BX19 TRS - carb issues

[Gordon]

Hi, hoping for some advice regarding my BX 1.9TRS with Weber carb 34-DRTC 14/100. In starting the car, no accelerator pedal, the car starts and revs up to 4000rpm then almost as abruptly drops back in the revs and stops. Re-starting the car, same things happens. If I very gently add a little bit of accelerator I can keep it revving - but very gently as otherwise its back up to high revs. It's lost its ability to run at idle.
I've replaced the petrol filter, checked the carb petrol supply filter and petrol supply flow, blown the petrol return to tank and also started the car with petrol return into a container - no difference. I've taken out the jets to see that there are no blockages, checked the idle cut out solenoid and other solenoid, taken the carb off and blown out all orifices, checked the floats and the dimension from float to gasket ( from the Haynes manual I've taken this as 7mm ) - all ok. The throttle butterfly valves seem to be working/closing as expected, as is auto choke. The idle mixture needle valve adjustment doesn't make any difference - fully in or out.
Previously the car was running ok so I haven't changed any the screw adjustments - choke, idle speed.
Not sure what else to check or what the cause of the problem is, so grateful for any suggestions.
Thanks.Gordon

[rutter123]

May be worth checking all your breather/vacuum pipes a split pipe may cause erratic idle, is the carb tight on the manifold, sounds like air leak symptoms.

[Way2go]

It's got to be air if the car is revving like that! I take it that you have a second throttle butterfly? Does this butterfly close completely on no depression of the acc pedal? Is it's closure spring fitted? Is the butterfly firmly attached to it's shaft?

[Tinkley]

I certainly had issues with my Solex and the autochoke mechanism. Hardened rubber parts on the L shaped sliding rod connected to the waxstat. Not sure if Weber is anything like that for the autochoke. Could also be the float getting too heavy/porous?. Needle valve sticking? or worn out? Ensure the vacuum from the first choke is correctly operating the second choke, you may have to take the carb off to properly check this, as W2G says is the (2nd) butterfly shut? at normal idle?
Also check the internal diaghrams for the accelerator pump as these can split. I got a kit from Solex to change most of these parts as they get hardened and do not like modern unleaded especially ethanol, which is now 20% of French fuel but only 10% over here......
Carb cleaner and a bit of low pressure air will normally sort out any aromatic gum from dried out unleaded. It's usually pretty orange these days from the 5% aromatics the fuel companies add.

[Gordon]

Thanks for your comments. I've checked the distributor advance tubing ex carb for any splits - ok, I've not replaced the manifold gasket but it looks ok and is tight ( when I put the carb back on I'll tape the gasket/manifold connection to prevent any possible leakage ). I've checked the primary valve opening as 0.85mm at 9degC ( Haynes manual gives positive opening of primary valve at 20 degC as 0 +/-1mm ) and the strangler choke control check as 13.5mm. I don't have a figure for what this should be but on a spare 15/100 carb I measure primary valve opening 0.65mm and strangler control check 15mm.
I'll replace the top cover gasket when I refit, and will try the 15/100 carb for comparision with the 14/100.

[Gordon]

Looking at W2G comments the 2nd butterfly valve is fully closed, and tight on its spindle. Can't see any problem with that venturi/throttle. The acc pump is certainly working so the diaphram is doing its job - as soon as accelerator moves more petrol is entering, the final pedal movement giving the petrol spray.
Cheers.

[NeilGP]

How is the float needle valve seating?
I just wondering if the petrol level is overflowing in the float chamber.

how good is the float vent pipe to the air cleaner. with this missing I have had the BX do weird things.

[Gordon]

Thanks for your help. Found out the cause of the problem -leaking mounting gasket to the manifold - so yes excess air getting in. It's been a good learning with the settings for the idle, strangler valve, needle valve, choke thermostat and cutting a new top cover mounting gasket. Almost as good as new!
Cheers, Gordon