Carburetor fault

[phoenix]

Freshly built engine when at idle only a little suction on front carburetor.. (filter etc removed,,,) . But when the air bleed screw is removed its much better. Whe reved suction seems fine . Where does the air bleed from and can I use an airline with low pressure to see if its blocked. 1984 S1 4.2.

[44DHR]

I am assuming by “air bleed screw” with the engine just on idle that you are referring to the “slow running volume screw” ? This a fairly large screw which is initially set at two full turns out from its seating.
The idle circuit takes fuel mixture from the main Jet through a passage to emerge behind the throttle butterfly. The mixture is adjusted by the aforementioned large screw. As the mixture outlet is behind the butterfly it is crucial that the butterfly is fully closed and the butterfly spindle attachments, both the return spring and throttle actuating lever are correctly adjusted.
The SU carburettor, (carb), is a simple but effective carb, but in multiple applications such as the three carbs on the E Type which need synchronising to get equal flow, it is the operating linkage which must be also correctly set.
If you are experiencing different suction on the carbs with them all on the factory initial settings, I would look at the butterfly spindle attachments. Firstly, on ensuring the butterflies are shut, the throttle return springs must providing sufficient tension, so check all three throttle return spring levers, (C14371), are set correctly to provide this degree of tension to keep the butterfly fully shut. Secondly, on the other side of the butterfly spindle where the throttle spindle levers assemblies, (C24927), are attached, these must be adjusted to all operate in unison. The reason they are called “assemblies” is that they are not just straight levers to move the spindles as they have a central pivot which allows for a certain degree of play between the main operating throttle shaft and where they bear onto the spindle. I think this play is intensional to allow for engine movement. It is again vital that the amount of play is equal so the 3 clamping bolts of these assemblies on each carb can be seen to move exactly at the same time.
I don’t have SU carbs on my E type as I run Weber DCOE carbs and the beauty of my 3 connecting links onto the throttle spindles from my main throttle shaft is that I can adjust and set these individually with a link which has a right and left hand threaded central body.
I have just helped set up my friends SU equipped E Type and I found that getting the throttle linkage to operate correctly was key to setting up and getting the E Type SU carbs synchronised.
I trust this helps.
Cheers,
Dave

[RICHOT]

Excellent and informative explanation Dave.

I think I’ll go and check mine again even though it seems to run smoothly.

Cheers,

Richard