rx7man13b

I have a 13b 6port with a Mikuni 44 carb and the other day I was driving and the car was running strong as always. I parked at a friends houseand 2 hours later I get in start it and it sounded a little funny when I tried to drive it had like no power was backfireing out the intake and exhaust, more from the exhaust. I only lived down the street so I got it home. The inside of the muffler was white instead of carbon covered which I think means its running lean. I checked the jets none were clogged checked out the fuel pump its pumping. What could possibly be wrong that makes the car run perfect for so long and run so lean after sitting for 2 hours? I changed the plugs and checked spark I was getting many sparks from the leading wires and single sparks from the trailing. I have an FC100 ignition thats fairly new but running lean cant be spark anyway. Is carb clogged somewhere besides the jets? At this point ILL PAY for any info that gets her back on the road. PLEASE HELP
I replaced fuel pump and filter with no success