2 stoke strimmer popping

[TheBrick]

Hi,
I am looking for ideas. I have a 2 stroke strimmer, (sthil HS80).

It runs but somewhat stuttery and every so often will pop. It easily stall if used on full throttle under use.

What i have done:
1. Adjusted the carb. Multiple times. No real difference.
2. Rebuild and cleaned carb.
3. Replaced fuel lines as they were a bit cracked.
4. replaced exhaust gasket.
5. Sprayed carb cleaner in exhaust and blow out with air line.
6. Pressure tested and vacuum tested block by removing and blocking off input and exhaust. So no head or crank seal issue.
7. checked the carb pulled vacuum and built pressure on fuel / return lines.
8. The big one, elbow on the original carb was damaged and non replaceable. Exactly the same issues.

I have not checked timing I don't have a strobe light but i am not sure if these symptoms seem consistent with a timing issue?

I have this "feeling" that it is exhaust some sort of extra back pressure? As mention I replaced the exhaust as this was damaged. The same symptoms with the new carb take me away from my first thought that it was fuel a problem.

Previously I have always noticed the exhaust was wet so possible build up of carbon inside? but no idea how I can clean it out, it is a cigaret box size and shape I think it loops about inside.

I have fiddled with the jets so many times slowly working my way though it seem like that is not the issue. Just looking for ideas?

 

[Cato]

If you've done the carb (replaced?) and exhaust and you're sure all else is OK then it sounds like crank seals to me. That causes it to run lean, which gives the symptoms you describe.

 

[tom2207]

try a fresh plug in it before you start stripping

 

[dave h 2]

My old man had a 30 year old Atco 2stroke lawnmower that ran like a dog, he tried everything except the exhaust, which turned out to completely coked up.

 

[TheBrick]

If you've done the carb (replaced?) and exhaust and you're sure all else is OK then it sounds like crank seals to me. That causes it to run lean, which gives the symptoms you describe.

I pressure and vacume tested. I think I did about 8-9 psi is that too low? I will give it another go. Thanks

 

[TheBrick]

My old man had a 30 year old Atco 2stroke lawnmower that ran like a dog, he tried everything except the exhaust, which turned out to completely coked up.

Did he just replace it or find a good way to clean out the coke?

 

[dave h 2]

Did he just replace it or find a good way to clean out the coke?

It split into 2 halves so was easy. If you can blow through yours easily it won't be blocked. If you can't liquid oven cleaner will probably shift it, just don't get it on you.

 

[Cato]

I pressure and vacume tested. I think I did about 8-9 psi is that too low? I will give it another go. Thanks

Sometimes the pressure test is a red herring as while it'll find a split/damaged seal it won't find a pressure loss when it's running which is how it shows up as wear.

It's not unusual to end up at crank seals by a process of elimination i.e. when you've tried everything else and you're sure that they (carb exhaust etc) are all as they should be.

 

[jo1010]

Sometimes the carbs don't rebuild well. Some of the carb kits are no good. And some of the cheap carb replacements are no good too (for one I had I had to search forums to find a good Chinese brand that actually worked). The Stihl parts are expensive though!.
Carbon in the exhaust and or rings can be big problem - the first because of blockage (can heat up the whole exhaust off the engine with Oxy fuel, or if its really corroded a replacement is cheap). The rings can jam in the piston with carbon...so need head off to remove and clean out. Running with more oil in the mix than you need might be the problem there.
I presume.you used new fuel filter with the new lines...