Joe Curto in NY is the SU guru.   He's been around for ages and usually has a good stock of any SU parts you need. and he rebuilds carbs like new.  https://www.britishsuperior.com/


Air in-air out. 

When it comes to good engine breathing, the other side of the issue is the exhaust system. When I got my Mini, a previous owner had stuffed a 1275 high compression engine in where its 998 used to be. But they kept the 998 intake manifold and a tired HS4, as well as the 998's exhaust system that has "issues" - leaks and back-pressure from a crappy after-market muffler. Along with an upgrade to an HIF44, she got an alloy intake manifold, a decent bit not extreme header and a RC40 exhaust system. NOW she can breathe! (I still probably don't have the best needle though!)

Thanks. When i had my HS4 carburetor it ran better around the same RPM.  Since the HIF44 is a "bigger" carburetor i thought it would be at least the same :)  I can hear the engine "struggle" more with the HIF44 above 3500rpm.

not all mini engines are high revving.
Depends on cam, breathing, etc.
The mini engine is a 2 valve per cylinder, siamesed intake, non-OHC pushrod anyway.

See my 1275 dyno run to give you an idea of how it is not a high revving engine

Thanks for the help guys! I ordered a new needle, just to eliminate. In regards to the fuel filter it now seems fine :) Tested it today and it goes like "rocket" in 1-2 and then in 3/4 it struggles a bit over 4000rpm.. Hopefully a new needle will fix it. Sprayed som starter gas around the manifold to check or leaks, but the rpm was steady so i guess its not that :) 

years ago, somewhere on this website, I discovered that a BCE was a commonly-used richer needle for a stock or Stage 1 1275. It made a noticeable difference in mine at higher rpms.  I tried a BDK but it was waaayyy too rich. Plugs black and soot spotted all over the rear bumper.


The OP said, "wont go past 4000rpm" sounds more extreme than what I had. My air-fuel ratio with the BCE needle at 3k rpm is 13:1, at 4k is 14:1, at 5k is 15:1. The dyno operator said that 13:1 in mid range was good for driveability (not bogging), and that trying to enrichen at higher rpms would not make much difference in "punch."  14.7:1 is stoichiometric, but 12.6 is for max power, 15.4 is for max fuel efficiency so he thought my readings were close enough. In daily use, 2000-4000 rpm is typical so I run a little rich. The fuel economy is not an issue. I dont often flog it above 5000 in order to preserve it, and it does not pull hard much above that anyway unless in 1st gear.

As for the fuel filter..... I long-ago installed an inline, horizontally mounted, see-thru plastic filter just before my carb. It NEVER fills up completely. Of course, I have only witnessed it at idle or revving in neutral, not on the road under load. 

I don't have any needle charts.
Hopefully someone who does and has experience selecting needles will pop in.

Took the needle out, says BEJ. Any advice for a richer needle?

The needle is easy to take out. You're going to have to change it anyway, the current one is obviously too lean.

Check if you can to see what needle it has. Ask the vendor if he knows.

Hey. Yeah, turning it clockwise :) No, i bought it of a guy in the community, brand new and never used. So expect it to be standard? Im pretty sure i did, yes. But will nevertheless recheck the gasket. Thanks! 

Are you turning the mixture screw the right direction? It is counter-intuitive, but you turn it clockwise to make it richer and counter-clockwise to make it leaner. That is because the screw is acting on a lever. When you turn the screw in, it pushes on the lever, which pulls the main jet down.

Do you know which needle you have?

When you put the air filter base on, did you make sure the gasket was the right way round and that the vent holes in the carb flange are not blocked? The vents allow air into passages that are needed for proper operation of the carb.


Have a look through this thread on the same subject:

https://www.minimania.com/msgThread/128088/1/1/no_power

Hei guys! So i bought a new hiff44, before i had the su hs4. Standard 1275 with a lucas 65d distributor. Set my timing today and went for a test drive. Car runs ok in idle, but when i pass 3500rpm the engine sort off "stops accelerating and wont go pass 4000rpm. If i pull the choke out it goes much better...The sparkplug color indicates a lean mixture so i tried adjusting the mixture up. It runs somewhat better, but still same problem. I turned the mixture adjusting screw againg and again, but after turning it almost a whole turn it does not seem to help. In the fuel filter there is about 1/4 of fuel and 3/4 of air. Could it be the mechanical pump not delivering enough pressure? Could not find any leaks.  is also a small hissing sound from the KN filter, but i guess thats normal since it sucking air? Any advice will be much appreciated.