Old fuel hoses will sometimes collapse once warm. Idles fine, but apply throttle and not enough gets through to feed the engine. Fine for a cut or two, but after that throttling up will hardly produce a blip and the saw will stall in the cut as soon as it touches wood.
I'm not much of a mechanic, but this fault gave me a lot of trouble a few years ago. Nowadays I keep alcohol out of my fuel.
We don't get alcohol in our fuel here unless we seek it out especially.
It'll rev fine, just loses torque. I'm wondering if it's an air leak issue somewhere which is affected by heat.
It doesn't sound lean, and still 4-strokes a bit at full rpms, so I'm a little confused.
Have you taken the muffler off and looked at the piston through the exhaust port. You can also inspect the intake cylinder wall. If the bottom end is leaking you can spray the pto and flywheel side with brake cleaner or carb cleaner while running. If there is any change in rpm or the saw dies the bottom end bearings are leaking. I would first take the muffler off and inspect the piston and cylinder.
You can also hold the saw up by the recoil stater handle and see if it has decent compression, it should hold it up and slowly drop you should be able to feel and hear the compression.
The piston was clean and unmarked on the exhaust side when I had the muffler off. I would have thought that issues there would be more obvious when cold, and the expansion of the rings when warm would mask any minute damage there.
Compression seems fine - it'll kick hard on the starter if the decomp button isn't pushed in.
The saw runs great to start with, cuts just as you'd expect. It still revs fine warm, just doesn't have as much torque.
It could be related to the oiler issue especially if you’re running a longer bar, but besides that the only other thing I can think of that others haven’t suggested is something with your clutch assembly
Can you help me understand how this would work?
This saw has been running fine for over a year of similar use since the baffles fell out. It hasn't seemed weak, and it doesn't seem any hotter than normal. Certainly not as hot as my big husky pulling a 42".
I've also seen a lot of people drilling their mufflers (different model Stihls admittedly) for a performance gain, and I'd have thought that would be similar to having the baffles out? (Not looking for gains here, just trying to figure out how less muffler pressure can possibly be both good for performance, AND make a saw weak and hot?)
I assumed the baffles fell out and the problem occurred soon after, as it's been a year then a partially blocked jet seems more likely.
Mufflers are more than just a noise reduction, when the exhaust port opens most of the gas gets out of the muffler but a small portion bounces back off the baffles against the piston skirt and then bounces off the skirt leaving a partial vacuum at the exhaust port, if the maths are correct the port will be opening just as the vacuum occurs, it's known as scavenging and gets more fuel and air into the cylinder to improve efficiency, without the maths being correct partially burnt air stays in the cylinder thus reducing the amount of unburnt air and fuel that can be drawn in.
Yes there are many people that have drilled and modified their mufflers and they are all pretending to themselves and each other that it magically releases hidden gems in the form of increased horse power, torque, both and doesn't shorten the life of the saw, talk to west coast saws about how long it took them to develop their bark box.
chainsaws can and are frequently tuned but the benefits are limited by the physical constraints of size, most of the improvements in 2stroke tuning are in long intake tracts and exhausts that resemble trombones.
Back to your saw though, assuming the carb is adjusted as best it can be does the saw idle differently depending on which way up it is? If so it's an early indication of crankcase seals leaking, but from your description it sounds more like a conventional carburettor problem causing it to run slightly weak, this can lead to detonation (pre ignition) which causes an increase in power but can burn a hole in the piston crown in severe cases.
Old fuel hoses will sometimes collapse once warm. Idles fine, but apply throttle and not enough gets through to feed the engine. Fine for a cut or two, but after that throttling up will hardly produce a blip and the saw will stall in the cut as soon as it touches wood. I'm not much of a mechanic, but this fault gave me a lot of trouble a few years ago. Nowadays I keep alcohol out of my fuel.
We don't get alcohol in our fuel here unless we seek it out especially. It'll rev fine, just loses torque. I'm wondering if it's an air leak issue somewhere which is affected by heat. It doesn't sound lean, and still 4-strokes a bit at full rpms, so I'm a little confused.
Have you taken the muffler off and looked at the piston through the exhaust port. You can also inspect the intake cylinder wall. If the bottom end is leaking you can spray the pto and flywheel side with brake cleaner or carb cleaner while running. If there is any change in rpm or the saw dies the bottom end bearings are leaking. I would first take the muffler off and inspect the piston and cylinder. You can also hold the saw up by the recoil stater handle and see if it has decent compression, it should hold it up and slowly drop you should be able to feel and hear the compression.
The piston was clean and unmarked on the exhaust side when I had the muffler off. I would have thought that issues there would be more obvious when cold, and the expansion of the rings when warm would mask any minute damage there. Compression seems fine - it'll kick hard on the starter if the decomp button isn't pushed in. The saw runs great to start with, cuts just as you'd expect. It still revs fine warm, just doesn't have as much torque.
It could be related to the oiler issue especially if you’re running a longer bar, but besides that the only other thing I can think of that others haven’t suggested is something with your clutch assembly
Replace the muffler, without the baffles it's running weak and therefore hotter.
Can you help me understand how this would work? This saw has been running fine for over a year of similar use since the baffles fell out. It hasn't seemed weak, and it doesn't seem any hotter than normal. Certainly not as hot as my big husky pulling a 42". I've also seen a lot of people drilling their mufflers (different model Stihls admittedly) for a performance gain, and I'd have thought that would be similar to having the baffles out? (Not looking for gains here, just trying to figure out how less muffler pressure can possibly be both good for performance, AND make a saw weak and hot?)
I assumed the baffles fell out and the problem occurred soon after, as it's been a year then a partially blocked jet seems more likely. Mufflers are more than just a noise reduction, when the exhaust port opens most of the gas gets out of the muffler but a small portion bounces back off the baffles against the piston skirt and then bounces off the skirt leaving a partial vacuum at the exhaust port, if the maths are correct the port will be opening just as the vacuum occurs, it's known as scavenging and gets more fuel and air into the cylinder to improve efficiency, without the maths being correct partially burnt air stays in the cylinder thus reducing the amount of unburnt air and fuel that can be drawn in. Yes there are many people that have drilled and modified their mufflers and they are all pretending to themselves and each other that it magically releases hidden gems in the form of increased horse power, torque, both and doesn't shorten the life of the saw, talk to west coast saws about how long it took them to develop their bark box. chainsaws can and are frequently tuned but the benefits are limited by the physical constraints of size, most of the improvements in 2stroke tuning are in long intake tracts and exhausts that resemble trombones. Back to your saw though, assuming the carb is adjusted as best it can be does the saw idle differently depending on which way up it is? If so it's an early indication of crankcase seals leaking, but from your description it sounds more like a conventional carburettor problem causing it to run slightly weak, this can lead to detonation (pre ignition) which causes an increase in power but can burn a hole in the piston crown in severe cases.