Ok folks after a few days waiting for parts, I have some good, bad, and slightly confusing news.
I pulled the welch plug out of the original Stihl carburetor. Everything under it was spotless clean. That’s something that I don't plan on doing again. I had already cleaned out all the passages leading to it from the top side except for the one that leads to the L mixture screw, and that one was already verified open by flowing carb cleaner through it. My torch nozzle cleaner tool is like a “tag wire” on steroids, and having multiple sizes in the set means I can generally be right at the passage size without either reaming it larger or leaving crud in it. Probably the best $0.50 I’ve ever spent at a yard sale… The fuel tank was spotless clean and gum free and the screen was completely clean as well.
I checked the high speed fuel check valve with tubing and it behaved as I expected. So I put a new rebuild kit in the carb (welch plug, needle assembly and pivot / spring, fuel pump sheet and gasket, diaphragm and gasket) and pressure tested the needle valve. It only bled down a little bit and it did that fairly slowly, so I deemed it good enough to try. I pulled off the recoil cover and loosened up and re-set the coil gap (still using the original coil) using the same piece of folded paper that I had used previously. I don’t think it ended up any different than it was, but I did it. From there I put the rest of the stuff back on, added fuel, and tried to start it. It started but wouldn’t rev above idle at all. It was different sounding than before, like it wasn’t getting air flow. So I took the muffler screen off and that made no change. Then I put that back on and took the choke assembly completely out of the air filter, and it ran like it was supposed to. I’d done that at least twice before, but without this particular carb kit, so I’m thinking maybe I had both internal carb gasket issues and a choke issue, and I’d never managed to have both corrected at the same time before. Regardless, at that point I was able to tune the low speed screw so that it accelerates instantly to high speed, and then tune the high speed so that it just barely wants to 4-stroke at free rev max speed. It no longer had the intermittent miss at unloaded high speed, either. I went and sawed with it for a while like that while I had it running, and it was fine. It would restart hot with 1 pull. It was back to being a valuable contributor to the ‘homestead’.
After I was done working it, I dug back in my saved parts pile and found the original 1985 air filter that I had saved that has the flocking worn out in many places. I took the choke lever out of it and put it and its spring in the aftermarket filter that I was using without any choke plate in it, and went to make a couple test cuts with it that way. That would start and run, but it was definitely down on power. So I think it’s still likely blocking air flow more than it should be when the choke isn’t on. The spring has plenty of tension but I am wondering if something in the internal filter shape isn't right. I’ve ordered a New Stihl filter and will install it when I get it. But if I have to install the aftermarket filter again without the choke plate in the meantime to get more stuff cut up, I’ll likely do it.
So the bad part is I’m not sure exactly what the original issue(s) were. I suspect some combination of the carb rubber parts and the choke lever, but regardless I have a path to the saw being useful again.
Has anyone ever tried to adapt one of the newer round air filters onto an 028 AV Super, even if it means having to run a separate choke lever and leave off the current air filter cover? If the replacement OEM filter doesn’t provide the necessary flow without removing the choke plate every time, I may pursue something like that. I really hope that’s not needed.
Thanks for everyone’s help in getting the saw back running!
