One Weber Carb running rich...

The 4 carburettors are well balanced, each choke sucking the requisite 4kg of air at around 1,000rpm
The fuel is circulating/flowing from tank to tank (i.e. the return is not blocked off to stop the pump clicking).
All the plugs are firing nicely.

When I was adjusting the idle mixture, 3 of the carbs respond - i.e. I can tell by the engine note picking up and dropping back as I adjust from lean to rich.
But the 4th carb doesn't respond. I detect no difference between having the idle mixture screws turned all the way in, to multiple turns out. When I rev the car during the process, the return to idle isn't what you would call 'snappy', the revs dropping slowly from say 2,000rpm back to idle.
When driven, the plugs are black - not covered in soot but just black, whereas the plugs for the 3 'good' carbs are a healthy brown colour. Also, I'm getting a fair few fat pops of unburnt fuel when I lift off. However, under acceleration the car is pulling like a train...

I rebuilt the carbs probably 10 years ago, and I was able to tune the car to a very good standard. I'm not worried about pulling this carb and taking it to bits but I'd be interested in any suggestions as to what it could be (ideally being able to fix it without pulling it!), I have my theories but my track record on diagnosing these issues isn't great, so I won't air them.

 

One quick check would be to ensure the individual choke lever for that carb is fully "home" and not hanging up. Every Weber carb car I've ever had, I disconnected/removed the choke cable as I find them more trouble than benefit.
Also, your inlet needle may not be seating or the float has sunk due to a leak.

 

Stuck float .

 

Just a reminder: you can check the float and needle/seat by removing the top of the carb in situ. No need to remove the carb completely. Hopefully the problem carb is one of the rear (right bank) ones. Much easier to reach

 

Yes, that's a good shout. And an easy one to check

 

It goes without saying that it's not the rear bank!
The choke lever is disabled but I will check the actual lever. Thanks

 

Somewhat high for a euro version -- should be more like 3.25~3.5 kg/hr. Those two cylinders may not be operating at idle (so the other 6 are increased some to compensate). One confirmation you can do is just (carefully) unplug the spark plug wire going to each of those cylinders at idle to see if the idle RPM drops and the engine "note" changes, or not.

That's is not the only issue for that -- the choke spools inside the carb can stick in the open position even if the lever in the cabin and the external lever on the carb are in the proper choke closed position.

 

Thanks Steve, great information. As always!

 

I forgot how fun the carbs were in my 308gts. Float I bet

 

When the engine doesn't respond to a mixture screw, its getting fuel from somewhere else.

Throttle plate position is set too high so its pulling from the transition hole, accel pump nozzle washer leaking or check valve leaking and the fuel wicks down the wall of the carb which requires a close visual inspection while its running. I have seen accel pump diaphragms with a taller center button cause fuel to drip from the nozzles. Choke circuit as mentioned earlier. A mistamped top gasket that omits the air corrector hole is another fun one I found one day. That was a real treat.

 

One quick check is to examine the idle jet for the clog and clean blowing out.
It is large enough to capture a grain

 

Then it wouldn't run rich if that was the case.