It doesn't sound as if that spring is causing the problem. Do you have a set of jet drill bits? Here is the specification of the M120X jets, as shown in the workshop manual:
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As you can see, the correct main jet for England is 0.85mm, and for South Africa is 0.90mm. According to the Toro manual, the correct main jet for the US was 0.80 mm, and the lean jet for high alititude in the US was 0.775 mm. This wide range of jet sizes suggests that in at least some countries, Suzuki was pushing hard for the leanest possible mixture, perhaps for emissions control reasons. It would be useful to know what size main jet your engine actually has. This is typically measured by pushing jet drill bits through it, preferably holding-end first rather than drilling end first, so you can't accidentally make it bigger.

The float height also needs to be checked. It should be 17.5 mm, according to the Toro manual.