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Carb confusion causing headache


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I'm about to put on a nice new set of carbs from Andrew Turner on my car but am having some difficulty with them. They're Stromberg CD150's, the early ones for the 1600 Vitesse with no springs under the lids.

My problem is the air piston in the front carb just refuses to drop with a nice thunk. The absolute best I can get is it rising to the top, then falling slowly and reaching the bottom after about 2 seconds.

So far i've tried re-centering the needle in the jet (a million times), checked the diaphram is sitting in its slot and even tried rotating the lid to see if that helps but all with no success. The rear carb is a little faster but not much better.

I had some air valve springs from a later set of CD150's lying around so tried putting them in and this seems to cure the problem....the air pistons fall with a nice thunk.

Are the pistons not falling as fast as they should because the diaphrams are new? What would be the effect if I put them on the car with the springs in them?

I'm a bit stumped as to where to go from here so any advice would be much appreciated.


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the 1600 spec does have no springs but should have a heavier damper weight ( the ring that clamps the diaphragm) ( about 3 time thicker than normal)

some aftermarket diaphragms are very thick and resist flexing, I  very much doubt Andrew would use these.
if you remove the dash pot and drop a screwdriver in to add some weight does this allow a clunk,??  dont forget that when running the vacuum produces a lot of load and will overcome any slight resistance .    

when centring the jet do this with the jet adjusted up flush with the bridge,  hold the damper down firmly slacken the holder 1/2 turn give it a tap and nip it up,  then back the adjuster down two to three turns ,


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Thanks Pete. I removed the springs and got the carb needles centered and the pistons falling as best as they would go, put them on the car and you're right, they freed up nicely and were working well.

Balanced them up nicely but they're running very very rich for some reason (even got black smoke from the exhaust). I took it for a drive and after about 200m it lost power and felt like it was running on 3 cylinders. I nursed it home and examined the plugs,  they're all sooty and black and no amount of turning the adjuster up to lean out the mixture seems to help.

Have I missed something?


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1600 needle is 7b no springs

piston springs richen the running mixture, stronger spring richer running.  not specified on the 1600

any dabbling with the fuel hose connections often release floating slivers of rubber cut off when you insert the bundy tube, these float back and forth with the fuel flow and then jam into the back of the needle float orifice, and the low fuel gets you down to 3 cylinders

if you are getting poor reaction to the adjusting nut there could be sealing washers missing, and fuel is coming up around the jet not through it.  set correctly with needle flush with the piston the jet for general start up and runable is the jet tube should be approx 1.5-2 mm below the brass jet holder ,

make sure no gaskets or air filter cans are upside down and blocking the various vent ports in the carb outer face.

I guess these have what I term the thames barrier choke , a ramp which lifts the piston and blocks the airway,  make sure they both fold down flat ,level with the bridge in the throat.   if its holding the piston up you wont be able to adjust anything


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