Mike raised some very salient and informative points above but, given the outcome that it seems to be running well on a substitute carb all seems to point to a problem with the rebuilt carb. Without having the advantage of being able to inspect and "fiddle" with it, I'd be suspicious that it's something to do with the needle valve or float. Have you checked the float level? I remember when I rebuilt the carb for my old Austin Ambassador I just couldn't get it to run well. Had it in pieces several times and just couldn't find anything wrong. Then I noticed that the float level given in the Haynes manual for that vehicle was different to that given in another Haynes manual for basically the same carb on a 1500 Allegro - SU HIF for those who remember them. The HIF stood for Horizonal Integral Float - I set it up to the Allegro spec and it ran beautifully. Wrote to the Haynes people - who I'm not knocking by the way, I've used their excellent manuals for years and this is the only discrepancy I've ever found - who wrote back saying I was correct and apologizing for the error (don't know if they tried to rectify it though?). I think you should check your float setting and maybe check the float doesn't have a wee hole in it?
On a more general note, and for the benefit of our less experienced people, I also noticed that Mike raised the "hairy old" subject of whether to clean spark plugs or not. I am firmly in his "don't wire brush them" camp. The argument goes that if you wire brush them then small traces of the metal the brush is made from gets rubbed off against the insulating "nose" which then encourages the current to track down the outside of the insulator to earth rather than jumping the gap to the side electrode. At worst this can lead to actual missfires but also I believe it can lead to a diminution of spark energy if some of the current "leaks" but still has enough "left over" energy to jump the gap but with reduced force? I believe that, a few years ago, the manufacturers (NGK for sure) stopped giving a glazed finish to the ceramic insulator "nose" (you can clearly see the nose is now matt in appearance on a new plug) I believe this type of finish attracts the metal particles even more?
If your engine is in good order and not allowing excess lubricant to enter the combustion chamber then modern spark plugs are, to a large extent, self cleaning. Just look at these plugs which came out of Becky (2010 1.2 Panda Eco Dynamic 60hp) last year at service time:
The first thing to notice is that they have, in common with many plugs today, extended "noses" which project the centre electrode well into the combustion chamber which allows normal deposits to be burnt off during running. Look a little closer:
You can see there's been some degradation due to heat - you can see the discolouring of the ceramic nose - and erosion of the electrodes due to spark jumping that gap - Look at the centre electrode and you can see it's corners are a bit rounded off, so far no metal has been leached from the side electrode. These plugs have been in the engine for a couple of years and are looking pretty much as I would expect them to look, which is - No great build up of carbon deposits. Some evidence of wear to the electrodes (metal gets burnt away by the spark over time) Threads looking good and undamaged thus indicating that the threads in the head are probably in good order (they unscrewed without jamming up too which is another good indicator). I'm sure I could have just stuck them back in and the engine would have run on them just fine but maybe with a little reduced efficiency. If I was doing this I wouldn't have done anything to them except check the gap. However they've served me well for the last couple of years and there's always the increasing possibility of electrical breakdown of the insulation due to age so now it's time for new ones. These are normal, some call them "copper cored" plugs, which my local factor sells very cheaply so that's what I run both my Panda and my boy's Punto 1.4 8 valve on (NGK ZKR 7A-10). I could choose to fit "long life" iridium tip type (ZKR 7AI - 8 I think?) and "forget" about them for 60,000 miles/10 years? but that's not for me as I like to pull my plugs every year at service time and examine them so they can "speak" to me about what's going on inside each cylinder. Also by removing them every year it pretty much ensures they'll never have time to corrode/seize in place.
For those reading who don't have much experience of examining plugs here's a new one - on the right - compared to a well worn one - on the left. Just look at how much the centre electrode has burnt away:
Close examination will show the side electrode is also suffering slight erosion as it's tip is slightly hollowed - I've seen them much worse than this with side electrodes almost burnt away and. usually the engine missfiring - which is why the customer has brought the car in of course - nothing to do with them having missed the last 2 services?!
If your plugs look like this:
Where the plug is heavily contaminated with black carbon (soot basically) Then you've got an over fueling (running rich) problem and it's not at all uncommon to find this will be due to the air filter needing to be renewed! Of course other problems can be the cause also but checking your air filter should be a first stop check.
Or
Wet and "gungy" with often heavy deposits inside the end. Oh deary me! this is due to oil being burnt. might be something "simple" like a blocked crankcase breather but more often due to a worn engine - commonly valve stem seals, sortable at a price - or, more seriously, piston/ring/bore problems which often means a new engine needed as labour to "fix" it will be prohibitive. By the way, Any "clever clogs" reading this will probably recognize this oily plug is actually from a 2 stroke but I didn't have an oil fouled plug from a 4 stroke to take a picture of - sorry! So, at the end of the day, If the plugs are nice and clean burning like mine above which you saw in my first picture then, if the electrodes are in good order, you can reuse them, just screw them back in. If they don't look "good" then, if it's just electrode wear renew them but if it's contamination you need to sort out the problem, no amount of wire brushing will sort that.
Just to finish off, again for the information of our less practical brothers (and sisters), here's what the business end of a couple of "long life" iridium/platignum/whatever plugs looks like:
You'll notice right away how different the centre electrode looks - very thin and "pointy". These centre electrodes are very brittle and should not be abused with gaping tools as they can easily snap off! The manufacturers claim the plugs are pregapped and should be fitted without attempting to adjust them. I still check visually just to be sure they haven't been damaged in transit. The NGK "jobbies" have a hard cardboard tube in the package which protects the threads and electrodes, I've never had a damaged one.
If course if your engine only has a long life plug specified - like my IBIZA - then that's what you should use, but I'm very happy to go on running "copper cored" plugs in our wee Fiats.
Finally, another thing which "bothers" me is the length of the treaded part of the typical plug on modern engines. Just look at those two Iridium plugs compared to a couple of older plugs:
As you can see the long life ones are on the left. To their right is first an N9Y (NGK equiv BP 6 ES) - found in so many cars form the 60's and 70's and a short reach from a wee DAF. I know modern plugs have sacrificial thread coatings (NGK call theirs Trivalent I believe?) but a thread that long, and I've seen longer, I think the KIA uses an unusually long one?
https://www.eurocarparts.com/search...Sy-ftHBzMAwvVerZfNYVZNO-FHPCmZzxoCHpgQAvD_BwE surely must be subject to greater possibility of seizure after 6 or more years? And yet they are telling us not to lube the threads with copa slip or other antiseize. On the strength of this I've been putting them in "dry" for a couple of years now so time will tell I suppose?
EDIT Ps. DO NOT clean plugs with abrasive papers, 'specially not anything with carborundum. and don't even think of using one of those old style sand blasters. In my opinion the chances of managing to ensure none of the grit gets left behind up in the recess is unacceptable and any grit which then gets into the cylinders will wreck havock|!