Hi, interesting developments.
Apparently there is a common problem with the FIRE where the oil spray tube (under the cam cover) gets clogged. I suggest you remove this spray-tube and get it blown through with an air-line (compressed air) although you could rinse it in engine degreaser and use a tyre inflator to blow air through it.
As for the bearings, get a second opinion on their condition. I'd be inclined to just clean up the surfaces a bit with wet-or-dry sandpaper, 400-600grade, and put it back together. But if they are badly worn, you might need the help of an engine shop - or how much is a replacement cylinder head anyway, second-hand?
I wonder how you got on with the idling problem... a few notes:
- that 'injector' you noticed at the bottom of the carb is probably a fuel-cutoff valve. Test it by unplugging it with the engine idling. If the engine stops, that's good - no difference, that's bad. Means that the fuel cutoff control unit is probably faulty and not switching the valve on when it should (it should only switch it off with the throttle closed at high revs). Quick fix: wire the cutoff valve to the positive terminal on the ignition coil (orange/red wires live with the ignition 'on') Edit: with Uno 45 FIRE, the valve is simply wired to the ignition anyway, so you might not have the fuel cutoff control unit: the fuel simply cuts off with the ignition.
- You don't have a throttle position potentiometer with a carburettor. However, there is a simple 'switch' (if you have the fuel-cutoff feature described above) where a wire is hooked up to the idle speed screw/spring. The idea is that this wire is grounded when the throttle is closed. It could be that this is not working properly, so clean it up and test (if you have a multimeter). Edit: if you can't find this wire, you probably don't have the fuel cutoff control unit either.
- Vacuum advance: test by sucking on pipe, there should not be a continuous flow of air. If there is, that gives two faults: an air leak, and a lack of ignition advance. Either of these faults will upset the idle. The vacuum advance unit is pretty easy to change and quite a common fault.
- Carb icing up: not an issue until you're actually driving again! Cold mornings/fast driving might give this problem - there should be a hot air pipe from the exhaust manifold on the front of the engine, to the air cleaner housing. Make sure this pipe is in place. Inside the air filter housing (undo the clips and remove to inspect) there is a thermostatic flap: you should find the flap closing-off the cold-air intake with the housing cold. If not, jam the flap somehow to ensure hot-air intake works.
Theory of carb icing: the small venturi inside the carburettor (tiny, for the carburettor in question!) causes an increase in air velocity and hence a decrease in the air pressure (Bernoulli's effect) to draw out the fuel. This decrease in pressure gives a rapid temperature drop. Moisture in the air can freeze, literally blocking the fuel jets.
Warm air collected from around the exhaust manifold has a lower moisture content, and of course the temperature in the carburettor will be increased also. Once the engine is warm, carb icing is less of a problem due to the heat conducted from the engine into the carburettor.
Older carburettors had engine coolant flowing around the venturi; but the Weber TLF carburettor doesn't have this. Though the 'dummy' pipe stubs are present, they are not hollowed-out.
Thanks,
-Alex
