1999 640, ids2 quiet core 10 discs, stock airbox cover, no snorkel but airbox top hole maybe 1" wider. I'm running: 4.5 clip 162.5 main 47.5 pilot I feel the main is one or two sizes low at this time. Other than that it runs pretty good. I can still hear it from 100ft away but it is a revving kind of sound and not a popping. One of the problems with more open exhausts is that it amplifies combustion defects and often that makes a bike sound much louder than normal. Seems if set the mixture solid at the first rich yellow (jet4power) I don't have any popping or uneven sounds, and power is nice and smooth. It's really crisp at off-idle, falls a little flat as my mixture drops off a bit higher in the throttle and solid yellow turns to green/yellow flicker. Needs more fuel there, probably a main or two. If that doesn't hit that area then I"ll need either another needle or I will have to tune the spring for a more appropriate slide position relative to that particular vaccuum.
I experimented a lot with the BST bought a jetting kit from Schneiders with the next larger emulsion tube/needle jet. That really worked well ! I drilled up the slide for quicker lift and the bike lifted the front in 2nd just by the throttle. I then shortened the spring but I did not like that at all.
I have a feeling that with the BST it is going to be a YMMV situation with the older bikes. My needle was toast, and my needle jet is also worn, as well as the slide etc, so it puts it all out of whack. I imagine others are in the same situation. As you mentioned, it could be a good idea to get another needle. Will have to look them up again to see what is available. The sudco ones mentioned on this site are a no-go. It could be that reducing the spring just gives it too much air, and that it would be better to stick with the stock or a stronger spring and raise the needle.
I used a new stock needle and the next step richer needle jet ! Stock spring and drilled slide Gutted the airbox following Schneiders recommendations. With that specific kit they recommend almost removing the top of the airbox + screen on the side. Their recipe is proven and tested and I can vouch for it
I have old head, not hi-flow. Schneiders Cycles is in Germany but are on the web (do a Google search), on e-mail info@schneidercycles.de, and their English is good. They know their stuff on tuning KTMs. The kit I got, and that really works is; | 640BST40Race | Vergaseroptimierungskit RACE | 79,00 EUR | Delivery Charge: 30,00 EUR Total: 109,00 EUR
Just did some clean up today... Nice freshly oiled air filter and 4k less altitude plus a "fixed" racing side cover and I think I should be revisiting my jetting I might even wash it!
Finally back to riding the LC4. Yesterday I did many test runs to try out clip positions. 1999 640, ids2 quiet core 10 discs, stock airbox cover, no snorkel but airbox top hole maybe 1" wider. I'm running: 4.0 clip 165 main 47.5 pilot The low end is perfect. Very crisp. On the jet4power this is a solid first yellow light. In the higher throttle positions it is borderline rich moving to the second yellow. The second yellow is where my motor begins to misfire a bit, so I've tapered down from 4.5 clip to 4.3 and now to 4.0 and the misfire is reduced to almost nothing. I think perhaps 3.9 clip would be more in the zone at higher throttle positions, or perhaps one less main. Not sure. I was going to try that but I friggen dropped & lost my 162.5 main when I was installing it while the pipe was hot. I must also say that some of my prior testing was evidently not accurate. I was using only the nylon washer to affect 1/2 positions but I found that it doesn't hold, and that you must use a shim. So I have been using some #4 washers I have, some 0.5mm thin and some 0.8mm. I enjoy the motor a lot now. The off-idle 1/4 throttle is crisp and responsive. It gets a little rumbly at around 6krpm, but I'm pretty sure that is just because I have the higher throttle position just a tad rich.
Tater- I've been following your Jet 4 Power ramblings for awhile now, and finally pulled the plug and ordered one up. I got her all hooked up and noticed the '03 640A with the high flow head, stock BST, 4th clip/165, desnorkled, desmogged, racing side cover, with an SXC Can was running was entirely too rich. It's pretty darned cool to be able to make more than one change on the carb, and take her for a spin to actually see how the changes effect the jetting. I've had the J4P installed now for about 3 weeks and have made changes then will go out and run a few hundred miles to get a good overall impression on the jetting setup, record my findings, then I'll make another change and record once again. Well, I'm shocked that I've settled on the 3rd clip, 157.5mj still desnorkled/racing side cover seems to be where she's happiest. I'm still tinkering with Loaded's fuel screw though. She's still just a bit fat in the meat of the crusing rpm's and since this is where I spend most of the time, I want the highest mpg's here and a solid middle green light. This area of the rev range is controlled by the needle position as well as Loaded's fuel screw. I can't go to a lower needle clip height as she gets hot too fast while sitting in traffic, so I'm leaving her at the #3 clip from the top. Am still turning in on the fuel screw. Hopefully I'll get that mid-green light at gravel road cruising speed of about 3,5k to 4k rpm's. Will report the number of turns out once I'm happy with the tuning. Here in Louisiana my elevation is a whopping 16 feet with humidity around 70% with temps already at 80 to 90's. The jetting is finally spot-on. On hard acceleration, she'll dip into the second rich yellow, moderate accelleration first rich yellow, steady cruising at about 70 indicated, she's mostly solid rich green with an occasional first rich yellow. 50 to 60 cruise at about 4k rpm's, she's mid green to rich green. At about 80 indicated, she's mid to rich green. But the cool thing is that no matter what speed I'm cruising at, if I give her a little or a lot of nudge with the wrist, she'll dip into the first rich yellow-which is where the "Max Power" is found according to J4P and will get up and scat. Idle, once fully warmed up is in the middle green. The milage is now knocking on the 50mpg door. For the last few partial fillups I've been getting pretty consistant 48 to 49mpg's. Which is a far cry from the upper 30's to low 40's before the J4P. I still can't believe I've settled on a main as small as it is while some of the BST cult is running much richer than me with the same airbox mods, at higher elevations. Now, I'd like to install this Jet 4 Power on my YZ 450 to get her dialed in but I'll have to find someone to weld the O2 sensor bung on that Titanium head pipe. Dirty
Yeah it's kinda weird about the BST. I was expecting lean dips, but it actually gives a rich surge when you wack the throttle. My motor doesn't handle the greens very well when it is under stress. Something about the G1 cam makes it a bit sensitive to leaner mixtures. It starts falling all over itself at about mid-green. When I have it too fat at higher throttle it starts a bit of misfiring. I've not entirely evaluated the main jet on it. I was going to start moving down from 165 but I lost my 162.5 so I had to order one. When I went from clip #3 to clip #4 I lost about 9mpg, went from 46mpg to 37mpg. It runs a lot stronger, but it is fat above 1/2. When you get the right mixture, it seems to achieve higher rpm at lower throttle. So I'm not all that concerned with moving leaner, I just want to get it to peak efficiency and see what economy is there. I run the idle on up at the first yellow rich. I couldn't get it there without at least clip #3.5 with a fresh needle. My original needle was reading about 2 clips richer than a new one! No wonder it ran like crap.
I did some taping up of my exhaust joints and it appears that the silencer flange is the culprit in my strange air-fuel readings. Evidently air is getting rammed into the flange bell and it kinda messes with things a bit. I know it messes up my meter readings, but I could also feel a matching motor reaction. Anyways, I have a new way to seal that flange. Not sure about the longevity, but it should be ok providing the temperature doesn't cause the silicone to crack too badly. I use this method on my two stroke cylinder header and it seals forever. I was kinda cheap and only used a couple of wraps. But it hasn't gotten crunchy yet. I expect it will to some extent, but I figure a few more wraps and it should be good indefinitely. This is high-temp silicone tape called stretch & seal (Nashua brand). It's at home depot in the plumbing dept. Wrap & then safety wire.
Update. I've been riding the LC4 for the past few weeks. I got sick of swapping the O2 meter back and forth so I bought one just for the bike. Here are my current settings: Sea-level, 1999 640, ids2 quiet core 8 discs, racing airbox cover, no snorkel but airbox top hole maybe 1" wider. I'm running: 4.5 clip 165 main 47.5 pilot One surprising difference is the effect of the racing airbox cover on the BST. It leaned out the mixture a lot. I think not so much because of the increase in air flow, but because as mentioned in some other threads, this airbox change results in a bit less slide movement with the BST. I have gone up to 4.5 clip from 4.0 to compensate a bit, but it is still running leaner than before, and not quite linear. I think it will require a weaker slide spring (or one or two clips clipped) to get the slide to track linearly again with the racing airbox cover. I will probably try that before I increase the pilot. I have the fuel screw turned a little more than one turn more than before the airbox cover.
I love reading threads like these. Tuning for optimum performance and efficiency can be bloody finicky on these lc4's, especially if she's done some mileage. Lotsa vibrations, combined with the original carby (worn slide, needle and seat etc), loose fitting or split intake/exhaust boots and those bloody header pipes can give erratic and inconsistent results. Still, I love fiddling with my beast to tweak a little more 'gusto' out of her. Not anywhere near as precise as what has been described in this thread but it does give some very definitive logic in areas I know I have overlooked! Good job potato and others! Just for an interesting comparison, this is what I am running: '05 640 Adv. FCR 41 mx Carb for Sea level up to 3500ft. @ 75%+ humidity. 170 main 45 pilot Needle (JD kit 007) Red on 5th from top Fuel mixture 1 and 3/4 turns out Still fidling with the pressure and duration of the pump, have a thin oring currently installed Full Akro headers and muffler (with insert) Removed snorkel Stock 16/42 gearing I've always wondered about installing the jet4power system and have been meaning to take my set-up through a buddy of mine who dynos race bikes and has some pretty trick anylizers, giving detailed graphic readouts (incl. fuel ratios) at varying loads/revs. After quite a bit of tinkering and adjusting, I keep going back to the above settings as they seem to give the best overall throttle response at all rev ranges and I can obtain 41 mpg when riding conservative and around 36 mpg when the right hand gets in the groove. I believe there is still a lot of room for better mileage without a 'noticeable' drop in performance but having a system like the jet4power and similar, this would definitely help to achieve this. Intereting to note (although I haven't measured actual temperatures), it looks as if the collector pipe (where the two headers form into one) runs hotter than either of the engine pipes. Could be wrong but it is 'blued' where the individual headers have maintained their nice titanium 'silver' colour. Dunno?!
IMO the realtime readout is more useful than knowing the exact fuel ratios. You can see air leaks, electrical misfires, fuel starvation. It would be interesting to hook back up a pumper to see how the pump behaves, but so far I've been satisfied with this BST40 once all the other issues have been normalized. That silencer seal shown above has worked great! Plus I put in some new coil wire. The four stroke is so tolerant that it is tough to tell incorrect mixture by seat of the pants. I started the day and the meter was almost off the scale lean, and I ended the day and it was about in the middle of the scale after a 1/2 clip change and another turn out on the fuel screw. I really coulnd't tell one way or the other at flat rpms, except that once the mixture was corrected the motor has more acceleration. I think about my past rides where the bike wasn't running well, and I wish I had a system like this on it. It certainly takes away a lot of the guesswork, but as trond says, you have to find the sweet spot of your motor and then try to jet for that light. Though I suspect that with most four strokes, the rich green or yellow is where to aim. One thing I enjoy is setting the proper idle mixture. That's usually a ballpark setting since the motor sounds exactly the same though a wide change in mixture, but here I can always return to the mixture that I want just by watching the light and turning the fuel screw. Doesn't work out so hot with the two stroke, but with the four stroke it is easy.
Dirty, this gave me some food for thought. i have been slowly going leaner on my 640 adventure. Look like i am going in the right direction.
Being a 3 year dead thread, you are unlikely to revive it. That being said, most 640 owners tend to go way overboard with "if a little bigger jet is good, just think how great a 180 will be!". I think you're finding out that's not the case. Maybe if you offered up where you started and where you think you'll end up, jet wise, a discussion may ensue...
