2-stroke carburetor advice

Greetings.
I wonder if there are any 2-stroke carburetor dudes out there who could shed a little light on my situation:
I have a 2-stroke Weed Eater string trimmer that I've owned for quite a few years. It has started and run flawlessly over the years, with nothing needed from me other than filling the gas tank occasionally.
But this year, it suddenly wouldn't start. Being a cheapskate (and loving to tinker) I disassembled the unit to try to determine the problem. During my tinkering I discovered that if I poured a few drops of gas/oil mixture down the carb throat, it would start and run for a few seconds, so I decided it was a carburetor problem. I tried pumping the primer bulb and that's when I realized that it wasn't filling with fuel as it normally did.
After studying the design for a few minutes, I concluded that the way priming works was as follows: the carburetor has two hose nipples on one side of it. One hose connects to the priming bulb, the other connects to the gas tank. A third hose runs between the gas tank and priming bulb. When you pump the bulb, a vacuum is created in the hose leading from it to the carburetor. That vacuum travels through the carburetor internals, and out of the second hose nipple, connected to the tank. As one continues to pump the primer bulb, this vacuum draws fuel out of the tank and sucks it into the carburetor through the second nipple. The fuel travels around inside the carburetor ("priming" it) and is eventually drawn out of the first nipple leading to the primer bulb. The fuel enters the bulb and from there is pumped back into the tank. This forms a sort of loop: as you pump the primer, fuel travels down from the tank, into the carburetor, back out of the carburetor through the other nipple, out to the priming bulb, where it is pumped back up to the tank again. Does this sound right?
When I disconnected the hoses from the carburetor, the one leading to the primer popped (indicating a vacuum), while the hose to the tank squirted out gas (indicating a pressure build-up in the tank.) These two facts led me to the conclusion that my carb was clogged.
I found a replacement on the web, ordered it and installed. Pumping the primer then caused it to fill up with gas as it used to, and the unit started right up. After a couple small adjustments it was running like new. I was delighted and (a little too) proud of myself.
But then on the third use, I couldn't get it started again. I disassembled it again, only to find the same symptom (no gas getting "through" the carb) so now it seemed my brand new carb was clogged also.
How far off base am I???
(thanks)
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Mr. Land wrote:

Hi, Gas tank cap venting? Tony
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It may be more complicated. Perhaps the gas cap isn't supposed to let air out (cause then if you push the primer, the air squeeze would go out the vent instead of pushing fuel). Just guessing, here.
But if the vent doesn't let air in, the system would starve for gas.
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Hello and thanks for trying to help.
Yes the cap on the fuel tank seems to be working properly. Just to be sure I tried starting with the cap loosened...no difference.
I am beginning to wonder if I inadvertantly put some very old gas/oil into the trimmer...which may have clogged the original carburetor (and subsequently clogged the new one!).
Is there a known way to unclog a carburetor? Perhaps soaking it in a can of some type of solvent?
Thanks again for any help...
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Did you put gas in that was not mixed? it would score up the cylinder. Look down the sparkplug hole with a flashlight to check if the sides of the cylinder is scored, if it is then time for a new unit. If not that then check my 1st answer.

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Could be cap, lines leaking air from cracks, lines clogged , filter clogged, air from a poor seal somewhere. Bad gas, drain it and it should work through, so how did it start with a few drops if it was bad. Use new gas and be sure you are not flooding it , ck for a wet spark plug. Choke is on ?
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Hmm, that would make sense if I hadn't already replaced the entire carburetor with a new one. As stated, the new one ran for a short while, then exhibited the same symptoms.
Thank you for your reply.
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Yes, I left that info out (figured my post was already too long.) Inside the tank there is an off-white cylinder-shaped filter attached to the end of one of the hoses (the one leading to the carb). As far as I can tell it just rolls around in there...it doesn't seem to be attached to anything other than the hose. It's definitely not felt, though...it has the texture of fine sand that's been solidly super-glued together to form a cylinder. I'll open up the new carb tonight and look for that screen. Thanks very much for the ideas.
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You all were so generous with your replies I felt obliged to post my results.
Apparently, the solvent I first used to try to clean the carburetor was ineffective. I re-cleaned it with a fresh can of carb cleaner from my local parts store...and the trimmer is now purring like a kitten (well, OK, a large, angry kitten.)
Sorry for so much bandwidth used for what turned out to be a simple problem.
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Thanks. Very few people ever get back with results. I did not contribute to this thread at all, but I'm just happy to see someone took the time to tell others the outcome. Ed
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