Here's my attempt to answer the questions asked before I went out in the shop tonight. I didn't think I'd be able to work on it at all but some other stuff cleared out for a day.
1. Share some pictures of the chain, the sharpening, and the angles you’re using. This is an Oregon chain that has only been sharpened a couple times. I use the Oregon file guide and a 5/32 file at 30 degrees I think, maybe 25, I'd have to look at the piece of the package that's in the saw case. I think it's different than the one on the MS180. I'm not good enough with sharpening to do anything more than what the package says to do. See the attached files.
2. Upload a video of the saw idling for a few seconds, then pin the throttle for 5-7 seconds before letting it return to idle. Repeat this a couple of times in the video. I attempted to do this today after doing the vacuum / pressure test and looking directly at the crank seals, but now it's decided that it doesn't want to accept any throttle input after starting. It will rev up on the fast idle but after that it will die and won't restart. I put the Amazon carb back on and it did the same with it, too. I did not adjust the Walbro carb since when I took the last video, but I did remove and reinstall it to put the block off gasket behind it for the checks. So something changed, and I don't know what.
3. Measure the skirt 1/2 inch above the base / bottom, and measure the cylinder diameter as well. While you’re in there, inspect the cylinder and piston for anything unusual. Will do, when / if I get to that point.
4. Disassemble the clutch, inspect it, and see if anything stands out. Nothing abnormal found in there (beyond the leaky oil seal on the oil pump. It didn't have any oil in the clutch assembly though). It says electronic quickstop on the cover, but this clutch looks nothing like either the standard or quickstop in the service manual that Charlie shared with me. Regardless, it looks like it's fine with minimal wear.
Check the coil gap, to my ear it sounds like the ignition is not advancing spark properly. In the last couple weeks I have seen many failed coils, inspect it carefully for warpage, very fine cracks in its body and the multi layered steel pickup legs delaminate and the wire lead and kill wire can rub through enough to kill spark when the saw is used...even worn crank bearings causing the flywheel to move away from the coil can cause weird issues. OEM is the best for coil replacement! Coil gap is good, I just set it when I put it back on after testing the aftermarket coil. No cracks or warpage of the metal I can detect. No play I can detect by hand in the crank bearings but I haven't put a dial indicator on it. There are no signs of the magnets having contacted anything to put scratches on them.
The high speed nozzle/check valve is stuck open. Where is this located? Something in the carb or somewhere else? I can't find it in the parts listing.
Yes. Make sure the clutch drum bearing is intact. It looks like it's brand new. No signs of scoring, heat, wear, etc. The assembly turns smooth like you would expect.
Make sure you don't have a crack in the intake manifold that opens when the saw flexes on the buffers. I got a new intake boot and will install it soon if I end up taking the cylinder off. With the old one I'm passing the pressure and vacuum test now without any leakdown now that I've tested off the impulse hose fitting rather than through the compression test adapter. Rotating the crank didn't make any difference either, and putting soapy water on the seal beneath the clutch didn't show any bubbles or suck in any solution during the test.
Did you ever probe the idle and transfer holes in the carb? Yes, every single hole. I have an old torch nozzle cleaner set that I use for this purpose so I always have something that fits tight to the drilled hole size. I watched the Walbro video too. Probably good for someone who hasn't had one of these apart before, but that's not me. I've cleaned and re-sealed them many times, so nothing new to me in this one. I've just never had a machine that didn't get fixed by either clean + gaskets or a new cheapo carb.
Is there a good pulse on the line? There is pulse, both pressure and vacuum. I can't say how much though, maybe 1-2 psi each way off zero? It moves pretty fast obviously.
Does fuel push out the hose with the fuel cap on? When the tank heats up after running, yes it pushes fuel out the carb hose when the inlet to the carb is disconnected. If the carb is connected, you can see and hear it bubbling air out the labyrinth passage past the grub screw in the vent tube.
I am sure the choke flap has been checked for spring tension. Correct, it has plenty of tension and it made no difference when I ran the saw (already warmed, a couple days ago) with the choke plate removed completely.
The spark stays strong when bogging? How would I accurately check this? Every inline spark tester I've ever used has had a VERY detrimental effect to the quality of the spark delivered while it was installed. Basically only good for a yes / no on spark, not spark quality. It has good quality spark when tested not running, grounded against the cylinder, and the China coil worked with the exact same symptoms.
The upper rear buffer is not contacting the plug wire when any pressure is applied? Nothing is hitting the plug wire.
The plug wire has no wear spots and is fully inserted into the module? No wear spots on the plug wire and no signs of it trying to pull out of the module. This saw likely has less than 20 hours of run time in its life, with me putting on at least half of that from about 2017 - 2023.
And the seals hold vacuum? (50% fail rate on that seal in an older saw). Correct. As mentioned above, it holds vacuum fine, static or while rotating the crank, when tested from the impulse tube.
So that's where I'm at as of now. Seemingly getting farther away from home the more things I check. There's got to be something silly I'm missing, but at this point I don't see how it could keep so many of the symptoms constant if it was carburetor or spark related when I've changed out both systems repeatedly.
