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Discussion Starter · #1 ·
I just bought a stock 06 KFX 400 from a guy that didn't know where to find the spark plug on it. It has sat for most of the last 2 years. I checked the plug before I bought it and it was nearly carbon fouled which lead me to believe that it's running rich so I picked up a couple spares before I went out this weekend. While running around the house I've noticed a couple times that in first gear it would stumble when going WOT at a low rpm.

I'm in Colorado. I live at just above 5200' and I ride between 6500' and 13000'+.

I took it out for the first time this weekend (between 8500' and 13325') and had a couple issues. I never got out of 3rd gear and spent the vast majority of my time in 1 and 2.

1. At or near WOT I was getting a stumble in 1st and it was bogging down in 2nd. Not severe, just enough to be irritating and know that I was having an issue.

After a while of this I pulled to airbox lid seeing how I was rich at 5200' and was only climbing from 8500. After I did this it was immediately better, but still not good.

2. Immediately after pulling the lid I did a couple 1-3 WOT pulls and every time I'd shift when in a high rpm and high load scenario it would fall flat on it's face, pop, and hesitate in the next gear for a few seconds. I pulled the plug again after these pulls and it was definitely lean so I just really stayed out of the throttle for the most part for the rest of the day and it ran alright.

I really don't like carburetors.

So my question is this. Does this sound like an issue that would be fixed by jetting and/or adjusting the fuel mix screw or do I have something completely different going on?

Sorry for my rambling post and TIA for any help.

Ryan
 

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I am in colorado as well and I have been suprised at how big of difference altitude makes on jetting. I have to jet conciderably leaner than everyone else and it sounds to me like the symptoms you are describing are caused from running rich. Usually if you are rich it will stumble at first and then pick itself up and take off as it cleans out. If you are starving for fuel it will stumble the whole time. Assuming you are running the stock carb anyway. To be on the safe side I would switch your petcock to "prime" and see if it runs any better.
 

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Discussion Starter · #3 ·
I'll try that. Thanks.

I'm pretty sure I was lean this weekend after pulling the lid. I'll post a pic of the plug.
 

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It could be the shadow, but it looks very slightly lean. Maybe one size on the main jet or a needle clip if yours is adjustable. As far as the fuel screw, did you read the sticky on how to adjust it? And how its linked to the pilot jet?
 

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Discussion Starter · #6 ·
Yeah, it was tough trying to get a pic to turn out in that light. In the sun the pics came out completely over exposed and in the shade you can't see nearly enough color detail. It's whiter than the pic shows.

I haven't done anything yet because I don't want to make it worse and/or screw up something that wasn't the original issue.

I read about the secret squirrel screw, but TBH, I'll have to read that post again a few more times before I can wrap my head around it. Also, since I'm riding so much higher than where I live, I'm guessing I'm going to want one of those thumb screws to dial it in every time I ride.

Thanks for the suggestions.
 

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Yeah I have a thumb screw, I race in co springs and berthoud and I live in Brush so I change elevations a lot. But if you were running lean after pulling your lid off I doubt you were running rich enough before to hurt your performance. I would try running with the petock on prime and see if that helps, sometimes they plug. Do a plug check at 5k feet and see what it looks like. If you get bored go through the carb and clean it all out.
 

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Discussion Starter · #8 ·
I've been busy today...

Here's what the plug looked like today after a couple 1-3 WOT pulls with the lid on. Ran pretty much perfect. Also, it ran the same whether it was on run or prime.



Then I tried pulling the lid and it immediately started stumbling, which I kinda figured it would do, so I decided to pull the carb, clean it, and learn a little about it in the process. I basically followed the jetting teardown procedures without actually replacing any jets. Everything was pristine clean. The jets looked perfect. Someone had been in that carb before. The idle adjuster screw had been messed with and I believe the bowl screws were replaced because they weren't soft at all and came out easily.

I put everything back together and decided to play with the fuel screw, which I found an easy solution to adjusting without buying anything. More on that later. I ran the RPMs up to what kinda felt like somewhere near the 1800 rpms that the guide says to use (not sure how you do this without a tach) and started going in on the screw until it started to die, which it never did. I got it fully closed and the RPM definitely changed but it never tried to die (were my RPMs too high?) and then I ran it back out until the RPMs hit peak but it never sounded any more or less crisp. It seemed to me that there was a large dead spot where adjusting made no RPM or audible change. There was nearly 1 full turn (from ~1/2-3/4 turn out to ~1-1/2 turns out) that it sounded the same and didn't change rpm. I put it somewhere near the middle of that dead spot (~1 turn out) and took it for another quick spin. It stumbled in the middle of 2nd gear and then picked back up again. It didn't feel as strong as before I adjusted it, but I had other stuff to get done so I just left it. For reference, when I started, the fuel screw was ~2 turns out.

Here's what the plug looks like afterward.



 

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Discussion Starter · #9 ·
I almost forgot..

The easy way to adjust the fuel screw... I just used a small flathead bit with no driver. I was able to make the adjustment without moving the carb at all with minimal fuss and I just threw the bit in with the stock tools so I can adjust it on the trail. Eventually I'll still buy a thumb screw, but for now this works.
 

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I was always taught to read the center ceramic circle of the spark plug and ignore everything else. If this is true, your inner circle looks bone white to me.
 

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Discussion Starter · #11 ·
From what I've been able to gather, the ceramic stays pretty clean because of all the detergents in modern fuels. I'm sure someone will correct me if I'm wrong. FWIW, there's no change in the color of the ceramic from new.
 

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Hey, brand new here and really only researching possible purchase of kfx/ltz but from my experience, if you turn the fuel screw all the way in on any of the carbs that I do have experience with and it doesn't die...then you need to go up a size in the pilot jet...I would start there no doubt.
 

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I almost forgot..

The easy way to adjust the fuel screw... I just used a small flathead bit with no driver. I was able to make the adjustment without moving the carb at all with minimal fuss and I just threw the bit in with the stock tools so I can adjust it on the trail. Eventually I'll still buy a thumb screw, but for now this works.
That's exactly what I use on the two I have with the stock carbs. I put a small piece of grip tape (like on a skateboard deck) on it and a yellow mark with a paint pen to index it easier.
I also had a piece of grip tape on my thumb throttle....sounds kind of dumb but it actually helps with thumb fatique a bit. I did it for one of the little guys in the group and he liked it so I thought I'd give it a try.

As for the jetting, have you checked the jetting database to search for comparable conditions?
http://www.suzukicentral.com/forums/17-maintenance-repairs-tuning/32548-jetting-database.html
 

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Okay, I just read through the whole "air screw vs fuel screw" thread and am now second guessing everything that I previously posted in that it may be the exact opposite on this particular machine. The bulk of my experience with jetting issues is with highly tuned mini race dirtbikes (my kids race motocross 50cc and 65cc) and all their bikes react the way that I described. It may be that I could be 100% wrong when it comes to these big displacement four strokes. Feel free to blast away at my ignorance!
 

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Discussion Starter · #15 ·
That's exactly what I use on the two I have with the stock carbs. I put a small piece of grip tape (like on a skateboard deck) on it and a yellow mark with a paint pen to index it easier.
I also had a piece of grip tape on my thumb throttle....sounds kind of dumb but it actually helps with thumb fatique a bit. I did it for one of the little guys in the group and he liked it so I thought I'd give it a try.

As for the jetting, have you checked the jetting database to search for comparable conditions?
http://www.suzukicentral.com/forums/17-maintenance-repairs-tuning/32548-jetting-database.html
That grip tape is a great idea. I think I'll give that a shot.

I've read that entire jetting thread and haven't found anybody posting that rides at these elevations with few or no mods. Someday I'll be lucky enough to have a pipe/silencer and airbox mod, :D, but I'll probably pick up another machine next year and kick this one down to the wife so it'll never be as heavily modified as most of the machines in that thread. :)

Okay, I just read through the whole "air screw vs fuel screw" thread and am now second guessing everything that I previously posted in that it may be the exact opposite on this particular machine. The bulk of my experience with jetting issues is with highly tuned mini race dirtbikes (my kids race motocross 50cc and 65cc) and all their bikes react the way that I described. It may be that I could be 100% wrong when it comes to these big displacement four strokes. Feel free to blast away at my ignorance!
No blasting here. I'm completely ignorant when it comes to small displacement and carburetors. I'm happy either way if your experience creates some discussion. :)
 
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