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SU Carb Piston question

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TRclassic3

TRclassic3

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Making progress on my high idle problem. Noticed this difference between carbs. When I lift the pistons manually to the top of it's range one of the pistons falls quickly while the other falls slowly. Both dash pots have the correct oil level, and this is with the damper in.
why the difference, and which is correct.

Thanks
 
First thing I'd do is take the dampers out of the top of the carbs(that'd be the dipstick looking things attached to the screw caps) and while they're out test the pistons. If they still react differently, pop the piston covers off and inspect the pistons and covers for any corossion or carbon buildup. If so gently use a scotch-brite pad or some steel wool with some carb or brake parts cleaner to clean them all up, then spray lightly with some WD-40 and reassemble and retest. If they still act differently I'd try centering the metering jets but I doubt that's your problem.
If taking the dampers out eliminates the difference, then I would inspect them carefully and /or order a matched set. If they are different (leingth, diameter, etc) then you'll never be able to get the pistons to react equally.
New dampers are fairly cheap through the usual suppliers.
 
Assuming they are set correctly and the engine is off, remove the air filters to look into the throats. If you push the pistons up - then let go, they should drop right back down with a click as they reach the bottom.
 
First of all switch the dash pots or pistons, check drop. Then check jet centering, the simultaneus clunk sound is correct. Make sure everything is very clean dash pots piston etc. I had a piston hang up last year, no idea why, it is the only time in almost 40 years of owning the car I took it home before I got to where I wanted to go. When adjusting the jet upward always remember to push it up just to make sure it is moving properly. I always check mine after pulling and pushing the choke. Probably a bit tight but better than leaking.
 
TRMark said:
First of all switch the dash pots or pistons, check drop.
Click to expand...

A good idea, especially if you have had the carbs apart and mixed up the covers and pistons. These parts were originally matched as a set and should be kept separate from the other carbs so they won't get mixed up. Might be as simple as swapping the covers but mark them (after you get this straightened out) to avoid future mixup. When I rebuilt mine, I used a center punch and marked two dots on one and three on the other, both on the cover and on the body next to one of the screw holes. I used 2 dots, not one, because one just looks like a scratch. I then put a little yellow paint in the dots to highlight them.
 
Hello Ed,

they should both fall at the same speed (and the damper has no effect downwards). Certainly the first check is inside the dashpots to ensure cleanliness. However, you should only, repeat only use solvent cleaners no abrasive material whatsoever as the piston to dashpot cover clearance will be increased. Try the test with the springs omitted as there is a huge variety of spring strengths.

As suggested you can also swop pistons to see if that helps.
If after swopping pistons (assuming the first two suggetions didn't work) the slow carburettor is still slow then it must be the needle binding in the jet? but I would expect that to give a clue by the idling speed being high or slow in dropping down to idle?

Alec
 
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