Could very well be, yes.
But then again, the Solex CISAC 28-32 isn't exactly the most predictable or adjustable kind of carb

I spent about 6-8 months to get it more or less right
Just another few steps to think of:
-Did you check float height when you fitted the new float?
-Did you check the carb base for straightness?
-Did you use new gaskets between carb and manifold?
-How tight did you pull the bolts that hold the carb on the manifold? (too tight will ruin your seal.... too loose and it'll be even more too loose in no time...)
Ahm..., now I can't think of anything else, or it might be the fuel filter in de carb itself, or even dirty jets... though that might be a bit far fetched.
[edit]
Oops, I'm mixing you up with someone else...
If it's not air coming in through unwanted holes, first thing to check would be your carburettor. Good chance your float is dangling completely under...
You can get a spare float at Renault dealers.