1980 Yamaha SR250G-R - Just wont idle - cleaned carb twice.

Discussion in 'Motorbike Technical Discussion' started by mmaunder, May 4, 2005.

  1. mmaunder

    mmaunder Guest

    I have this beautiful SR250G-R yamaha that was sitting in a garage for
    years. I don't have any mechanical experience with bikes - only
    outboard boat engines. I'm busy trying to get it running again. Dumped
    carb cleaner in the gas tank and got it running, but it just wont idle.

    I have opened up the carb twice today, cleaned all the jets, sprayed
    carb cleaner through all jets and made sure it was spotless, got rid of
    any signs of rust etc. And every time I put it back together, the bike
    performs exactly the same. Starts, idles fast, push the choke in and it
    just dies. I can open the throttle wide and push the choke in and it
    runs, but very roughly, especially when I start closing the throttle
    again and it dies at low throttle.

    Any tips would be very very much appreciated.

    mmaunder, May 4, 2005
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  2. Your idle jets and the idle mixture ports are probably really clogged
    up with gum and varnish. Crud in the carburetors is about 90% of all
    problems with modern motorbikes. I've explained how to *thoroughly*
    clean out carburetors at least a hundred times in this NG. Google up
    "[email protected] +idle jets" to read umpteen posts about the subject...

    Be sure the vacuum slide is free to move too, spray some carb cleaner
    on it to dissolve any gum or varnish keeping it from moving...
    krusty kritter, May 4, 2005
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  3. mmaunder

    mmaunder Guest

    By Vacuum slide do you mean the diaphram assembly in the center of the
    carb that slides up and down according to the level of vacuum in the
    carb? Here's a diagram of my carb - Are you referring to number 11?

    mmaunder, May 4, 2005
  4. Yes, number 11 is the vacuum slide which has the rubber diaphragm
    attached. You want that vacuum slide to move freely so it doesn't tear
    the rubber diaphragm. Honda will charge you a lot of money for a new

    Number 12 is the pilot air jet. It probably won't get plugged up, I'm
    just pointing it out for reference. Almost straight down on that
    drawing you can see a tubular section of casting in the carburetor
    body. If you look down on it, you will probably see that it looks like
    there's a plug in that hole.

    Since the drawing doesn't show the idle mixture screw and its
    accompanying spring, washer and rubber o-ring, it's probably right
    there under that plug. The plug is supposed to keep owners from
    "tampering" with the idle mixture. Shop mechanics will refuse to touch
    it, saying they will get fined for messing with it.

    But, many knowledgeable tuners have been drilling that plug out for the
    last 25 years so they can get to the idle mixture screw, clean that
    hole and adjust the idle fuel air mixture.

    First, a small pilot hole is drilled in the plug. Then a small sheet
    metal screw is screwed into the plug. Pulling on the screw with a pair
    of pliers removes the plug.

    Then the tuner can slowly *tighten* that idle mixture screw, counting
    the number of turns until the idle screw just bottoms out. The idle
    mixture screw can then be removed, taking care to save the spring,
    washer and tiny rubber o-ring if there is one in the hole.

    Then the tuner can squirt aerosol carburetor cleaner down the hole and
    the spray will come out the pilot air jet, the idle jet, and the idle
    by pass holes down stream of the throttle butterfly.

    If you squirt through the idle mixture screw hole with your finger over
    the three idle mixture discharge ports downstream of the throttle
    butterfly, the aerosol cleaner will have to go out through the pilot
    air jet and out through the idle set, cleaning out the tiny idle
    mixture passages.

    The spring, washer and o-ring are reinstalled on the idle mixture screw
    and it is threaded back into the hole until it bottoms lightly. Then
    the tuner screws the idle mixture screw back out the exact same number
    of turns he screwed it in when he was counting turns. So the idle
    mixture setting is back to what it was from the factory.

    When you start the engine up, warm it up and then open the throttle
    rapidly and close it rapidly. If the idle RPM settles back down to the
    normal speed, you're done with that screw. But, if the RPM stays high
    for several seconds after you close the throttle, the idle fuel air
    mixture is too rich or the idle speed is too high. Turn the idle speed
    screw down first, then try the mixture strength test again. If the idle
    speed still hangs up too high, turn the screw 1/8th of a turn clockwise
    and repeat the idle mixture test, adjusting the idle speed screw and
    idle mixture screw until you get the fastest smooth idle for the smount
    of throttle opening as determined by the idle speed screw.

    Good luck on cleaning and adjusting your carburetor.
    krusty kritter, May 4, 2005
  5. mmaunder

    Tostada Guest

    Make sure you check and replace the fuel filter on the petcock too.
    Tostada, May 5, 2005
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