AFAIK, it should be possible to cut the flow of fuel through the main jet by screwing the adjustable needle fully home. As Chazza says, you do sometimes see a worn ring around the needle, caused by it hitting the lip of the jet, when the needle is adjusted all the way in.

If you have screwed the needle fully in and the vehicle does not notice the difference, I would say you have the wrong jet or at least need to re-evaluate the fitting and how it is meant to work.

I've got a Stromberg with an adjustable jet in my Dodge and just like the idle mixture screw, you can adjust it so that the engine runs smoothly but screwing it in or out from that point, causes the engine to falter. I think I've got a 60 jet in the Dodge, but can't check as I'm a few thousand miles away ATM.

Would it be possible to add a pic of the adjustable main jet fitting ? I've got one at home that is different to others I've seen and though I've pondered over its operation, can't fathom how it was meant to work - wonder if yours is the same ?


Jack

P.S. BTW I thought it worth mentioning that if you can't fix the problem, I would recommend reverting to an original Landy carby ie a Zenith or Solex. I think the former is less complex and the one on my Landy (2A) returns about 22 mpg on a run. If they are in good shape, these carbies are a very acceptable fitting.