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Thread: XR650R carb mods: posted here.XR650R Carburetor Fix' Go

  1. #1
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    Default XR650R carb mods: posted here.XR650R Carburetor Fix' Go

    From borynack.com/XR650R/xr650r_caruretor.htm
    PICS HERE:
    http://www.xr650rforum.com/share-you...ix-go-t711.htm



    XR650R Carburetor Fix' Go Back To XR650R Page


    Remove the Choke flap with the spring off the choke plate!

    Loosen both screws, then remove one. They have crimps on the end of them to make sure they never fall out so, they are a little hard to turn at the end. Push the flap down out of the way and put the screw you just took out back in with locktite on it to make sure it never comes loose again. Now take the other screw out and remove the flap. Put that screw with locktite back in and you are done. The reason for this that some have had this flap plate break apart and the small pieces go into the engine making a mess. With the carb jetted with a 68s or better yet a 70s the bike starts better for most and the choke still works great (maybe better) full on.


    Rubber Plug (magnified)

    Knowing what the AJCV (Air Jet Cut Off Valve) does might help in knowing what will happen when you disable it. The pilot and needle jets have small holes (emulsion holes) in them that adds air to the fuel that is drawn through them. This atomizes the fuel. Makes the mixer right is a nice way of saying it. This air comes through the air jet....at about 19~20psi of vacuum the diaphragm is pulled back (vacuum comes through that small hole that we are plugging) against the 18+pound spring, in the AJCV and the little plastic door that the diaphragm held open; closes cutting off the needed air. This is designed to soften deceleration...The AJCV makes the idle on deceleration richer, except that there is no extra fuel added, the AJCV does nothing more then turn off the air NEEDED, to make the pilot jet mixture atomized right. There might be more fuel pulled through the pilot, do to the loss of air but, I don't know how much that would be. The un-burnt fuel will pop as it hits hot fuel still burning in the exhaust. When this happens, the rich mixture makes a dead cylinder, less vacuum on the intake stroke and still burning on the compression stroke. Somehow this makes less engine braking compression. . Why you would want to soften the engine braking on a race bike is beyond me. Mikuni, Edelbrock, and the FCR carburetors do not have this...maybe because they were made for race bikes. If you are going to ride the street then this would be good but, you might have gotten a XR650L. As in what you use to plug the hole is up to you RTV works ....I got lucky and found these real small rubber plugs that were to be used for one of my kids toys.....he lost them...they look like little top hats and fit perfect in the hole.......hole plugged no vacuum diaphragm doesn't move bam disabled. Leave the spring in or this won't work. There is fuel that leaks into this small vacuum hole when the bike is laid over (crash/bike falls over) so, whatever you use has to be fuel proof. You should readjust the idle mixture screw once you do this modification.
    There are a lot of carb's that have the mixture screw being an air screw but, this is mostly on the small 4 strokes and two strokes. If the idle screw is in the front it's a fuel mixture screw like the XR650R is as you know, an Idle fuel mixture screw. If it is in the back it's an air mixture screw. The big thing that is needed to make the AJCV disable trick work is to jet richer pilot (because you can) and adjust the idle mixture screw to work right. Turns out from full in....
    2~2.5 for the 68s, 1~1.5 for the 70 and .5 to almost 1 turn for the 70s. depending on the bike.The other reason is to load the carburetor up for smoother idle while coming to a sudden stop and still being able to run the bike lean to pass AQME standards (smog). The bike does not pass smog if you run it rich enough to run right. Having this AJCV lets Honda run the pilot lean but, not have the bike stall coming to a stop as often.
    The big confusion comes from some of the first guys on some groups, getting my modification backwards and cutting off the tit to the diaphragm, cutting the air from the air jet off all the time making the bike run bad. There would be no air going to the pilot emulsion holes at anytime. Some did this and had to buy new diaphragms to fix it and are sour on any talk about the AJCV. Make sure it will resist fuel, fuel will get in that hole if the bike is stalled and on it's side. The diaphragm when pulled out by the vacuum, closes a plastic block cutting off the air from the air jet. AJCV is not needed or wanted for most racing. Having the air to fuel mixture right at all times is the goal for most.
    http://www.cyclewareables.com/pages/.../aircutoff.htm
    http://www.cbxman.com/detail.aspx?ID=598
    http://www.hawkworks.net/manual/4.html


    Setting the Float on the stock Carburetor:
    Float level should be 16mm from gasket flange to bottom of float
    with carb inverted. I ran mine almost 17mm to help it stop peeing on it's kick stand on a hill.
    Jetting
    The difference between pilot jet with and without the "S" after the number

    Starting a flooded bike.

    There are two drills to starting a downed pig But, First if you crash...lay it down, fall over (that last one is all the time for me). Make sure you don't have GAS coming out of the air box. This is where most of the argument will come form.

    Most will tell you to hold the decompression lever in, while holding the kill button and kicking the bike through 3~5 times, up to ten times to clear out the fuel. Then at times if it was a real good spill there is so much fuel in the cylinder this can still not work and you start all over again.

    The Race way:

    You pick yourself up off the ground in a daze wondering where the bike is. Find the bike and pick it up. Fight the pain and hope you are pointing the right way. You pull the throttle wide open and grab the front brake! This is important....it stops the throttle from moving while you kick the bike over. Find TDC and kick as hard as you can and go! I have had a few argue with this but, once it is done without the throttle moving it works!

    The main thing to know is the bike has way to much fuel in the cylinder to start so, you need to add lot's of air to get the fuel to fire. If you kick the bike over slowly to clear the engine and the spark lights the fuel the bike is on fire! Starting the bike the Race way pulls everything into the cylinder not back firing into the air box.

    Some of my good friends that I ride with didn't believe it and it did not work for them. But, once they got the front brake thing down it was almost every time one kick...two at the most....

    Hold the throttle wide open, holding the front break.....find Top Dead Center and kick!





    Once you use a pumper the stock carburetor just isn't any fun! If you don't mind the trouble of the initial setup (lot's of jets) the Mikuni was two years trouble free for me and it's a better (smoother) carburetor. I have owned and ran all three. I made the stock carburetor work....it can work but, it is nothing like a pumper...If you haven't ridden a BRP balls to the wall, drop offs, popping up and over walls, just looking for the edge, then you might never know the difference. The Edelbrocks biggest fame is how easy it is to tune and its ability to get the Oinker started and keep it running, no matter what! I have my Edelbrock pored out to be a 40mm X 42mm and it rips on my bike. The Mikuni TM40 can be tuned to perfection but, that takes major time and effort but, it is worth it! It is power on everywhere all the time, never lagging always spot on through out the throttle movement. But, a pumper is not needed for the casual weekend warrior or fire road rider but, it still is nice even there!.
    Michelangelo
    noob



    Joined: 2009-10-04
    --------------------------------------------------------------------------------
    Posts: 12
    Location: Utah
    XR650R Year: 2001
    Last edited by Michelangelo; 10-18-2009 at 06:27 PM.
    2001 Yamaha TTR-150L
    2005 CRF450X
    1998 Buell White Lightning

  2. #2
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    I've not dived into my new bike yet to see if this has been done. Be curios to hear from anyone who's done any of this.
    2001 Yamaha TTR-150L
    2005 CRF450X
    1998 Buell White Lightning

  3. #3
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    Keihin FCR 43

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