Take the screw out and make sure the hole is clear. Screw should not have any scoring.
There are 2 holes in the bore, one at the idle screw and one above. The above hole should be clear.
The idle tube should be clear. This is the tube that screws into the float bowl with the top off.
There is an air bleed hole in the bowl close to the top of the idle tube. Make sure that is open.
Perhaps the throttle shaft is worn and leaking too much air. Use carburetor cleaner and spray around throttle shaft. If the RPM smooths out, you found a vacuum leak and will have to re-bush the throttle shaft.
Was the throttle plate removed & replaced? It could have been installed backwards.
A strong smell of fuel could indicate a flooding condition.
A rough idle can also be caused by a vacuum leak. Spray carburetor cleaner around the mounting gasket, intake manifold and any other vacuum source.
Adjusting Idle Mixture
- Engine should be at operating temperature
- RPM should be at factory specification. Probably around 500 for the YF cars.
- Hook up a vacuum gauge either to the carburetor, or intake manifold port.
- Start with screw about 1 1/2″ turns out.
- Turn screw in 1/4 turn, wait for the RPM to catch up (1 second).
- Keep turning in 1/4 turn until vacuum starts to drop.
- Turn screw out 1/4 turn.