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SU Carburetor TR4a


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Hi All

I have an issue with the rear carb on my TR4a, in that the Piston is sticking when i tighten the three dashpot screws (no matter how i tighten them).  The piston was sticking, so i cleaned them and replaced Jets, Needles etc. but this issue just will not go away.  I can obviously get them refurbished, but wondered if anyone has any ideas on how i can solve this issue?

In addition, i can lay my hands on a pair of SUs for a GT6 and wondered if this is a simple fix or more complicated than i need it to be?

Many thanks

Gordon

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The piston  and dashpot are supposedly a 'matched pair' selected for fit Gordon - is there any chance your pistons have been swapped over between the two carbs so they are now un-matched?

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12 minutes ago, RobH said:

The piston  and dashpot are supposedly a 'matched pair' selected for fit Gordon - is there any chance your pistons have been swapped over between the two carbs so they are now un-matched?

Hi thanks for responding.  I did check that but it appears to be the dashpot causing the issue, as no matter which piston i put in, or carb i try it with, i get the same issue.  I was wondering if the dashpot has been over tightened and become distorted in some way?

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If it only happens when you fully tighten the screws, could it be a burr or something on one of the mating edges  -  either the dashpot or the carb body?

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It is possible for the dashpot to be warped or made slightly out of round by rough handling. Look for signs of rubbing or abrasion on both the piston and the dashpot. it is possible to lightly polish the inside of the dashpot where it sticks using a Scotchbrite pad or very  fine sandpaper or metal polish. Go slowly and remove all grit from the polishing compound as you check your results.

Check out this old thread:

 

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try a kitchen synthetic scourer on the inside of the dashpot to clean off years of residues...if it only sticks as you tighten the screws then a quick fix is to shim that joint with some aluminium foil.

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Remove the needle from the offending piston and dash pot pair.  Refit piston, spring and dash pot.    .......    does it still stick after lifting and dropping in situ?
 

  If not ....your jet centring to the needle is suspect and needs reviewing or adjusting.

Peter W

Edited by BlueTR3A-5EKT
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