The AHA! Moment...
That special moment when suddenly everything becomes clear, and the solution that has eluded you is finally within your grasp! Sometimes, perhaps often, it comes along as an accident - but whether by accident, skill, or just good luck, it's always nice when it comes!
I was working on another String-Trimmer this afternoon; This time a McCulloch trimmer that uses a Zama-brand carburetor. This was a remanufactured unit, and the customer complained of it leaking fuel, suspecting that it was leaking from the gas-cap. I don't think so... I noticed fuel dripping from the air-cleaner, and when I removed the air-cleaner and pumped the primer bulb, I saw even more fuel flooding out of the carburetor. Following the fuel-lines and watching how the fuel moved, I could see fuel flowing up one line, and trying to move down another which was bent at a sharp angle because it was too short. I cut a new length of fuel-line to replace that, and it seemed to me that the lines were reversed on this machine as well. It was at that point that one of the lines suddenly popped out of the fuel-tank, much to my surprise!
At first, I thought that the line had broken off, perhaps from being old and brittle, but that wasn't the case. What I had thought was the fuel supply line turned out to be the return line! I had thought it was the supply line because I had seen fuel moving up the line rather than down the line back to the tank. Now this was starting to confuse me, so I pulled up a PDF file I'd downloaded from the Zama web-site to see if it would shed some light on the connections issue. It did, and it also cleared another problem! It seems that I was mistaken about the primer bulb being used on the supply end of the fuel-line system. From what I've now found out, these primers are actually purge systems, and they go on the return end of things! They dont' suck fuel from the tank and push it into the carburetor - they suck fuel through the carburetor, and drain it back into the tank!
When you stop to think about it, this actually makes a lot more sense: If you're trying to push fuel through the carburetor, you're forcing it against the needle-valve that's supposed to stop the flow of fuel until the engine demands it. Yes, you can exert enough pressure on the system to do this, but it's really not a good idea. By sucking the fuel through the system, you're moving the fuel in the proper direction, but by creating a vacuum on the other side of the needle-valve, it willingly opens to allow the fresh fuel to flow through! Once I had the lines in the right places, fuel easily moved through the entire system: From the tank, through the carburetor, through the purge-bulb, and back into the tank. That's when the AHA! Moment hit me!
THIS is probably what the problem with the $*(&$*$*&%!! Weed-Eater is! Right now, the connections on the Weed-Eater are trying to push the fuel through the system instead of sucking it through. The tiny fuel-pump on the carburetor can't suck the fuel through the purge-bulb hard enough to keep the fuel flowing, and that's why it keeps shutting down! I still have to actually put this theory to the test, but I am 99% certain that I have now found the problem! I'll have to test it tomorrow!