ORIGINAL: Spacemonkey71
I can tell you with almost 100% certainty what your problem is: K&B .40 series engines don't run worth a CRAP with 15% fuel in them!!!! [:@]
How do I know this? Because I've got a K&B .45 and I was seriously PO'd when I couldn't get it to run for more than a minute or two at a time, then I spoke with someone about K&B engines any my frustrations. It just so happened that he had owned several of them over the years and he told me they WILL NOT tolerate fuel above the 10% line. I drained the tank, flushed the lines, and put in some "sweet pink" in the tank and I was flying inside 5 minutes. No kidding! It had just about EVERY problem you described (plus a couple of extras like not being able to target the sweet spot on the idle). The K&B engines are built like bricks, but they are SUPER finiky in their diet.... It makes no sense that 15% fuel freaks out an engine that runs great on 10%.... it's the strangest thing....
But while you're at it, there's no harm in replacing the fuel tank and fuel lines. I had to change the tank+tubing out on my Sr.Falcon that hadn't been fired up in 15-20 years! The brass clunk was so oxidized it was GREEN!!! [X(] and the sludge in the tank was like something out of a bad sci-fi movie.... or duck pate'....
Don't give up on that engine, K&B makes a dang good engine that lasts a heck of a long time; but it usually takes a little while to figure out how to get it running
Nonsense. I've got several K&B 40's of various vintages (no .45's though) and they love 15% fuel as that is all I buy. I run both K&B carbs and Perry carbs on them without a problem and have been running them since the 70's. They do HATE to be run too lean with the ring in there though so watch out for that. BTW, I also have K&B 61's with and without pumps and even one with a reverse crankshaft & pump that runs great on 15% fuel. No, I'm not stuck on K&B's as I also have OS's but K&B's were my first engines and I still like them.