SU Chokes stuck down

[44DHR]

Further to my previous post “Choke cable replacement” where I am working on a friend’s 1966 4.2 FHC 2+2, I have now fitted a new choke cable and trying to understand why it broke through the chokes being locked solid.
Incidentally, the replacement choke cable I ordered from my “usual” was too short in length as well being too large an external diameter to fit into the hole in the dashboard outer cable clamp - as was another that I ordered from them, so I simply ordered a metre of Bowden cable from Car Builder Solutions at a third of the “usual’s” price, which was perfect.
On removing the SU carb suction chamber and piston, I could see all three carbs had their jets in the retracted choke position within the jet bearing. I tried to pressurise the fuel system and fuel was rapidly leaking out of Number 1 carb jet around the jet needle. I can only assume the jet had been pulled down when the choke was engaged and become stuck within the jet bearing and the return spring in the base of the carb could not overcome the stuck jet to cause the fuel to leak around the needle.
On removing Number 1 carb - which was fun hard up against pedal box housing - I found the jet firmly stuck in the jet bearing.
I understand that all three jets were replaced about two years ago as there was a problem with the diaphragms, so what has caused these new jets to stick firm ?
Is there any work that needs to be undertaken to the jets within the bearing to avoid this problem ? I understand that the jet bearing needs to be centred as the pistons fell smoothly onto the bridge, so I don’t believe there was a problem with the jet bearing alignment.
Any thoughts what caused the jets to lock solid ?





Now to remove the other carbs !
Cheers,
Dave

[mgcjag]

Hi Dave......As you mention if the jet bearing isn't centered and locked the jet could get stuck.....the needle running in the jet could also get worn....any wear signs on the needle?.....Steve

[44DHR]

Hi Steve,
Thanks for the reply. The needles were replaced at the same time as the jets were replaced a couple of years ago.
I didn’t notice unusual wear on the needles, but I will now fully check when we return to the garage next Tuesday where the car is presently stored. The car has been very infrequently used and was driven to a local garage to display and be potentially sold, so it has been sat there around 8 months to cause the issues and with the fuel possibly going stale.
I will have gently tap out the jets from the jet bearings and see the origin of the binding. As there is a diaphragm to retain the fuel below, I was wondering how critical the jet outer diameter has to be, i.e. if it was cleaned with oil lubricated 1200 wet and dry, would that cause a problem with it’s clearance to the jet bearing wall ? Also, when the new jets were fitted, how clean internally were the bearing walls ? I have a set of carb cleaning brushes and graduated cleaners, so I will make sure that internal surface is also clean and has no verdigris.
I think you have hit on a point that the jet bearing may have initially be centred, but if that nut on the jet bearing was not fully locked, the bearing could have subsequently moved out of alignment, so once cleaned up the jet bearing will be fully checked, centred and locked.
Thanks again,
Cheers,
Dave

[bitsobrits]

Fuel varnish can be a powerful adhesive, especially in a tight clearance situation such as the jet to jet bridge guide. I would avoid using any abrasives to clean things up. Just clean everything throughly with petrol (or an ultrasonic cleaner if you have access to one). The sliding "L" shaped piece (brass tube with steel "foot") which moves the jet can also seize in the carb body. Again a good clean and a bit of thin oil helps there. (I use ACF50-not sure if that is available where you are).

[ralphr1780]

been there with same issue on the 3 carbs but all stuck in the upper position, hence could not pull the choke cable enough. Removed pistons, sprayed carb cleaner and let soak, then couple of gentle taps with a brass rod and all back to normal operation.