To all the engine experts on this board,

I've had my GS for a while now and already know that it's a cold starter and runs very lean stock. (I have the 04 F by the way.) However, when I first got the bike, and till this day I noticed that the bikes seems to stumble when running at low RPMs. This happens at all temperatures and fairly consistently, although I notice most when I first start riding because I'm usually giving it a little throttle at stops signs/lights in order to keep the engine from stalling. Basically, when I have the RPM's around 2K (or just below) the engine will pop and sound like it stumbles on the left [I meant as you look at the bike head on so I guess this is actually the right, side with break pedal not gear shifter.] cyclinder (that's just what it sounds like.) I pulled the plugs today and both were tan with a little bit of white on the tips suggesting that the bike is running lean which I would expect since its stock.

I've searched the site and found a few posts about having a short in the coils/ignition wires and I'm thinking (I know very little about engines but this is just from what I've been reading) that I could be getting the short at lower RPM's. At higher RPM's the engine runs fine and does not stumble/misfire. I'm not sure if this is true, but it would seem that at lower RPM's the alternator would be putting out less charge and or slightly more irregular current and if I have a slight short in either the ignition wire or a bad coil then that's when one of the coils doesn't fire properly and the engine stumbles. Not sure if that's correct, but that's just my assumption.

I've recorded two videos so that you guys can see what I'm talking about.
http://www.wamail.net/~aevans/Stumble1.MOV
and
http://www.wamail.net/~aevans/Stumble2.MOV
In both videos I've warmed up the engine and am giving it slight throttle to keep the RPM's right around 2K. You will see the tach bounce and hear a small popping sound, that's the stumbling that I'm talking about. Once in the video I start to take the RPM's above 2K and so I let off the throttle a bit to bring them back down, but the rest of the bouncing is from the engine stumbling or misfiring.

Can anyone suggest some things that might be causing this to happen and how I can test each one to see what the problem is? I'm pretty sure this has been happening ever since I've owned the bike. Oh yeah I'm currently running 92 octane, and have seen the problem with 89 and 87.  I don't think that this popping is the engine knocking due to pre-ignition of fuel/air mixture as I've read that the L-Twin is not a high enough compression engine to cause this problem, but I'm just including this information to rule that out completely.

Bad coil, short in the electrical (I'm going to test the charge in the coils shortly, but I wanted to post this first)
Carburation problem
fuel flow problem
Bad spark plug (I haven't tried to replace the plugs yet either, but I do have 2 replacements although they are the NGK DRP9EA-9. Ones in right now are DRP8EA-9 which are stock. Manual recommends DRP9EA-9 if engine is overheating, which I don't think is happening.)
Other

Thanks in advance for any help that you can provide!!!

I updated my post as I realized that I put Left cylinder instead of Right. So I cleared that up, it sounds like it stumbles on the right side (side with the brake pedal.)

up the pilot and mid-main one size and I bet your problem will go away.

If you apply the choke a little bit does it smooth out?  If so, I agree that bumping up the pilot and mid jets would probably fix it.

-M

Why would larger main/pilot fix the problem? I've already stated that this problem does not happen at higher RPM's (like 2.5k or higher). Also, maybe I didn't mention this, but the same thing happens when I have the choke on. It doesn't happen when the choke is fully applied because the RPMs climb up to high. However, when I first start the bike and have the choke on about 75%, the bike still pops/stumbles/misfires when the Idle is at 1.8K. I know that increasing the mains/jets will solve some problems and is generally recommended for this bike, but I guess I don't understand why enriching the fuel/air mixture will keep the engine from stumbling/misfiring/poping. Applying the choke also enriches the mixture?(I think), and the bike still stumbles with the choke, so I'm assuming something else is wrong. Am I wrong in my assumptions/ can you explain why upping jets/mains would make this go away???

Do you have an air leak on one side?
When you are shifting and you cut the throttle, does the rpm drop quickly or does in take a second to fall from 3/4k down to idle?

Not really sure. I shift quickly and don't pull the choke all the way in. RPMs drop normally, as far as I can tell, but I haven't paid particular attention in some time. How would I detect an air leak, and where would the leak be? (Between the air box and the carbs or the carbs and the cylinder?)

I've got the same problem with my bike, but I haven't gotten around to fixing it yet. I get a similar sound below ~2k rpms, but mine is more of a clicking sound than a popping sound. Mine happens both hot and cold, in gear or in neutral. The rpms definitely drop when it happens, and if I'm in gear the bike will lurch, especially in first.

brett what year is your bike? My bike sounds different which is why I try to include the different videos, but it does sound like a clicking sometimes. I've never had it lurch forward when in gear. My RPMs usually just drop and when it's the worst the bike stalls. I wonder if you have the same model year as me (2004F) if there is a common problem. My bike is still under warranty so if there is a problem that I can isolate, then I will take it in and have them fix it, however I want to make sure that know the exact problem and can be sure that they actually fix it.

Has the bike had this problem since you got it or has it developed over time?  It really sounds like an air/fuel issue in the pilot circuit of the carbs.  If you're feeling adventurous you could pull the carbs and give them a thorough cleaning to see if it helps.  It also couldn't hurt to go through the ignition circuit with a multimeter to look for irregularities.  There's a tutorial here for checking the resistance of the coils: http://gstwins.com/gsboard/index.php?topic=19546.0 and I know I saw a post with more thorough checks somewhere on this site. 

Let us know what you find out,

-M

Mines a '94. I've never had mine stall because of this. I've got my idle at 1.3k rpm, is yours lower? As for the cause, I've had a hunch that it may have to do when I took the case off covering the ignition. My buddy lowsided and had to replace his, and we were looking at mine to see what it should look like. Reassembly was very simple, but now that I think about it I don't remember this problem happening before that. My clicking also sounds like it's coming from the right side of the bike, but I can't pin it down other than generic right side of the engine. I'll try checking the resistance once I can borrow/steal a multimeter from the lab.

As for the "lurching," I suppose I should be more clear. If I'm in first around 1.5-2k rpms, it goes that I hear a click and the rpms drop real quick causing the back tire to all but stop, basically suddenly braking. Then the rpms kick back up real quick and I get the same feeling as I gave it too much throttle. This all happens in about a second. Basically, my point is that the only variable that seems to matter is having the engine at 2k rpm - moving or not, choke or not, gear or neutral, it doesn't matter.

thanks for the clarification. I don't see it happen that much when the bike is in gear, but then again I'm usually pretty heavy on the throttle and don't leave the engine around 2K when riding because my bike will stall.

I haven't checked the resistance of the coils yet. I just bought new sparks plugs yesterday.  (I had another pair but they were DPR9EA-9 and I currently have DPR8EA-9's on the bike now.) I should have time tonight to install them, and if the weather isn't too crappy give it a test run. I checked the coil post and it was for pre 99 models. Here is the link;
http://gstwins.com/gsboard/index.php?topic=19546.0

I don't know if the 04 model is the same. I was hoping that some of the engine experts would be able to offer more information so hopefully they were all out riding this weekend and will get back online now that the weekend is over and everyone is heading back to work!!!

3f

Thanks to everyone for their replies so far.
MANDRES,
Sorry I missed your post yesterday. The bike has had this problem every since I got it. As this is my first bike ever, I didn't really know much about bikes so I just figured that the popping was normal and would go away once the bike was warmed up. The problem still exists, but it hasn't gotten any worse over time (at least not yet it hasn't). I've been trying to avoid touching the carbs because that's something that I've never done, and I want to be able to ride my bike since the weather is getting nice. Also, my bike is still under warranty so if there is a problem that needs to be fixed I can get the dealership to fix it. The only thing is I need to be 100% sure what I ask them to fix or else they won't do it.

When I bought the bike it was slightly used. The previous owner put 200 miles on it and then returned it saying that there was something wrong with the engine (a tick is what the sales guy said.) The sales guy said that they had the suzuki people look at the bike(since it was still under factory warranty) and they opened the engine and tested it and found that it didn't have any problems. Like I said I don't know much about engines and for all intents and purposes the bike works fine. Now that I've learned a little bit(mostly from reading posts on this site) I think that the original owner was talking about the same thing that I'm describin here, and so I assume this has been a problem all along. However, if I just take the bike back they will waste my time, do the same thing as they did in the past, and then give me my bike back exactly as it is and I'll have missed out on a month of good riding.

Is there any way to prove that this problem is carbuerator related? I have about 2.5K miles on the bike now so I'm sure a carb clearning wouldn't hurt. I would love to clean the carbs myself (I'm not going to re-jet without some professional help.) I've read through the carb tutorials on the site, but it just seems like there is a lot of stuff that needs to be set exactly and I don't want to mess my bike up. I don't mind paying people for work as long as I know that the work is actually needed and will fix the problem that I'm trying to fix.

Also, I forgot to mention, but when I first got the bike the best gas mileage that I've been able to get is 48-52 mpg. I haven't checked lately, and will check the next time I fill up, but this seemed a little low considering that my bike is stock and alot of people get 55-60 mpg with the stock jetting. Also, these ratings were still during break in when I was keeping the RPMs around 5K as directed so I would assume that the 48-52 mpg would be max. I know that durring the winter months my mpg was really low, but I just figured that was due to long warm ups and riding with the choke on for extended periods of time due to cold temperatures. Not sure if that helps isolate the issue anymore, but I just thought I'd add that to the discussion.

Thanks again!

Replacing the pilot jet with one  size larger, and the mid-main with one size larger will most likely solve your problem because your problem is most likely caused (mis-fires/stumbles etc) by the bike being too lean at idle and slightly above.  These are the areas that the pilot jet and mid-main are controlling the mixture.  If your bike is an 01 or later you are already LESS lean at full throttle than earlier bikes - with the 04/5/6 being less lean than the 01 or 02 bikes - the went with larger main jets in 01 and upped it again in 04.  It is still lean at full throttle if you do anything with the air filter or exhaust.

mjm
Thanks for the response. Okay, so to sum up what you are saying is my bike stumbles near idle because the mixture is too lean, and that if I made the modifications to pilot jet and mid-main that would enrich the mixture and allow the engine to operate correctly. That sounds reasonable, and would explain why this problem only happens when I hold RPM's just under 2K. Over 2K and the engine is getting enough fuel to not stumble. My only concern is that this happens even with the choke applied. Doesn't adding the choke enrich the fuel mixture? Also, this doesn't happen as often when the bike is just at idle (it does happen but not consistently) so if this is just a problem of being too lean, then why doesn't it stumble at idle up to 2.5K?

Also I did replace the air filter, but I used an OEM suzuki air filter, and I'm also running the stock exhaust.