Hey guys,

I hate that a post like this has to be my first post on the forum, but here I am... lol. Allow me to elaborate on my problem...

I bought a 2007 S40 with just a little over 11,000 miles on it back about 2 months ago. Up until about two weeks ago it was running just fine (I've had it since the end of august). Although I've had it for a little while now, I haven't been able to take it on the open road quite yet. This is because I had to get an M endorsement put on my license (I've rode dirt bikes my whole life but wanted to get into riding on the street) and it needed new tires. There was a small hole on the outside portion of the drive belt so I replaced that with a used belt in very good condition as well.

After I installed the belt (around a week and a half ago), I hopped on it to take a ride around my neighborhood to see how everything felt. I had been riding for around 2 minutes when I accelerated through a turn and got a backfire. I turned down a hill, and as I pull in my clutch to come to a stop at the bottom, the bike dies on me. Lights and everything were (and still are) working fine. The engine cranked just fine. It also acted like it wanted to catch and start but only if I held the throttle wide open. I was pretty low on gas so I thought I might have just needed to fill it up. I ran back to my house to grab my gas can and filled it up. I put the carb on prime for a few seconds to let the bowl fill up, and it changed nothing. So I pushed it to my house and left it alone the rest of the evening because it was getting a bit late anyways.

Fast forward to the next day, and it still wasn't starting. This prompts me to pull the carburetor out to see if it needed to be cleaned. While I had the carburetor off, I checked the petcock because I know it is a common problem-causing item, but it was working just fine. I also checked the gas tank and fuel filter, both of which were spotless. I thought it might have been the battery, but I charged it fully and still had the same issues.

The carburetor itself was very clean, but I cleaned the jets and such anyways. However, I was unable to get the top part of the carburetor (where the diaphragm is) off. The screws are practically welded in. I've tried to get them out with PB Blaster and it does nothing to free them. The same is the case with the 3 TEV screws on the side of the carb near the mixture screw.

The next day I reinstalled everything, as well as gave it a new spark plug and tested the coil for spark (everything looked fine) and it still wouldn't start... until randomly it did. After it started, it idled and ran like nothing ever happened. It felt the same and sounded the same as it did since the day I got it. I let it run a bit and then left it for the night. I had an appointment scheduled to get new tires installed the next day, so I started the bike that morning (started just fine) and ran it around the neighborhood. About the same length of time passed as the first time it died on me and it died again. It went back to not starting. but almost starting if I cranked it while holding the throttle wide open. I loaded the bike up to get the tires changed and got it back a few days ago. This led me to think it was the coil overheating. I replaced it, and no change. I also tested the battery and the readings showed that it was good.

With that being said, this is what I have found out:

If the bike sits for a a good few hours (like overnight), it will start if I hold the throttle wide open while it cranks on a full battery. Before I never had to touch the throttle to get it to start. I can keep it running by modulating the throttle anywhere from around 1/3 open to wide open, but it absolutely refuses to idle anymore. The odd thing is that it will run at an "idle" speed if I hold the throttle open enough, but it will die as soon as I try to get it to idle without holding the throttle open. If I spray starter fluid down the airbox intake while I'm holding the throttle open to get it to "idle", the rpms increase.

To me this sounds like a lean issue with the carburetor's idle circuit not providing enough (if any) gas. The weird thing is that I cleaned it, and the pilot jet was perfectly clear! I will make mention that I messed with the idle mixture screw quite a bit after I got the bike because I drilled out the rivets in the exhaust to make it a bit louder until I can get a nice exhaust like a Delkevic one. The mixture screw used to be rather easy to turn, but now it is almost seized where it will not turn. I believe this started right around the same time it died on me the first time. I think this might be the key to my issue.

I was wondering if anyone who sees this post has experienced this issue or knows someone else on the forum who has? Does anyone have a clue what the issue is? It seems that everything I believe might be the problem ends up not fixing it at all.

The coil, battery, petcock/filter, and the parts of the carb I can get to all look fine/work great. I just don't understand it.

If you need more info, please just ask. I'll give you any info I can if that helps you figure out what might be my problem.

PLEASE HELP!

MrMangoTime wrote on 10/28/24 at 17:05:37:


ah... the old louder trick.  and you're right, it's lean and you probably need larger jets.  holes need bigger holes.  see the tuning posts in the tech section.

Quote:


See, that's the thing though. It had been running fine for a few weeks with the holes drilled and no changes to the carb itself OTHER than the mixture screw adjustment. That's why I'm so confused. I'll take a look at the tuning section.

Thanks for the advice on the screws as well, I'll grab my vice grips and see if I can get them loose.

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.

ThumperPaul wrote on Yesterday at 16:42:28:


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.

MrMangoTime wrote on Yesterday at 20:25:38:


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.