4. 4
   Replace the mixture nut and the jet. Screw the nut up as far as it will go and push up the jet as far as it will go. You are now ready to center the jet.
5. 5
   Remove the two securing screws on the piston cap, because it can bind the piston. Notice that it will fit by rotating it 180 degrees. The cap will also fit on either carburetor. If you get them mixed up, this will also be a factor in whether the piston rides freely. That is if they were centered correctly before you pulled your carbs apart.
6. 6
   Seat the needle in the piston so that its shoulder is flush with the piston body. Screw it down tight. Place the piston in its keyed passageway. Check to see if it is sliding freely and drops with the ‘click’.
7. 7
   Place the piston cap on, don’t forget the spring. Seat it and press down on the top of the cap. Lift the piston and drop it. Click? Probably not. You can rotate the cap and the jet or try the other cap till you get the click. Start tightening down the mixture assembly 13/16’ locking nut along with the piston cap screws, continuously checking for the almighty click. Thoroughly cleaning carbs and shining the inside of the piston cap will do you no harm.
8. 8
   Back off the mixture screw, pull out the jet, noting its orientation, and replace the tension spring, mixture nut and then push in the jet. Again, check for the click.
9. 9
   Screw the mixture nut up tight then back it out 12 flats or two turns. You are now ready to begin setting up your carbs.

   
   

Tips

• Make sure your float chamber is setup correctly. The level in the float chamber determines the level around the jet.
• Double check to make sure you are using the correct needles and seats.
• Lean mixtures can toast your valves. Follow the workshop manual.
• Unadjustable lean mixtures: look for an air leak
• Unadjustable rich mixture: center the metering needle.
• Learn how to tune with a vacuum guage. Chiltons in the public library.
• Of course you do all of this with the mixture linkage disconnected.
• Save yourself a lot of future bother by marking the piston caps. They fit on either carb and also rotate 180 degrees. When you put your carbs back together and fit them on the engine manifold; you gotta get the click. There is no adjustment for this per se.

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