Strange lawnmower issue

I have a 10yr old Craftsman push lawn mower (model #917.377091) with a 6.5hp B&S engine that I haven't used in 3 months. Pulled it out yesterday and put some clean gas in it. After a few pulls, it cranked up and I was able to mow the front, sides, and a few rows in the back yard. I noticed it didn't seem to be running a full speed and would not fill up the bag with shavings (I had to run it in mulch mode). Even then it would occasionally stall unless I tilted the front up. I let it sit for about 20 mins and then when I started it, it would run very rough (like it was misfiring) - even when idling on concrete. I decided before doing the remainder of the back yard to replace the spark plug (it was still the original) and air filter (it was filthy), but the condition was the same (i.e., running rough). The strange thing is if I tilt the mower up it sounds perfect. As soon as I lower it, the rough sounds return. And as soon as I start mowing it stalls before cutting hardly any grass. I checked the fuel line and gas tank, but there are no obstructions. I took the air filter compartment off and intake on the carb looked good. I took the carb casing off and the float&hinge look good and I didn't see any debris. I also tried starting without the gas cap off, but same issue. Any ideas? I haven't taken the carb completely off to insepct it thoroughly (only the casing), so I'm wondering if there may be something else. It's just strange that if I tilt the mower's frot off the ground about 45-degree angle it sounds great (no rough noise, more RPMs - perfect), but as soon as I lower it, the issue comes right back. Like there's a leveling issue of some sort??? Thanks.