Older XT250 Won't Run W/O Some Choke

Discussion in 'Old's Cool' started by stevemd, Dec 11, 2012.

  1. stevemd

    stevemd Adventurer

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    I have a 1980 model with a professionially rebuilt carb per the previous owner. Bought it with the running issue described in thread title. I pulled the carb, checked the rubber/metal carb to head spigot for possible cracking/leaks, checked the main and pilot jets for obstruction, and reinstalled with a fresh gasket between the spigot and head.

    It will start easily with choke and as it warms, the choke can be decreased to about 75% off but no more, it will die. Trying to rev it while pushing the choke to completely off doesn't work, it will die, even when warm. I take this to mean it is getting too much air. Should I experiment with the air pilot adjustment? It is located downstream of the slide. It is currently 2 turns out from all the way in.

    I confirmed it has the correct main jet but I didn't check the needle position. The accelerator pump works. I don't know if the float level is correct and it might be the last thing to check anyway.

    Any advice?
    #1
  2. DR-Zamboni

    DR-Zamboni Been here awhile

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    too much air could also means not enough fuel: Its a ratio kind of thing. So before going further think about where fuel could be blocked, rather than how too much air could get in.

    So since you cleaned everything, and checked for blockages, I would start playing with that pilot. bring it out a half turn, most people do that anyway during the winter months. See if you get any change at all. If you do then you know somewhere to start.

    Throw a new plug in there too... never know if it maybe is fouled from running lean or just plain is old and dirty.
    #2
  3. stevemd

    stevemd Adventurer

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    Thanks, that was in the back of my mind, not enough fuel. The float could be misadjusted, shutting off fuel to the bowl way too soon. Plug looked good, correct gap. Starts easily with choke on but I have heard other stories of running issues unexpectedly resolved via new plug. Will play with the pilot tomorrow.
    #3
  4. ttpete

    ttpete Rectum Non Bustibus

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    That's typical of a plugged up pilot jet. Maybe the "professional" who rebuilt the carb missed it.
    #4
  5. troidus

    troidus Long timer

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    And it could be that the jet is clear, but there is crap in the transition slot/port(s).

    There was mention of an idle air screw. Is there also an idle mixture (fuel) screw that's maybe not open far enough? Also, will it take throttle with the choke/enrichener engaged, or does it always die when you give it gas, regardless of choke setting?

    The first thing I would check is the float level, because that changes the behavior of every other fuel circuit in the carb.
    #5
  6. ttpete

    ttpete Rectum Non Bustibus

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    Float level doesn't really affect the idle circuit as much because the fuel comes from much lower in the bowl. The fact that it won't idle without fuel from the enrichener tells me that it's not getting much if any from the idle circuit.
    #6
  7. stevemd

    stevemd Adventurer

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    I can rev it with the choke on or 50% on. There is no idle mixture screw although I guess the air pilot contributes to idle mixture. There was a small amount of crap in the bowl. I will try different pilot adjustments and if that doesn't help, out comes the carb again.
    #7
  8. manic mechanic

    manic mechanic Been here awhile

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    I'd be willing to bet that you do have an idle mixture screw, but it's hidden under an aluminum plug/cover, courtesy of the EPA. I usually drill through the plug, use a sheet-metal screw to remove it. Then determine the position of the mixture screw, remove it, blow solvent then air through the passage. Replace the screw with the o-ring, backing washer, spring. Remember to also clean off the screw tip.
    #8
  9. alexem

    alexem Adventurer

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    <!--[if gte mso 9]><xml> <w:WordDocument> <w:View>Normal</w:View> <w:Zoom>0</w:Zoom> <w:HyphenationZone>21</w:HyphenationZone> <w:punctuationKerning/> <w:ValidateAgainstSchemas/> <w:SaveIfXMLInvalid>false</w:SaveIfXMLInvalid> <w:IgnoreMixedContent>false</w:IgnoreMixedContent> <w:AlwaysShowPlaceholderText>false</w:AlwaysShowPlaceholderText> <w:Compatibility> <w:BreakWrappedTables/> <w:SnapToGridInCell/> <w:WrapTextWithPunct/> <w:UseAsianBreakRules/> <w:DontGrowAutofit/> </w:Compatibility> <w:BrowserLevel>MicrosoftInternetExplorer4</w:BrowserLevel> </w:WordDocument> </xml><![endif][if gte mso 9]><xml> <w:LatentStyles DefLockedState="false" LatentStyleCount="156"> </w:LatentStyles> </xml><![endif][if gte mso 10]> <style> /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Normale Tabelle"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman"; mso-ansi-language:#0400; mso-fareast-language:#0400; mso-bidi-language:#0400;} </style> <![endif]--> Not really solving yr prob:

    I did a ride together with a xt250-driver, when he switched of the engine he he gave fullthrottle.
    After that the motor always started easy…….
    #9
  10. MCCOYBOY

    MCCOYBOY Adventurer

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    If that doesn't work out for you let me know. I can put it in my ultrasonic cleaner and if there is any plugged passages it will clean them up.
    #10
  11. tommymerle

    tommymerle advwanabee

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    how old is your fuel?
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  12. tommymerle

    tommymerle advwanabee

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    also, loosen the gas cap to see if vent isn't working anymore. No vent=slow fuel delivery.
    #12
  13. stevemd

    stevemd Adventurer

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    Thanks!
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  14. stevemd

    stevemd Adventurer

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    Fuel is fresh. Thanks for tip re. tank cap.
    #14
  15. Vankaye

    Vankaye n00b on the move

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    #15
  16. stevemd

    stevemd Adventurer

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    Thanks Van. Tried some things after work yesterday: pilot out 3 turns vs 2. Gas cap loosened. Air filter pulled partially out to let some unfiltered air in (pool of filter oil in bottom of air box). No change worth noting, cannot back choke off more than %75 although as before starts easily hot or cold with choke full on.

    Next step is more pilot adjustment in/out and if that doesn't do it, out comes the carb.
    #16
  17. MCCOYBOY

    MCCOYBOY Adventurer

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    You have a blocked pilot jet. Pull the carb and either clean or replace the pilot jet. No response in pilot screw means no fuel flow through the pilot jet.
    #17
  18. troidus

    troidus Long timer

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    The jet may be clear, but the passageway clogged. Try blasting through the pilot jet with aerosol carb cleaner to see if you can get a good mist from the idle port and the transition port(s). Maybe alternate with compressed air from a blow-off nozzle, using a rubber tip for a good seal against the jet well. If blasting with cleaner doesn't do it, it'll be time for the ultrasonic bath. Pull off all your rubber parts before dunking it.
    #18
  19. stevemd

    stevemd Adventurer

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    Update on this plus Season's Greetings:

    Cleaned carb back in and looks like the pilot was clogged as will now run w/o choke after about 30 secs. Noticed that when I let it idle, rpms would rise for no apparent reason then come down a bit. Will try adjusting pilot screw.

    Need to figure out if the multi-meter I have will read 1 cylinder rpms and where to take the readings off. Coil?

    The Yamaha shop manual says to check the float level with the carb on the bike, bike upright, clear hose from the carb drain looped up alongside carb, measure how far fuel level in tube is from bowl/carb gasket. What a PITA.

    Noticed there is a metal screen between the oiled foam air filter element and the carb. I would like to take it out to clean it but looks like the air box has to come out and I'm guessing the mono-shock has to be pulled to do that. Oh, well, a man's gotta do what a man's gotta do.:1drink

    Thanks for advice to date and beyond. :wave
    #19
  20. someonesdaddy

    someonesdaddy so be nice!

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    realize this post is over a month old, so i don't know what you figured out on this, but i picked up an 81 XT250 last weekend that had been not loved and not ridden for a good many years. carb was real cruddy and all honeyed up pretty good. pulled it apart and soaked it in carb dip on wednesday. pulled it from the dip yesterday, rinsed it in warm water and blew all the passages clean. everything absolutely clean and clear and all passages fully functional. ordered a Keyster rebuild kit for it and put it all back together today with a new float needle assembly (key note).

    fired up second kick with the choke on full. did the same thing yours did, with the thing wanting to stall and not run without the choke on, and when i backed the choke off (before i reset the float) it idled high then stalled.

    scratched my head for a second and remembered how the pilot jet did not sit that low in the fuel bowl, at least nowhere as near as the main jet - at least 10mm less if not more. knowing first hand that the passages/circuits were all clear i figured the fuel level in the bowl must be too low and the pilot jet was not submerged enough.

    pulled the fuel bowl (carb still on the bike) and checked the fuel flow by opening the petcock and lowering the float slowly till the fuel started flowing. without having any factory specs in front of me, it looked like the float was probably set too low, so i bent the tang down a bit to allow a higher fuel level, put the bowl back on and sure nuff, the bike ran way better (went 55mph with no problem) without the choke on, but still wasn't real solid on idle regardless of any adjustments to the idle mixture screw.

    after doing a little research on the web today, i found the float level to be 12.5mm +/- .5mm, so im gonna pull the carb again and set it on the bench to that spec. if its off (too high) when i pull it apart then that would explain a lot, and should remedy the problem.

    moral of the story, your guy probably did a pretty good job rebuilding your carb but didn't set the float to the correct height, which would explain why your pilot circuit seems dead, unresponsive or way too lean.

    let me know how it turns out on your bike.
    #20