It is not the CFM.They all should pull 17 to 21in.Hg of vacuum.
.take your air cleaner off.Start your engine,put parking brake on with wheel chocks for safety.Get out by your running engine and rev it up to around 1500rpm.Put your flat of your hand over the top of the carb.After that,tell me that it will not suck vapors(containing particles of dirt).
.BTW,,,, how does your hand feel? grin

There is enough good in the worst of us and enough bad in the best of us that it does not behoove any of us to criticize the rest of us.
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Be yourself. If you are ever lost, It will be much easier to find yourself if you know who you are!