HS2 Carb adjustment screws and mixture nut setting

[mk1sdl]

Hi,



I am having problems setting up my carb properly.



As far as I can tell, there are three things that can be turned to adjust the settings..



The mixture nut that sits underneath the carb below the jet
A sprung screw at the front facing slightly downwards
A sprung screw screw on the left (when facing the carb from the bonnet - no 1 cylinder to the right) which faces upwards


Even with the mixture nut wound all the way in, the engine revs still rise when lifting the piston carrying the needle

The engine revs won't drop below 1000 rpm even with the sprung screw at the front (fast idle??) wound all the way out

What does the other screw set? It appears to push up against the choke cam..



Are there default settings for each of these to use as a starter point for getting idle speed and mixture set up properly?





Many thanks!

[dklawson]

Hello and welcome.

Answering each of your specific questions and explaining the tuning process would involve a lot of writing. Instead, I have included a link below to the Burlen website. Burlen is the current owner of the SU intellectual property.
http://sucarb.co.uk/technical/

On the left side of the screen in the link above you will see tabs. Select HS Type Carburetors and the window that opens will give you options for articles on how the carbs works and how to adjust them.

Also see the link below for a PDF copy of one SU tuning manual.
https://tecb.eu/onewebmedia/Tuning_SU_Carbs.pdf

I assume your Mini is new to you. You should buy a service manual if you haven't already got one. Most manuals will have a fuel system section that discusses tuning SU carbs. In the meantime the information at Burlen and in the PDF I linked should be a good starting point.

[mk1sdl]

That's great - many thanks!

Yes it's very new to me and the Haynes manual is a bit limited on the specific functions of each of the adjusters.

Thanks again.

[dklawson]

Since this is new to you I'll add that as you learn about these carbs, don't think you have to focus on information that specifically mentions the Mini. The HS series carbs were used on many cars through the 1960s and 1970s. While there are sometimes differences in the carbs from one make car to another... and while there were changes added to the carb design over the years... the basics of the HS carbs remains the same. Videos and documents you find on the HS2, HS4, and HS6 carbs may each be informative regardless of whether they are for Minis, MGs, or Triumphs.

There are lots of skilled carb tuners on this board... I am not one of them. Read a lot before you start experimenting with the mixture adjustments and be sure to ask questions here!

[DB1380]

mk1sdl.

If you need more reading material, get a Haynes SU Carb manual. Well worth getting and a mine of information.

Dicky.

[dklawson]

If you need more reading material, get a Haynes SU Carb manual. Well worth getting and a mine of information.

The original Haynes SU carb manual may be out of print. It appears to have been replaced by the "Haynes Weber Carburetor Manual" which includes sections on Weber, SU, and Stromberg carbs.

[whitewithblackroof]

is the choke stuck out, give the jet a bit of a push up also make sure the choke cable end is not outside of its location on the carb as this can cause it to stick
just to add to this, the choke on the SU is done by the jet being lowered allowing more fuel to air ratio if stuck down it could be rich even when the jet is wound up fully

[AndyPen]

Those choke cams - if I read you right, can become very sticky and stay open as the surface is rather rough.

Have you tired disconnecting the choke just to see what happens?

I am assuming you've lifted the piston(s) to check they drop fully with a nice clunk, and have plenty of oil in the chamber so the resistance is felt?

Are the plugs sooty??

[DB1380]

If you need more reading material, get a Haynes SU Carb manual. Well worth getting and a mine of information.

The original Haynes SU carb manual may be out of print. It appears to have been replaced by the "Haynes Weber Carburetor Manual" which includes sections on Weber, SU, and Stromberg carbs.

I didnt think of that Doug. Shame really as it is a good source of information. I got mine at an Autojumble many moons ago...think it was £3. lol

Dicky.

[TECH396]

I'm sure someone has mentioned it, but make sure you start with correct float levels, before you consider other adjustments.
Also, pull up on the jet after you attempt to Lean off the mixture. They have a tendency to stick down, or Rich, even after adjustments.

[mk1]

Also check for air leaks, these can cause the symptoms you describe.

The technical section on the main MK1 site is usually a good place to start.

http://mk1-performance-conversions.co.u ... l_carb.htm

Both this;

http://mk1-performance-conversions.co.uk/hs_tuning.pdf

And this have been on there for years.

http://mk1-performance-conversions.co.uk/tuning_su.pdf