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.