GA C-150 PLT EXPERIENCES IMC IN VFR FLT.

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I FILED AN IFR FLT PLAN WITH GRB FSS FOR A 63.7 NM FLT FROM LNR TO FEP. THE CEILING AT LNR WAS ABOUT 1200 AGL; FIELD ELEVATION 717 MSL; VISIBILITY VFR PLUS. I DEPARTED VFR AND CALLED ZAU TO PICK UP THE CLRNC. THE CTLR SAID HE COULDN'T PICK ME UP ON RADAR AT MY 2000 OR SO FT MSL ALT. HOWEVER; HE ASSIGNED ME A XPONDER SQUAWK. THINKING HE WOULD COME BACK WITH A CLRNC WHEN HE KNEW THE AIRSPACE WAS CLR; I PROCEEDED VFR ENRTE TO FEP. I BELIEVE I CALLED BACK ONCE AND THE CTLR CAME BACK SAYING HE STILL DID NOT HAVE ME ON RADAR. I CONTINUED ENRTE IN LOWERING VMC CONDITIONS. AT SOME POINT; I BEGAN MONITORING ROCKFORD APCH CTL; WHO I KNEW WOULD TAKE THE HDOF FROM ZAU. I STILL EXPECTED TO HEAR FROM SOMEBODY; SINCE I FILED IFR; CONTACTED ATC; AND HAD BEEN SQUAWKING THE XPONDER CODE GIVEN TO ME BY THE ZAU CTLR ON INITIAL CALL-UP. ABOUT 2/3 OF THE WAY TO FEP; THE CEILING AND VISIBILITY WENT DOWN CONSIDERABLY AND I COULD NO LONGER MAINT VFR. I CALLED RFD APCH AND ASKED IF HE HAD A CLRNC; AT THE SAME TIME; I INITIATED A CLB INTO IMC. THE CTLR SAID HE HAD NOTHING ON ME AND CONTACTED ZAU. THE RFD CTLR CAME BACK SHORTLY WITH MY CLRNC; AND I PROCEEDED TO MY DEST WITHOUT FURTHER INCIDENT UNDER IFR. I SHOULD HAVE GOTTEN A VOID TIME CLRNCS FROM FSS. I SHOULD HAVE REMAINED IN THE VICINITY OF LNR UNTIL THE PROB WAS RESOLVED. THE FIRST CTLR SHOULD NOT HAVE ASSIGNED ME A XPONDER CODE AND THEN IGNORED ME. I ASSUMED I WAS GOING TO GET A CLRNC WHEN SEPARATION WAS ASSURED.