Float Chamber

[David53]

I recently fitted a rebuilt carb which has improved the running quite a bit. However, I still think it's running too rich. Mixture nut screwed right up and lifting the pin causes revs to increase slightly.

I assume I now need to replace the needle with a leaner running one. Not sure what's in there but will pull it out and check. Can you easily remove the piston and needle by taking the bell chamber off the top? ie: No need to remove carb from manifold?

[kittyfell]

All needles are the same at idle, so it's more likely your richness is being caused by over-high fuel level. Try dropping it slightly. Yes the bell/piston/needle assembly just lifts off. It's also always possible the needle is not fitted correctly, perhaps too far into the piston. The shoulder should be flush with the base of the piston.

[David53]

Interesting you should say that Kittyfell....when I first fitted the carb and switched on petrol spurted out the top of the float chamber....I assumed they had missed a washer so I fitted and extra washer under the nut to seal. stopped the spurting petrol but I am now wondering if the top of the float chamber is meant to be selaed tight or is it meant to have some breathing? In which case was the float not working properly and stopping the flow meaning that at idle the carby is being flooded? Can anyone tell me?

[newagetraveller]

As the petrol level in the float chamber rises the float should also rise and cause a valve to shut which prevents more petrol entering the float chamber and keeps the petrol level in the float chamber ( and hence the petrol level in the jet ) constant.

If the petrol level in the float chamber is so high that petrol is coming out of the top then it is probably also coming out of the top of the jet which is why the mixture is still too rich. You need to fix the valve in the float chamber to stop it flooding.

[kittyfell]

That's it, the float chamber should be vented, indeed some carbs have a little fitting with vent tube that fits on top. Does sound like faulty needle valve. When running along it won't be too bad because the engine uses up the fuel, but when idling there will be too much fuel. Remove the bell and switch on ignition. Look down inside the carb and you should see the fuel about 1/4" below the top of the jet. If it's higher than that or even possibly overflowing then the needle valve needs renewing or perhaps just bend up the lever in the float lid. It's all detailed in the workshop manual.

[newagetraveller]

The other possibility (as you have an early Minor) is that the carb has a brass float in it which has sprung a leak and sunk to the bottom of the float chamber in which case you need to get all the petrol out of it and solder up the hole.

[David53]

OK.....It now appears that the float chamber valve is not shutting off completely which is resulting in the petrol spurting out of the vents on the top of the float chamber and thus also resulting in the jet being flooded at idle.

I think what I need to do is to adjust the lever which sits on the top of the float and pushes the valve closed. In the manual it says the method to do this is to use a 7/16" bar under the lever as a gauge. Not having a 7/16" bar can I just bend it gradually down until it shuts the valve off? What happens if I bend it too far and the valve shuts off too quickly?

[kennatt]

if you bend it too far the carb level will be too low and when on full throtle will tend to cause the car to lack power,just bend the lever back a bit, To be honest its more likely that the valve has got grit underneath which is stopping it from closing.take it off and clean it out. If its a rebuilt carb was a new valve and needle fitted have a close look at the needle if its new there should be no sign of scoring on the needle or valve,sometimes you can reseat them by carefully grinding them in with valve grinding paste. Or simple fit a new set very easy to do.Take the top off the carb and with every thing attached blow down the fuel inlet and slowly lift the float with your finger to see where and if the valve is closing.If it is then you will burst a lung trying to get air past it. If you can still blow then the valve needs attention.Its quite a common problem with su carbs.sometime the other way round stuck shut and no petrol in the chamber,good luck

[mike.perry]

Not having a 7/16" bar

Have you got a 7/16 drill bit or the metric equivilent?

[David53]

Oh good thinking Mike!! Why didn't that occur to me?

[kittyfell]

Yes, drill shanks are very standard measuring devices. But I agree, assuming the float is not 'sunk' then a worn/damaged valve is more likely than just a wrong setting.