I tried to be member at Adv-rider but some moderator has to approve me and I don´t have time to wait, I am to frustrated Well it is not a Husqvarna but I there is much knowledge here so I will give it a try… I have a KTM 640 Adventure from 2003 whit the original BST carburetor and a modified SXC 625 silencer and I have opened the side of the airbox and all the emissions crap is gone. It stutters at 3500-4000rpm (1/8-1/4 gas) and I don’t know what to do. It feels like I have tried everything. Current jetting is: Mainjet 160, pilot jet 47.5, lowered the needle clip one point downwards. Air/fuel screw 2.5 to 3.15 turns out and no difference. The temperature is about 7 Celsius and I am at sea level. I have searched the index tread at adv-rider and followed the instructions there but it the stuttering is still there. So what to do? One more question: If the silencer is to “open” could it affect the exhaust pulses and make the bike stutter at low speed? That’s the only thing I can think of right now…
Your carb is a "CV Carb" right? those are not exactly for performance when compared to a Keihin FCR (more tunable/ more proven/ more knowledge about). CV carbs "sometimes" can not be tuned perfectly partially due to a lack of Accertator Pump- though they are more stable for altitude and temps. Defining what you mena buy "stutter' may be helpful- are you riding it at the time? are you twisting the throttle (like to wheelie)? Thoughts: verify no vacume leakes, verify pilot circuit.
When I am cursing along and use even throttle the bikes dies some times for a split second. It feels like there is a shortage of fuel. It works great when I accelerate and uses more than 1/4 of gas. No vacuum leaks and no air is getting in at the wrong place as I can tell. Verify pilot circuit Yes I guess there is a problem at the low speed circuit, but I can´t figure out exactly what´s wrong and it is driving me crazy.
You raised the needle one clip and it had no effect? (raising the clip/lowers the needle(leaner), or lowering the clip/raises the needle(richer) You opened the fuel screw from 2.5 to 3.15 and it also did not have an effect? So you went richer on both and it had no effect on what seems like a lean condition… you sure its not just the chain kicking back?(just a thought) If the bike didn't have these issues and sat- you may just want to open up the carb and clean all the jets and cicuits and verify the float spec. To verify your Pilot jet and tune the fuel screw: Eddie Sisneros has suggested this procedure for years: seems like the most common jetting issue that comes up are pilot circuit related.the following is a sure method to choose the needed changes.with the bike warm and idleing turn the fuel screw in till the idle drops/misses.then go back out till the idle peaks/smooths.this should happen between 1 and 2.5 turns on a fcr carb and 1 and 3 turns on a cv.if you end up at less than 1 turn you need a smaller pilot jet.more than 2.5 (or 3 turns on a cv) you need a bigger pilot jet.choose the appropriate size and retest.If you don't get anywhere with my suggestions and no one else offers more- my buddy AL who owns AdvancedLandspeed (small shop in WI) has had 640 Adventure forever- he may have some tips- Email him and tell him where you are from and your issues- tell him Brad from the Wisconsin Dual Sport Riders thought he may have a tip or 2.... I'll PM his email... Good Luck
I don't have any more patience at the moment so I change the silencer back to the original and re-jetted the carburetor to its factory settings. Now the bike works fine again Maybe I will give it a try later one...
