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FZR Strange Idling issue

Discussion in 'Yamaha 250cc In-Line 4's' started by deanodean, Nov 15, 2008.

  1. deanodean

    deanodean New Member

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    Hi there,

    I have some strange idling issues with my fzr. I've been searching through the forum looking for info , and there seems to be alot of stuff it could be but nothing that grabs me specifically.

    The story: I Bought an fzr and it developed overheating issues. It turned out to the head gasket allowing the coolant to escape through cylinder 3.
    Either way, it had to sit for a while until i figured this out and had the head machined and so it ended up developing carby problems which weren't helped by the fuel filter breaking and the inside part of the fuel tap coming off so all the junk in the tank flowed straight in to the carbs.

    I opened the carbs and cleaned them and that fixed the problems with revving i was experiencing. Having put the bike together , everything runs great now except when im trying to idle.

    It does some weird things ! When you rev the bike and close the throtle it will take a while to return to idle, instead revving hard (at say 3k ) and then eventually for no reason at all it will drop to idle at a slowish pace. Once it drops to idle, eventually it will slow done too much and the engine will stop altogether. I can hit the starter and the engine will start straight up so it doesn't flood. When it is idling, if i give the throttle some it will return to this much higher idling speed and stay there until i either put it in gear and use the clutch to slow the engine or just weight it out until whatever changes and the engine slows down.

    It also has a regular "pulsing" of revs , so while its sitting there it will regularly have a tiny increase of revs, like someone sat there and regularly gave it a tiny twist of throttle.

    I thought perhaps I need to synchronize my carbs but i didn't think that that could cause the engine to die when its idling ?

    You can adjust the throttle screw but it doesn't have consistent results. I can turn up and the bike will idle nicely, but after a rev it settles to a much too high speed and you have to turn the throttle screw back down.

    It didn't do this before it developed the heating problems and now , apart from that, goes perfectly.

    Any ideas ? Would really appreciate it.

    Thanks

    Dean
     
  2. dave

    dave Well-Known Member

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    It is a difficult question you ask.... I can only suggest a couple of things..

    That you check the valve clearances.

    Also check the rubber boots between the carbs and head for leaks as it may be sucking air into the engine, bypassing the carbs. That would have worst effect when you have most vacuum at idle. I take it you had the carbs off the head to do the machining..
     
  3. deanodean

    deanodean New Member

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    Hey , thanks for the reply.

    Yeah I did take the black pipes between the carbies and the head off for it be machined.

    I will take a look.


    Thanks
     
  4. x5466

    x5466 New Member

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    hi
    i had something very similar with my bike i just played with the idle (going by sound id guess its at about 800rpm-1200rpm) and it seems ok except when the choke is on
    im looking into it myself till then im not using the choke and just giving it a few revs when its warming up
    will let you know if i come up with a answer
     
  5. dontz125

    dontz125 Active Member

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    A hunting idle, pulsing high and slowly coming back down, is often a sign of running lean. Check your carb boots, check the o-rings on the starter plungers - mine were completely shot, and sucked wind like you wouldn't believe.

    X5466 - by adding fuel (putting 'choke' on) you solved the lean condition, but there is now more air (leak) and fuel (choke) than will allow the bike to idle nicely - it wants to run away and play...
     
  6. dave

    dave Well-Known Member

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    Just for your info - idle speed of the FZR is meant to be 1600rpm.
     
  7. deanodean

    deanodean New Member

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    Someone earlier mentioned checking the value clearances. Can anyone tell me what other sorts of symptoms you can get from a problem with the valve clearances ?
     
  8. dave

    dave Well-Known Member

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    Poor compression, hard starting and poor performance, backfiring thru carb. And inevitably, burnt out valves. It seems to be the inlet valves that wear the most.
     
  9. dontz125

    dontz125 Active Member

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    There's a FAQ listing on the FZR Archives about the intake valves. Basically, the metal is just a little too soft for the speeds the valves achieve; as the springs age and soften, the valves bounce off the seats and 'tulip', sinking deeper and deeper into the head. This tightens up the clearance between cam and bucket to the point the valve never quite seals enough, or long enough. Compression fails to build, fire vents out through the intake tracts (unlikely; if the valve is damaged to that point it's unlikely to sustain ignition), a lousy day is had by all. Street bikes (600s) can start having problems before 20000km; it's not unheard of for racers to replace intake valves and springs each season.

    If you can't find anything wrong with carbs or spark, check your intake valves.
     
  10. x5466

    x5466 New Member

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    and i started the bike up the next day and realised i well underestimated that it probably more like 2k idle
     
  11. deanodean

    deanodean New Member

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    For a quick update.

    I built the super dooper carb synchronising device mentioned on the net involving two glass jars, some oil , a joining pipe and two pipes out.
    Once I got the consistency of the liquid inside correct, the tool worked brilliantly. The carb with more vacuum would pull the liquid in one jar up at a gradual rate.

    Anyways, i synced all the carbs together and low and behold the idling was a nice consistent beat. It was sitting there on the driveway idling away for 15 minutes straight without dying ! But alas I got too excited.

    When it was idling on the driveway i didn't have the air box or tank cover on so the thing could suck air easily so it idled really well. As soon I put the air box and tank cover back on, low and behold back to the idling nicely for thirty seconds and then a slow die. When you measure the vacuum at idle its so low that the guage can't settle on any number and just random throws the needle right around the gauge.

    I am taking a guess that at low RPMS the engine does not have enough vacuum to pull a sufficient amount of air through the air box and air filter to stay alive and suffocates itself.

    Its also still settling from riding along to about 4k rpm for 15 or so seconds before it settles to idle and then eventually dying. Some gremlin is playing around with revs.

    I'll check the compression tomorrow of the next day. I might also take the air box out and take it for a short ride around the block to see my theorem has the slightest ounce of dignity to it.
     
  12. deanodean

    deanodean New Member

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    I tried my theory of not getting enough vacuum to pull enough air in -- as i suspected, i don't know what i'm talking about. It died anyway.

    I pulled the bike into the garage and whipped out the gauges. At Idle, the vacuum gauge oscillates wildly and there is no chance of getting a correct reading.
    I did some reading about using vacuum gauges to diagnose engines and i found that apparently a rapidly oscillating gauge can imply a valve problem.

    Next I whacked on the compression gauge. Each cylinder seems to make around 600 kPa. I looked up the compression and its supposed to be around mid 900's ?
    My old man came along and suggested pumping oil into the cylinder then doing the test. Apparently this can show if you have poor compression because the oil comes along and seals the rings up temporarily. With some oil pumped in there i get readings around the 1600kPa mark.

    Regardless, I have what i would consider poor compression ( i would appreciate if anyone else has an opinion on this ? The engine has done 40k worth of work ? )

    Considering that as soon as this thing gets some revs it runs perfectly fine, doesn't blow smoke, doesn't back fire or anything else, what do you all think i should check ?

    Valve clearances or get new rings ? Any ideas how to get a hint ?

    Would appreciate any opinions. <!-- s:alcoholic: --><img src="{SMILIES_PATH}/icon_all_coholic.gif" alt=":alcoholic:" title="Alcoholic" /><!-- s:alcoholic: -->

    Thanks
     
  13. dave

    dave Well-Known Member

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    The 250 compression should be around 160psi(~1100kpa?) new or 100psi (700kpa) just about won't start. Check valve clearances first as that is not so difficult.
     
  14. dontz125

    dontz125 Active Member

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    If your idle was set before the sync, one cyl may have been carrying the load. Now that all are synchronised, you may need to bump your idle screw (the big one in the middle of the carb bank, not the idle MIXTURE screws on each carb).

    Your compression test must be done with the airbox off and the slide in question held open; it's not enough to simply sit there with the butterflys open if the slides are choking off the flow of air!

    +1 to checking valve clearances. If your intakes have tightened up, they don't seal properly - bye bye compression.
     
  15. deanodean

    deanodean New Member

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    I blew out the pilot air jet ( seen when looking down into the carby) and now it seems to idle ?
     
  16. say_wat

    say_wat New Member

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    You what?
     
  17. Turkey

    Turkey Member

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    Help me im haveing the same problem what do you mean?? Is that the hole when looking into the carb?
     
  18. my67xr

    my67xr Well-Known Member Premium Member

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    With the air box removed, the air pilot jet is the one to the left with the slotted head when looking down the carby throat's from the back of the bike toward's the front.

    Sometime's you can clear out any blockage's by placing the palm of your hand over the intake side of the carby firmly.
    It create's a vacuum through the carb and can suck through anything that's blocking a jet.
    Then crank the bike over for a couple of second's, your hand might get a little fuel on it when you crank it
    Do the other carby's the same, then refit the airbox and try to start it.


    If this helped get your bike started then you need to remove the fuel bowl's and check if there is anything like dirt etc in them that came out when you cleared the blockage, clean it out and refit the bowl's.
    Should be ready to go then.
     
    Last edited: Jun 15, 2017
  19. Turkey

    Turkey Member

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    Would it be possible to use compressed air and simply blow down into the pilot hole?
    The bike runs fine when using throttle, how ever it will idle for a second and then drop to a lower rough idle and some times stall.
     
  20. my67xr

    my67xr Well-Known Member Premium Member

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    I would be starting a new thread and ask question's there, this thread is 8 year's old now, put the thread here
    Put all the detail's of your bike up, eg FZR250 1990 3LN3
    And give us as much information about the bike, engine carby's as you can, eg bike has sat for 5 year's, or carby's just been rebuilt, engine just had new ring's piston's etc etc

    You can use compressed air but you are better off doing it with the carby's off and apart
    Blow out all the channel's, jet's and tube's etc
    Check the hole's are all open especially the tiny hole at the bottom of the thread's where the pilot jet (fuel) screw's into

    But i would be checking your fuel level's first
    If it's aq 3LN3 then you want the fuel; level to be around 9.2mm above the line on your fuel bowl's
    Place some clear tubing on the drain barb's and have it pointing upward's above the carb top level, and undo the drain screw
    The fuel will run out and up to where the current level is, check it against the line on the carby
    Also check the level's with the fuel pump on, or engine running it may be different

    Yamaha_FZR1000_1989_BDST_FuelLevel.jpg
     
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