Hey guys,

Almost completed my rebuild of my twin 1 1/4 su's.

However, I've come across a slight issue. The pistons are struggling to 'clunk' once being raised. I'll pry it open slowly, push it right up, however it doesn't completely fall down to sit flush (both fall back down to within a quarter of an inch from the bottom).

If I pull the choke out to have the jet as far out as possible though, the pistons fall completely and 'clunks'.

As far as I can see the needles sit dead centre. So I'm thinking the carbies might have the wrong needle fitted? Both the jets are brand new, however I've no idea how old the needles are.

Could it be something else?

Any assistance is greatly appreciated! Just so keen to get these on the mini and get driving! (or move to the next problem...)

It sounds like you need to centre the jets.
You need to loosen the jet carrier and re-centre the jets.

Cheers Morris,

I'll give that a try tomorrow, I wouldn't have thought there would be that much leeway in those carriers to make them not centred though (said like a novice)? Are there different ones required for particular jets?

There is enough adjustment to allow for production variations. Also enough to catch the novices the first time they play! We are playing with very small fractions here!

My preferred way is to set it as I assemble the carby.
Remove the jet adjusting nut and spring.
Loosen the jet carrier so it is almost finger tight.
Hold the jet in position.
Lift and drop the piston a couple of times to push the jet carrier to centre.
Slowly tighten the jet carrier as you still keep lifting and dropping the piston to ensure it stays on centre.
Put everything back together.

If the carby is fully assembled you can still do it but it takes a little more effort to move the jet carrier with the nut and spring in place.
Some people will push the piston down with a pencil or screwdriver to help it move the jet carrier.

if you have the carb upside down and spin the brass seat (bearing) the jet slides into, you can see how much the bore can offset very clearly. You can adjust by a very large amount to get it centered.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

Thanks Folks!

Worked a treat over the weekend. Pistons are all good now. Was hardly a hassle re-centering the jet carriers!

Glad to hear it.

You need the SU jet centring tool. Its a piece of round bar, one end is the diameter of the needle shank and goes into the piston. The other end is the same diameter as the jet outside diameter. (not the 0.090" hole size in the jet)
I've had one since the mid 60s and it comes with a spanner, 2 height check gauges to check piston height and mixture strength when the engine is idling (you don't have to struggle to reach the piston lifting pins) and a plastic wallet to keep everything together along with the instructions.
Fit the small end into the piston where the needle goes.
Fit the suction chamber and piston with the piston fully down on the bridge.
Fit the jet carrier into the body so that the tool fits inside the jet carrier.
Tighten the jet carrier retaining nut and that's it.
When you fit the jet into the jet carrier and the needle into the piston it will be perfectly centred in the jet.
There is only 0.001" - 0.0015" clearance between the needle and the jet.
If you have the spring loaded needle set up you have my deepest sympathy.
Any system that is setup to actually wear the needle and jet is not one of the best efforts from BMC/Leyland!!!

If you have twin carbs, make sure you don't mix up the suction chambers and pistons. They are matched and should be kept together as a set.
Mark the orientation of the suction chamber and body as well.
I've found that if you have them 180 deg out the piston can stick as it touches the suction chamber wall.
Do not be tempted to clean anything with steel wool or other abrasives. There is a very close tolerance between the piston and suction chamber.
A bit of Brasso on a rag is all you usually need.

RonR

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68 build Mk1 Cooper S
72 Clubman GT project. Total restoration required