Darren the last thing to alter or mess with when a Mini is not running correctly is the carb.
So if you have not done so check / replace and adjust everything else first. Points and condenser (if fitted), spark plugs, vacuum line, valve lash, Ignition timing. These are all common things to be done when buying a Mini and should be done for peace of mind and reliability.
Once all the above is done then proceed onto the carb. These 1.5 HS4's are easy to work on check for throttle shaft wear and if equipped with a waxstat jet throw it away and buy a regular conversion kit with a new jet. Check the needle for wear if you have the stage one kit installed you will need a richer one if not already installed, look on the shaft there will be a sequence of 3 letters telling you which needle you currently have. Then a float and needle and seat valve and it should be good to go. 

Absolutely no need to buy a new carb if you have worked on other carbs before.

Darren,
Before you head off on a long-distance trip down the rabbit hole of carb selection, tuning and frustration, there are some simple things you should do first.

We recall the vendor said the car was running fine before you purchased it. It also ran well enough to be loaded and offloaded in various places. Lethbridge is 907meteres above sea level and is a relatively dry climate. You however are at sea level and in a humid climate. Add to that the current temperatures.

The carb and timing may be tuned for the thinner air - richer (or leaner??) and the spark retarded.

Another thing you are likely to be experiencing is carb or in take icing. From the picture of your engine bay, I can see two potential problems:
1. Your air cleaner housing is missing its warm air duct and air temperature control. This is a collection of pieces that draws intake air from around the original cast-iron exhaust manifold where the air in cold weather gets warmed to improve driveability. These parts are probably no longer available and probably wouldn't work as well with a LCB header installed. 

2. You have an aluminum alloy intake manifold with a built-in warming tube that is not connected. As originally built, your engine had a 'siamesed' cast iron intake/exhaust manifold. They were designed as one unit so that exhaust heat would be conducted to the intake side to keep it warm and prevent icing. To compensate for this feature, the aluminum intake you have is built with a heater tube. The heater hose from the engine head should be connected to this tube so that the hot coolant provides warmth for the intake manifold. Some guys in warm climates don't use this feature as they do not experience icing.
Connect it up before touching the carb at all.

3. Way back when Minis were made for Canada, the carbs were fitted with two electric heaters - one a heated manifold spacer to improve cold-engine operation by warming the air/fuel mixture as it left the carb, and the other a heating band that ran around the dashpot to warm its oil at least until the engine reached operating temperature and possibly longer. Your car does not have either of these because it was not built for Canada, but it does suggest that if you are trying to run the engine with cold, stiff oil in the dashpot damper, you WILL experience driveability problems - lousy throttle response and poor power. 

As  Mur suggests, your engine will run fine on a worn SU carb. (My car came with a 1275 sporting a 998 intake/exhaust manifold and a very worn HS4. It was a terrible combination that didn't let the engine breathe properly, but it ran just fine when I drove it for the first time all the way from Toronto to Ottawa.) Your SU carb was probably built for your engine and likely had the needle changed as part of the stage 1 upgrade package, so it is probably fine. 

So, to sum up, you need to connect the heater hose to the manifold to eliminate or at least reduce carb icing and try replacing the dashpot oil with something much thinner.

Google is your friend - use it, as we won't all provide all the answers on a plate all the time, occasionally a bit of self help is required...

https://en.wikipedia.org/wiki/Zenith_Carburettor_Company_(British)

 

"are these carbs notoriously difficult to rebuild ? Or fairly easy ?"

"PS- I’ve built about 6 stromberg carburettors and found them easy to work on and fix."

If you can rebuild a Stromberg, as  you can rebuild an SU.  I think that's a sensible answer at this stage.

 

Hi werewolf. I tend to agree. Pay once and likely solve the trouble without much effort. Thanks bc. Much appreciated. mur, I do intend to check the entire ignition system before delving into the carb. This is normal practice imho , when troubleshooting a poor, erratic running car. First few pages of a Haynes manual explains this. So just please be patient. I’m doing this research so I will know what to expect if the ignition system tests fine. Question answered ? PS - you will read somewhere that I mentioned my trouble shooting normally does start with the ignition system. Spark first, fuel next. Bog standard auto troubleshooting.

Alex , I honestly don’t recall you posting such a thing. Please tell me where to find that post. Secondly, read thewerewolfs post in this thread. But basically I’m doing reaserch on how to best attack the carb if the ignition system proves to be O K. Bear in mind that it is 6 a m where I am, so too cold for for MY shop, thus, I’m learning about the card options while I would otherwise be idle. Make sense to you ? I feel that you haven’t answered my question. Please read it and see if you agree. Thanks.

Britnics my car runs terribly. It just barely runs, with bogging taking place all over the throttle range. Warwming the engine up does not help. As I mentioned, I am familiar with Stromberg cards , which are a variable Venturi carb, just like the S U is a variable venturi card. So I have some previous experience with them. No, I’m not an expert, and I’m not a mechanic, but I do know that you test with the ignition system, and if you have good spark, at the correct timing, then you move on to the Carburetter. Thank you for the tips.

Guys and gals, I answered these questions out of sequence. My apologies. I’m blaming it on the fact that I have had only 1/5 a cup of coffee so far. Now as far as I can see, Alex, you did not not answer my question. Not a problem , but if you read my post I think you will agree.

I think there are about 5 or 6 threads of mine that should be locked, because some are referring to obscure posts that could take a person ( me ) an hour to find. Please, keep this post open, so I can get more feedback. But am going to try to get 4 or 5 of my threads locked because they are so off topic that they are not relevant.

Thanks for the help everyone.

A Stomberg is essentially an SU clone to avoid royalties (Google the history) with the diaphragm instead of the piston.  If you can do one, you can do the other.

As already pointed out, a bit more info as to why you're thinking you need to rebuild or replace would help as an SU is a very resiliant device.

I'll ask what's leading you down the rebuild/replace road?

First off, what is there? Dual HS2s? Single HS4? Single HS6?

Then, what are the symptoms? Won't start, stalls after a minute, runs 100+ miles an hour or nothing?

I've never liked Stromberg carbs with the float bowl at the bottom of the carb and those stupid rubber diaphrams that fail every 5 years. And as mostly fitted to American market cars with the temperature compensator which either works or doesn't.

And you adjust the mixture by moving the needle in the dashpot, vs the SU fixed needle with the jet moving up and down in the carb body

The SUs have a couple of failure areas, some covered in the normal rebuild kit and others requiring more work. First off are the needle valves in the float chamber. Easy to replace and included in standard rebuild kits.

The tube from the bottom of the float chamber to the jet used to be in the rebuild kits.

Jets/needles. In the 70s (late 60s???) the needles went from being fixed to spring loaded, so they aligning them in the jet wasn't critical. It does mean that the needle normally hits the side of the jet so it starts to wear the jet bore out on one side.

Finally, the throttle shaft starts to wear in the carb body. If it was caught soon enough the solution is a new throttle shaft. If the wear is too bad then you're into a rebushed carb body or replacement.

If you do a replacement the needle/jet in your old carb should get you close.

Cheers,

Jim

I was hoping for a post that said: this is what I saw, this is what happened when I moved the choke cable, this is what happened when I adjusted the mixture.

Instead I read that you were going to change the spark plugs. That surprised me and made no sense.

The car’s engine is a 998, and it will run reasonably well on the original carb with a worn throttle shaft at 100,000+ miles. I can’t imagine a rebuilt or brand new carb running noticeably better than the one you have on hand —if it was in correct order and tuned by a competent mechanic.

To be clear, a different carb fitted to your engine will still need to be tuned not just to set the mixture at idle but to take advantage of the improved breathing afforded by the header.

Buying new parts is not going to solve your problems.

 replace..  just buy direct from the UK. www.burlen.co.uk  HS4.. later bc

Hi All. To get right to the point, what do you all suggest ; a carb rebuild or a carb replacement. I figure that a replacement be the most likely to succed “ right out of the box “. But perhaps they all need to be tuned , even from new ? Speaking of new, are there new ones on this Mini Mani site, or are they rebuilt ? I really don’t know which way to go. I will of course need the S U carb rebuilt book ( can’t remember the name ) , but are these carbs notoriously difficult to rebuild ? Or fairly easy ? And ate there some “ check this and this “ first items ? Any help at all will be appreciated. Thanks friends, Darren.

PS- I’ve built about 6 stromberg carburettors and found them easy to work on and fix. Generally one half day would have my carb rebuilt and me driving again. This was always on dual carb set-ups. Also, my old Honda 500 four had 4 mikuni carbs, and they were also dead simple to rebuilt and repair. Am I more worried about the S U’s than I should be ? Thanks again, Darren.