There have been a lot of comments from folks who had Solex carbs on the F-134 engines concerning fuel economy, mixture and overall performance issues. After a lot of fooling around, the best fix in many cases turned out to be a Carter to replace the Solex.
One contributor said the best way to fix a Solex was to throw it as far out the shop door as you could, then go find where it landed, dig a hole, bury it and then forget where the hole was. This might be a bit extreme, but Solex just does not have a good rep with M38A1's or other F-134 engines. There is a reason they cost a lot less ..... .
But, your results may vary.