You try give it some gas to see whether it will keep runing? If it will run, your idle screw might be too low, need to tighten up to increase the idle.
If that is not the problem, try closed the choke just a little to see whether it will run better.

Check the screw of the carb is tight. I assume you check the fuel lines for leaks also.

If all else fail, use the carb kid, change the gasket. use carb and choke cleaner spray to spray through the jets. Use compress air canister( Duster from computer store) to blow all the holes and cracks inside the carb dry to get rid of all the dirt. Then reasemble back and see what happen.

Thanks yungman. I have tried the choke and throttle in every possible position, but no luck. I will try the duster trick.

You might have an air leak somewhere. I'm not sure how the trimmers are put together but I imagine pretty similar to chain saws. One of my saws exhibited similar but not the same sort of problems. It was a hard starter but for a time I was able to keep it going by messing with the H, L and idle adjustments. I knew it was pretty far off spec but was happy I had it running. Eventually it would not run anymore. It turns out it had an air leak. Not at the crank seals as is fairly common if a leak is an issue. Instead it was my spark plug and decomp valve that were not sealing. To find this I had to do a pressure/vacuum test on the engine. If you can do a press/vac test, you can pinpoint/eliminate a lot of problems quickly.

Hi Masiman, what do you use for testing pressure/vacuum? That is not something people usually have, I don't!!!

I think before going that deep, I first assume the engine was find 2 years ago. Old gas likely clog the carb and Ocmoses did the right thing of opening the carb and clean the inside. My guess is that it is more likely that the gasket is not sealing, need new gasket( from the rebuilt kid). Also a wise car mechanic told me, never just use carb cleaner to wash the internal parts only, alway use compressed air to go through all the holes and cracks. The computer duster ( compress air can) is the best. I use it on my and I got it to run perfect the first time. You have order the kid, do this first before getting the vacuum tester. JMHO.

I have a mity/vac pressure/vacuum tester that I bought for just this. It is also handy for bleeding brake systems. You can make homemade versions that are just as effective if you are so inclined.

Be careful using compressed air. There are check valves in carbs that can be ruined by the compressed air. I would think those computer dusters would be okay, but I have often read not to use compressors directly into carbs. Also be very careful doing manual cleaning of the jets, i.e. wires. The jets are finely machined and you can ruin them if you are not careful.

It seems like he has done the carb cleaning part, that is why I suggested the pressure/vac test. If the kit and second carb cleaning don't work, I'd recommend trying a new carburetor to eliminate the carb as the issue.

Thanks, Where do you hook the mity/vac pressure/vacuum tester?

The duster is a lot more gentle than the big pump!!! and it is very directional.

To do the press/vac test you have to seal the inlet and outlets allowing only the press/vac tester to insert or remove air from the crankcase. On the saw I did, I had to seal the exhaust port by the muffler and seal the intake port by the carb. I cut out a piece of plumbers rubber and made holes that fit the mounting studs for the carb and muffler. Muffler can be done easier though if you just loosen the muffler and slip a piece of rubber between the port and muffler and then retight the muffler. In my case, I took the muffler off, slid the rubber onto the studs then remounted the muffler.

The carb is a little different. Because I wanted to use my impulse line as the press/vac input, I had to remove that part of my carb. I slipped the rubber onto the carb mounts and remounted the carb minus the carb's impulse input piece. I then hooked my tester up to the impulse input to the crankcase and did the test through that port. Optionally, you can purchase adapters that will cover the carb input and the impulse line. The press/vac tester hooks directly to the adapter. If I have to do another one, I might purchase the adapter.

The pressure test for my saw states that it should hold pressure (0.5bar/7.5psi) for at least 20 seconds.

The vacuum test for my saw states that it should hold vacuum (0.5bar/7.5psi) for 20 seconds. During that 20 seconds vacuum should rise to no higher than 0.3bar/4.25psi.

This was for a Stihl MS026.

It is important to do both tests. A seal may work one way but fail the other. I think it is that they are more likely to fail a vacuum test than a pressure test, but my memory might be wrong on that.

If you can't identify by sound where the leak is, you can use soapy water at the seals to id the leak. Additionally if you still cannot id the leak, you can dunk the engine into a tub of water to see where the leak is coming from. You are better off doing a pressure test than a vacuum test if you use the dunk method. I don't think you would see any signs of a leak with a vacuum test in water. If you have electronics on the engine, you may want to remove them before dunking. It may not matter, but I did not want to chance making things worse so I took off my ignition module. I dried let my saw sit for 24 hours before reassembling.

If you get a tester make sure you can do both pressure and vacuum tests. There are testers out there are meant to do vacuum only for brake bleeding.

Once these little carbs get gunked up they can be a bear to get clean again, if you know someone with the same model try swapping carbs. Steve

Thanks Masiman. That sound like a lot of trouble!!! Do they sell adaptor for different engines?

In Ocmoses' case, I think it really depend on how old is the trimmer. I am not familiar with the model, if it is 20" like the the HC151, it is only $279 new and I've seen customer returned in OSH for $140!!! All depends on the condition of the blades before I would invest a lot of money.

I guess that depends on what you consider trouble :). I like learning to work on engines so it was not trouble to me. If I had no interest in learning then I agree it would be alot of trouble. In fact it probably would have made me swear off used equipment, maybe even Stihl products in total. What it did do for me was make me appreciate how my saw was put together. Because I had done it to mine, I was able to do the test literally in the field on a friends MS660. It took about 20 minutes but it was much faster than the 2 hours it took my first time.

As for the adapters, I do not know how universal they are. I think the Stihl adapter will fit all Stihl saws, maybe even all Stihl engines. I think because there are only a few carb manufacturers that maybe the adapter will fit a number of other makers products.