The engine’s a bit of a drip...
Oil is dripping from the intake manifold on my BMC 1.8 where the air filter is bolted to it. There is also a fair amount of smoke on start up at times although this clears after a couple of minutes. Could valve stem oil seals be likely? Also, when ticking over for a while the engine seems to ‘miss a beat’, could the injectors need servicing? There’s also a light weep on the fuel manifold from where it goes on to the injectors. The engine starts and runs well but if I rev too much, I get black smoke.
QAFirst, have a good ‘mirror and torch type’ inspection of the rocker cover gasket. They are not the easiest to renew correctly, especially on the manifold side.
After that, check the crankcase breather system for blockages. As far as I can see, the system on the 1.5 and 1.8 are very similar, although Calcutt seem to add a breather from the oil filler cap on some engines. The oil mist separation on these engines is not the best so it is fairly common for oil from the breather system to drip out of the bottom of the air cleaner.
As long as the engine is starting well from cold and developing sufficient power, take the breather hose off the air cleaner and route a new length into a small milk or drink bottle in the engine tray. If it is oil mist, I doubt it would be worthwhile having the engine overhauled.
If the oil really is trickling down inside the inlet manifold then valve guides are about the only source, but it is usually oil mist from the breathers. I think that what you call the ‘fuel manifold’ is the leak-off pipe that links the tops of the injectors. The most likely cause of such a weep is old, hard copper sealing washers. A new set of washers will probably cure it. If the weep is at number four injector, inspect the brazing where the pipe is secured to a small banjo union because sometimes the fuel filter vibrates and breaks the pipe or braze.
These BMC engines use pintaux injector nozzles which have a very small spray hole on one side of the nozzle. These spray when cranking to direct the fuel into the hottest part of the swirl chamber. The hole tends to block fairly easily and, when they do, cold starting suffers. Your smoke on first cold start of the day is almost certainly just vaporised fuel (white) or a mixture of partially burned and vaporised fuel (grey). For a BMC this is very common and not worth worrying about as long as, in all other respects, the engine performs well.
I explained the above because of the slight misfire. Taking that together with the smoke suggests it might be worth having the injectors overhauled, but before you do that, check the valve clearances and remove each glowplug and test it across a battery. Only consider the injector overhaul once you are sure those items are okay. I am not sure having the injectors overhauled will have a noticeable improvement though because an odd misfire on idle is not that uncommon.