Can you put the horse in front of the cart and outline what you are dealing with.

Assuming it is and A or A+ engine, what size and build is it (Stock or otherwise)?
Are you trying to set up a single or multiple carb system?

For example, a HS4 might be too much for an 850, or dual HS4s might be too much for w stock 1275.

SU carbs are generally set up with the jets Alex notes - .090 for HS4 or .100 for HIF 44. The jets are proportional to the carb throat diameter. Varying the fuel delivery is done by changing the long needle that goes into the jet, not the jet itself. Basically to get a leaner mixture, you would use a thicker needle. There are hundreds of different needle sizes. They are also selected to give different air/fuel ratios at different engine speeds. So, if the needle you have provides a good idle mixture but is rich at higher rpm, you need a needle the same diameter at its thicker end but thicker towards its tip.

But before mucking with different needles, you need to make sure the carb is set up properly:

1. Is there oil in the dashpot damper... what grade of oil?
2. Is the dashpot moving freely with the damper piston removed? It should drop with a small but definite "clunk".
3. If the dashpot doesn't fall properly, the needle and jet may not be centred and may be sticking. There are two types of needles depending on the carb type - fixed or spring-loaded, AKA self-centring. There is a process to properly centre the fixed type,
4. Once the dashpot and needle are correct, you can then move on to setting the idle mixture. This is done by adjusting the height of the jet. The lower the jet, the richer the mixture. done at a idle speed specific to your engine.
5, The mixture adjustment will also depend on the condition of the carb itself. If there are air or vacuum leaks anywhere, you will be adjusting the idle mixture to compensate for those leaks. A common leak in a used carb is at the throttle shaft bushings. You may also have intake gasket leaks. If you have tuned to compensate for leaks, then your mixture will be good at idle but rich at higher rpm even if the needle is the perfect choice.


Lots to learn!

An HS4 uses a .090" jet assembly.

The .100" from an HIF44 if completely the wrong shape to even fit.

Can you clarify a bit what you are asking about and what size engine you are working with?

This is going to be a bit of a weird one. Just wondering on everyone's opinion that uses the carbs on smallest jets sizes to get and what size range engines they would suit. Trying something at the moment but is currently just fouling plugs and overfueling under throttle.
Any help would be much appreciated.