Half throttle backfiring doesn't sound like a sync problem. My first impression would be that the power enrichment system is not tipping in properly or not at all. Are the carbs Zeniths?

Whoops I didn't read all of the post. Are these vacuum diaphragm slide type solexes?

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

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