I ran into a strange problem with the carb on my Mac powered weedeater yesterday, and I thought I would share what I found with you guys. Maybe save someone some grief some day.

It had run a bit "ratty" the last time I used it, (months ago), so I figured I better clean/check the Walbro carb before using it again. So yesterday I pulled it apart and cleaned it. The regulator diaphragm was probably useable, but a touch stiff, so I put a new one in it. I did notice when removing the carb that one of the rubber fuel lines was like it was glued to the brass carb inlet tube, and came off in pieces. The hose itself seemed ok, but I had to scrape pieces of it off the tube where it had actually glued itself on.

I then put it on the machine and tried to prime it with the primer bulb. Most people don't understand how the primer bulb works. It actually pulls fuel through the carb, and squirts the excess back into the tank. When you use the bulb, you are not pushing fuel through the carb, you are drawing it through the carb. This causes the diaphragm to open, and fills all the passages in the carb with fuel. No fuel gets injected into the engine as many tend to think; all it does is fill the carb. Well, the carb wouldn't fill. I took off the primer and inlet hoses, and connected them together and the primer bulb itself was working fine. So I pulled the carb back off, and tore into it again. This time I noticed that the regulator metering lever was way low. So low that the diaphragm couldn't open the needle. I thought "that's strange, as I know I had it set right". So, I reset the lever height, put it all together, and tried again. Same darn problem; it wouldn't draw fuel through the carb. So I took it off AGAIN, and again found the metering lever bent down and far too low. That is when I reasoned that the needle valve itself had to be sticking in its seat. I had removed the needle when I cleaned the carb and it seemed fine. But upon examining the tip with a magnifying glass, I could see that the rubber tip was slightly swollen and actually sticky/gummy feeling. I cleaned the seat area again, put a new needle in, and then the carb primed perfectly. The machine then started on the second pull, and a slight tweak on each mixture needle had it running perfectly.

At first I thought it amazing that the primer could cause the metering lever to bend like that. But after some more thought, I realized that even if the primer only pulled .5 atmosphere, the diaphragm is nearly 1 inch square, so that would be around 7 lbs. of force pushing on the lever. The needle was stuck on it's seat and wouldn't move, so the lever bent.

It was a pain at the time as I wanted to finish and use the machine, but it was an interesting experience, and I hope this post helps keep someone from "tearing their hair out" with a similar problem some day. Apparently additives in the gas turned that rubber needle tip to "gumby", and caused it to stick so tight it wouldn't move. It would move when I grabbed it with a needle nose to remove and clean, but would then stick so tight the metering lever couldn't budge it!

AV8TOR