MrMangoTime wrote on 10/29/24 at 20:25:38:ThumperPaul wrote on 10/29/24 at 16:42:28:Sounds like your air/fuel mix screw is royally messed up if it won't move smoothly and gets stuck.
Can you gently turn it all the way in clockwise (don't smash it in there, just gently until it stops turning)? Then back it out 2 full turns. Try that.
I broke a carb trying to get a seized air fuel mix screw out. Be gently.
It has progressively gotten harder to turn every time I have tried to adjust it since around the time it died on me until now. I'm worried I will literally "screw" up the screw head if I try to turn it with the force it needs to actually move any small bit now. When I first got the bike it was very easy to turn like a mixture screw should be. I'm almost half tempted to just get a new carburetor all together like a more simple Mikuni VM38 if I can 100% confirm the carb is the problem. I saw some tests between the stock carb and a few others such as the VM38 here on the forum and it seemed to perform better and be a little bit smaller in the process.
The same thing happened to my A/F mix screw when I went to replace it during a rebuild/cleaning. Given your symptoms and when the problem started (after you fiddled with it), I’m 99% sure that’s the issue. Wherever you got it stuck at, the engine doesn’t like that a/f mixture. Its sounds like it’s stuck in a “too lean” setting. See if you can get it to move counterclockwise 1/2 to 1 full turn. Get some penetrating oil in there and gently move it back and forth while working it outwards (counterclockwise) about 1/2 turn. Final adjustment (optimal setting) might be more like 1 full turn out from where it’s stuck right now. You can optimize and tinker later, but you gotta get that screw to move to tune it.
I could be wrong about “too lean” based on your symptoms. It’s possibly too rich. If it starts more easily when it’s cold, it’s getting the richer a/f ratio it needs when cold with the too rich setting. The bike will struggle to start if it’s set too rich AND you pull the choke (too much fuel to air).
Question: When you fiddled the a/f screw, do you think you partially moved it inward or outward?
Directing guidance without knowing where it’s presently set makes tuning it and providing guidance tricky. Ideally, you’d want to get it seated all the way in and then turn it out 2 full turns (that’s the base tune starting point that will at least get the bike started rather easily and idling somewhat smoothly). From there, the optimal tune is usually within 1/2 turn in either direction unless the pilot jet has been replaced with a smaller or larger one.
If you think you got it backed out 1 turn fighting with it, try to get it moved back in 1 turn. And vise versa, if you think you screwed it in 1 turn, try getting it backed out 1 turn. In essence, you’ve lost proper tune fiddling with it.
After stripping the head, I resorted to various methods to remove it including trying to tap it out. There’s no room to work down in that rabbit hole and I screwed the pooch (stripped the threads and killed the carb). You can try using some penetrating oil or PB Blaster penetrating catalyst on the screw. Let it sit overnight and do its job. Then work the screw backwards and forwards to remove it (baby steps). If you can get it out, you’ll want to get a new screw and o-ring in there.
I ended up replacing my stock carb with a Keihin-style PWK38 and I had a Mikuni VM36 on a different S40 I had. They both work great, but you will need a different throttle cable (or modify the stock one), a different air snorkel intake or air filter, and you need to have some carb tuning knowledge. It’s not difficult, but it’s not a simple plug ‘n play either.
If you aren’t familiar with working on carbs, I’d focus on getting that seized A/F mix screw out of there. You’re not the first person it’s happened to. It’s happened to several people. Apparently old gas gets gunked up in there and acts like frinkin’ loctite on the screw. In addition to penetrating oil or PB Blaster, you might try soaking some carb cleaner down the rabbit hole. I considered a mild acid, but that would be too harsh on the aluminum.