Correct Alex.
The best manifold to use when using a HIF on a Mini is the Metro one, some of the aftermarket ones will not work without modifications and the ones that do fit are really tight for space on the bulkhead.

Because the HIF doesn't fit.

Contrary to popular belief, the engine was moved forwards in the subframe to accommodate the HIF44 carb when the 1275 was reintroduced and not when injection popped up.  I don't think I've ever seen a Mini with an HIF38 factory fitted, that's not to say they weren't, just that I've ony ever seen them from a Metro.

Thanks, Malsal. My typo on the year. Any idea on why Japanese models got the HS type?

Dan it is a 1990 model a 1996 would be SPI unless converted. All Japanese spec 998's that i have seen came fitted with HS 4's but not that it matters as the HS 4 and the HIF 38 both run 90 thou jets so the AAU needle will work in both.

its a 1990 rover mini Checkmate edition, 998 cc and single su carb, last edition before they went to modern upgrades

It probably has no coolant drain plug, I siphon/suck out what I can from the heater take off on fan side of the head, then remove an end from the lower radiator hose or just loosen it for the rest. Not a lot of fun to get at the lower hose.

Being a 1996 model, wouldn't it have a HIF38 (AKA HIF 4)? Just wondering which type he has.

Most Stage 1 kits ( intake manifold, exhaust manifold and exaust )come with an AAU I believe.  This is a richer than stock needle for the HS4.  A little richer alternative is the AAA ( as I recall ....damn, wish MintySU site was still up ).
Best thing to do is also swap to a Stage 1 sort of set-up......cheap 20% HP gain

I used an AAU in a very well breathing 1130 with a 1 1/2" SU....... works great !

OK Good!
If you were mucking about in 1975, you have experience! The basics haven't changed!

Great info, will put it on and do the checks to see how she responds to the change. I remember in 1975 when I changed the carb to a Webber down draft, added a header and a total free flow exhaust system all at the same time.   Now that was a nightmare, it ran like a British Leyland tractor

Kermy underestimates what is available here.

There is no simple answer to the needle you need.

If you add a K&N cone filter (or even a standard-shaped one to your air cleaner), it will low air better than the stock filter system. It will also be noisier!

To start with, even if your engine and carb setup are completely stock, your engine has aged since it was built. Who knows how it might have been modified or rebuilt since it was first installed. For example, if a valve job was done, the head might have been skimmed to ensure a good seal when it was reinstalled. The valves also might have been modified from stock. To the needle it was built with might not be optimum. Add to that carb wear.
 
What other modifications have been made - do you have a stock exhaust system right from the head back, or has an aftermarket system been installed? 

However, just changing the air filter style might not be all that drastic a change. The simplest was is to go ahead and change it and see how the engine runs. Then test drive it to see if performance is better, watching for stumbling at various rpms and throttle imputs. It should run a little leaner, but the carb is adjustable for that reason. If it does hesitate or stumble try adding a very small bit of choke to temporarily richen it to see if it smooths out or gets worse. It may be sufficient to richen the mixture with the needle you have. Your spark plugs will also provide indication of how the engine is running. If they are sooty before you change the filter (probably are), you may be running a little rich to begin with. After changes, the plugs might be a more suitable colour or they may begin to indicate excessive leanness.

If you have access to a diagnostic centre that can run your car on a dynamometer (AKA rolling road) and analyze your engine's tuning and performance, that will give you a good indication of how it is tuned. The report will likely also show you need work tuning the timing, or need a carb overhaul.

If that centre happens to be familiar with SU carbs, they may be able to suggest an alternative to the needle you have, or even might have a selection of needles to test.

That being said, there are other tuning issues that may come into play when changing something on an engine. The ignition timing may need modifying. You may need hotter or cooler spark plugs. You may need a different weight of oil in your carb's dashpot damper.

Before dynanometers and tuning centres became available, engine tuning was done "by feel" - by test and observation.

So, go ahead and try it. You can experiment to see if it improves performance and if it doesn't, you can always put things back the way they were. Just adjust and test one thing at a time - otherwise you end chasing two different issues at a time.

There're smarter people than here at mk1-forum.net.

have a 1990 Rover mini Checkmate single SU carb, 998cc that I want to put a KN cone filter on, I read somewhere that I might have to change the needle in the SU to account for a greater air flow. is this true and what needle will I need.  Also where is the coolant drain plug, need to drain the coolant and flush the system but the manual I have refers to it but does not show where it is.
tks
Lester from Leicester