Which carb is best on LC4 640

Discussion in 'Thumpers' started by losiu, Sep 19, 2006.

  1. qwertylolman

    qwertylolman n00b

    Joined:
    Jul 31, 2017
    Oddometer:
    4
    Location:
    Kiev, Ukraine
    Hi, mates

    I need an advice. I have 640adv 2007. And i have some problems with bst. So i was thinking, maybe i should switch to fcr by chance? I'm not sure if i want to change an exhaust, cause stock is pretty noisy, and i don't want a noisy exhaust. So, maybe there is no need to make sidecovers for airbox? But if i will not change exhaust and leave airbox as is, is there any benefits from fcr? Only thing i would like to improve is to make better response on throttle. Should i setup fcr or rebuild bst?
  2. Tripndrag

    Tripndrag Tripndrag

    Joined:
    Oct 28, 2007
    Oddometer:
    89
    2003 625 SXC with the BST here. Follow the BST Bible and you will be happy with your BST. The top airbox cover was gone from my bike, I followed the BST Bible and I could not believe the difference! (The only thing I didn't do was cut the spring in the instructions. You really need to purchase the fuel adjustment screw as well if you can find one, to fine tune unless you want to take the carb on and off.)

    I sold my XR650 with the power up kit to get this SXC because I wanted/needed a street legal dirt bike. I had purchased a New Old Stock 2006 KTM 950 Adventure to replace my street bike and was ready to make the switch on the dirt bike side. I almost didn't purchase the 03 SXC when I test rode it because it was not impressive at all. They are hard to find where I live, it was just so cheap as well I just couldn't pass it up and reading other posts I was kind of going on blind faith in what people were saying about them being gutless and not impressive in stock form. It really felt like a 250!

    I left everything stock and just followed the BST Bible and couldn't be happier. There is a pretty big difference on an XR650 from stock to the power up kit but this KTM was the largest difference I have ever felt from stock to the modifications on the carb only lined out in the BST Bible. I have had this bike for around 8 years and I love it. It took me 3 tries to get the gearing right, bar risers, seat concepts seat and this bike is awesome for me. The gearing and the seat really brought things together, got rid of all the vibrations and buzzing at the speeds I ride. I was really hard to ride in the mountains with the stock gearing and I really didn't need the 107MPH top speed. I ride the woods, sand dunes, roads, wheat fields, town, everywhere. From sea level to 10k feet the thing has run flawlessly. You can put a giant rear tire on these bikes as well. Getting that gearing to where I could go 55-65 at an RPM that didn't buzz and rattle you off the bike when you were on the road and still be smooth at the trail riding RPM's was really hard. The first gear is really low on this bike so trying to get the gearing where I didn't ever have to use 1st gear, work on the road, town and trail was a chore. This is not the same for everyone. You do not want to be trying to go between 1st and 2nd on this bike. The stock foot pegs were like bolts, so uncomfortable. The stock seat was also so uncomfortable. I had to 1 bun that thing, it was rounded, super tall and just didn't feel good to sit on. These bikes are tall in the stock form and the seat height makes them even taller. The Seat concepts seat is a few inches shorter, flat and super comfortable! It also looks great. You have to send them your seat pan but it is really affordable and totally worth it.

    Mine came with the giant black 5 gallon tank but I went back to the stock tank. I have done quite a few 150 mile mountain rides and haven't run out of fuel. The first few times I carried a small 1 gallon reserve tank on the back tiny luggage rack but I ended up never needing it. I was really scared one time getting to end of a long loop finding downed trees everywhere. Going over and under 6 or 7 I finally walked ahead and found it only got worse so I had to turn around and do the ride all the way back out and then get home on the hi-way portion I took to get there. I think that ended up being close to 170 miles.

    I owned a previous XR 600 that had the bogging issue I read about earlier in this post. In the sand whoops and big top speed g-outs in the wheat fields is where it would happen and I fought trying to fix that issue for a long time, jetting, pipe tuning, float heights, carb adjustments, very frustrating. This was back in the days where you couldn't find the answer on the internet. My other friends said their bikes didn't do that but when we traded bikes their bikes did it as well when I rode them. I ended up installing a PC Racing carb vent kit and that fixed the issue! I have never experienced this issue on the KTM, the thing runs perfect all the time.

    I would love to ride a 625 with the pumper carb but I don't have any issues with my bike and the way it is setup now. I don't think it could get better.

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  3. cbig

    cbig Rift- Raft, SCooter Trash

    Joined:
    Sep 30, 2003
    Oddometer:
    1,612
    Location:
    Whittier, CA
    My sxc has the fcr on it. Runs fine, better at some altitudes. Minor spark knock if opening up fast I haven't fully dialed out, very seldom hear it, tried raising needle didn't help.

    Bike runs fine, got it that way with full titanium akro exhaust, do dad. 901 miles, 2800 bux 4 years ago. It's at 14k almost and am thinking of tearing it down to check piston, I understand from some posts the design isn't that great and an aftermarket wossner does the job. Will look into carb after that. I'll handle the rocker arm bearings and water pump then just 2 be sure.

    Pretty damn versatile bike. Yesterday pounded pavement from Borrego up to Palomar. Bike gave good account of itself for the locals, then came back via dirt. Put passenger pegs on for my boy or wife on occasion when motorhome camping. It's as much fun as my Harley ever was on the twisties, vibrates like one too!

    Carb is getting 49 to 51 mpg. 20160102_102708.jpg
  4. meat popsicle

    meat popsicle Ignostic

    Joined:
    Feb 9, 2004
    Oddometer:
    14,566
    Location:
    Circumlocution Office of Little Dorrit
    There are threads in the LC4 index on the switch between the constant velocity BST (stock) and pumper FCR carbs. As I recall you will have to open up the airbox for the FCR because it needs to pull more air. You should open up the airbox a bit for the BST but don't go crazy because its a "tuned" airbox and the BST wants laminar(?) airflow. FCR doesn't really care as I recall.

    I'd say you should make sure you have the BST working properly before deciding if you need more throttle response. The downsides of the FCR are reported to be less fuel economy (but probably not much if setup properly) and more sensitivity to altitude changes (BST stays functional across a wide range of altitudes where the FCR needs jet changes).

    Oh yes, getting the FCR to fit can be a bit of work. Certain models of the FCR reportedly fit easier than others. Aftermarket exhaust header was noted as problematic in some cases.
  5. gunnerbuck

    gunnerbuck Island Hopper

    Joined:
    Nov 19, 2005
    Oddometer:
    6,633
    Location:
    N.V.I, B.C.
    14 K isn't much to warrant changing the piston, I bet your rings are still in spec too... I put 175,000 KM {100,000} miles on the OEM piston and the piston is still serviceable, it just wore out the rings a couple times... I put that piston on the shelf, installed a new wossner piston along with a fresh nikelsil cylinder and gave it a full valve job... Since installing that Wossner piston the bike is burning a little bit of oil between changes which has never happened before until the top end had many miles on it.. I tore it down and installed new rings along with new valve seals to see if it would improve things, but it still was using oil... Next time I tear it down I will either put the oem piston back in or go for a new Pro-X piston of which I have been running in my spare bike for 25K miles without issue..
  6. Hahanoulis

    Hahanoulis Adventurer

    Joined:
    Oct 20, 2018
    Oddometer:
    17
    Location:
    Athens