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Last year I purchased a used 2006-2007 Craftsman 2-stroke straight shaft string trimmer with blower attachment. It ran great for awhile, but I developed issues with the way it runs. I replaced the fuel lines, primer bulb, and "ignition switch" last fall, and also cleaned the carb about 3 times last year.

This year, it ran great for awhile, but once again started having running issues. I disassembled the carb and cleaned it in an ultra-sonic cleaner, then cleaned it more by hand.

Today I decided to pull off the muffler and try it out. It ran great! It throttled up fully, idled nicely, and was all around great (but noisy without the muffler!)

As soon as I put on the string trimmer or blower attachment, it won't throttle up worth a darn. No matter what I do to adjust the high or low side jets, it acts the same. But if I pull off the attachment, it runs fantastic!

What is the likely cause of this? Is it perhaps a compression related issue, being that this is a 8-9 year old 2-stroke? It really runs great with no attachment and no muffler! That's the odd thing about it....put an attachment on and it bogs horribly when attempting to throttle up!

It needs some back pressure to run properly. You found the issue was a plugged muffler. You can either clean the screen in the muffler, along with the exhaust ports on the head, or just pick up a new muffler. One of basic maintenance check lists for 2 stroke trimmers is to occasionally check to make sure the muffler screen and exhaust ports are clean.

L8R,
Matt

Yes, but it operates just fine without the muffler and NO attachment. If I put an attachment back on (blower or trimmer head,) it runs like crap and won't throttle up.

It also runs like crap WITH the muffler on, WITH an attachment. Also will not throttle up.

I have not attempted to put the muffler back on and run it without an attachment. I'll do that right now and see how it responds.

I do know for sure that the exhaust port on the head is clean. I can't get inside of this muffler, though. The screen appears to be inside and cannot be disassembled without cutting it open.

Without the head on there is no load on the engine so it runs fine
With the head on there is a load so it runs bad.
Blue smoke exhausts are a black art.
Too much back pressure & the fresh charge can not get in so it chokes & dies.
Not enough back pressure and the fresh charge goes in one side then strait out the other so it is like operating a 4 stroke at idle .

There are a lot of bugs that like to build nests in blue smoke exhausts & plug them up.
There are a lot of beetles that like to crawl up the muffler & cark it.

I have re-installed the muffler. Leaving the bolts slightly loose MAY make a bit of a difference, but not significantly. When I blow through it with my mouth, there seems to be virtually no restriction in the muffler itself.

I notice that if I put the head on WITHOUT the trimmer line, it runs fine. But if I put the trimmer line in, it bogs down.

Right now it has been running in my backyard for 10 minutes with no trimmer line, just idling away.

But there are also times when I can't restart it for the longest time. Nothing seems to be consistent with this old trimmer.

Anyway, it idled away in the yard until the tank ran out of gas. No string was inserted into the trimmer head.

Who knows, it may take 20 minutes to start it next time. Perhaps the diaphragm(s) in the carburetor are old. They don't look bad to me, but perhaps they are still too stiff. I really don't know, I didn't put a rebuild kit in the carburetor. I have only cleaned it multiple times.

Sounds really like it it is not getting sufficient fuel.
However give up your obsession with the "runs without " bit
As in the previous post, without a string there is nothing resisting the engine turning apart from its own compression and the resistence of the internals of the shaft.
Once ther is a string hanging out of the head, it has to push it's way through the air so it puts a load on the engine. A longer string applies a bigger load.

The fact that it will run no load suggest that the electrics are OK so that leaves fuel & compression.
Now I am fresh out of fairys to pop over there and put a spell on your trimmer so you will have to do some work yourself.
Do you have a compression tester ?
Do tou have a pressure pump ?
Do you have a vacuum pump ?

Start by going over the the Walbro site Parts & Service down at the bottom riht are some links to service manuals and tech tips, click on them download the manual and read the tech tips all cube carbs work the same way regardless of brand and there are basically only 2 types the butterfly & the rotary.
Make a coffee or two and read he entire manual at least twice then go back & have a look at your carb.

My initial thoughts are the high speed jet is blocked but go through according to the manual.
It has big clear colour diagrams that can never be bettered by text alone.

I do not have a compression tester, but I do have a vacuum pump, both electric and manual.

I will read up more about this Walbro carb. But perhaps it is best not to invest any more money into a 9 year old 2 stroke. I don't know.

Just curious but what did you use in you use in the ultrasonic cleaner?
Have you tried a rebuild kit? The more times you take it apart and tighten it back up the more wear on the gaskets leading to a weaker seal.

I just had some ultrasonic cleaner that is normally used with electric facial shavers. But my ultrasonic cleaner crapped out within about 15 minutes of use.

However, I have cleaned this carb so many times with brake cleaner, carb cleaner, dish soap, water, and a tooth brush. It is spotless. However I have NOT done a kit on it with replacement gaskets and diaphragms. I am not sure exactly which model carb this is, otherwise I would look for a rebuild kit online.

What is the brand of carb, Walbro?

Yes I do believe it is a walbro. It has a 2-screw mounting plate for the primer bulb.

I "modified" the adjustment area for the high and low screws by removing the screws with pliers and then cutting off the aluminum that was meant to shield them and force you to use a special screw driver to adjust them. If that makes any sense, perhaps a photo would be worth 1,000 words!

Zama also has those, those are probably those splined adjustment screws.
Here is a video on walbro model location #'s: https://www.youtube.com/watch?v=EHZUA6MA62o

Everything is fine with the adjustment. I just cut the aluminum "shroud" off so I have unrestricted access to the adjustment screws.

Are there any clues? The exhaust seems to be very oily - of course, that could have been caused by using a fuel mixture that was overly rich was oil.

At the same time, with the foam air cleaner off, you will occasionally see fuel spitting out of the intake. Do these lead to any clues? Piston rings?

No reason to post anymore, I got rid of this trimmer yesterday. A guy said that he was willing to take on a decent project and really had a liking for Craftsman yard tools, so why not! The problems are no longer mine.

Perhaps I'll learn my lesson this time and not buy any more used 2 strokes! At least I was still able to use it for half of a season and still made a slight profit on top of the initial purchase price. I'm satisfied that it is GONE! Spring is the best time to get rid of things like these.

Again I say...good advise and should be in a "two stroke sticky."