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Help FZR250R 3LN5 rough running and slow to rev

Discussion in 'Yamaha 250cc In-Line 4's' started by sumthingdarkside, Mar 31, 2020.

  1. jmw76

    jmw76 Well-Known Member Premium Member

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    Well, it sounds like a lot of progress has been made. Many things ruled out. Which is good. Unfortunately, we are now moving into the territory of looking at more serious things. A compression test should definitely be done. Maybe a burnt out valve broken piston ring, broken valve spring or worse.
    Have you tried spraying water around the intake manifold boots while the engine is running? This might give a clue to possible undetected air leaks.
     
  2. Linkin

    Linkin The Apprentice Premium Member Dirty Wheel Club Contributing Member

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    If it's not too hard, take the carbs and exhaust manifold off and have a look at the valves. Checking clearances would be a good idea as well.
     
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  3. sumthingdarkside

    sumthingdarkside Member Premium Member

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    would the leaky air boots actually have a chance at causing it to run as poorly as it does? also if it was lean wouldnt the plugs be coming out light coloured instead of dark like they are, it definitely is running rich. I think what i'll do is purchase some new air boots, wait for them to arrive, and pull the exhaust off and have a look at the exhaust valves. definitely all of the intake valves are in place and where they should be.
     
  4. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Have you re-checked the EXUP since it started misbehaving, I recall when my cable snapped it was a bugger riding it the last 5kms home.

    I was going to suggest worn emulsion tubes are why it's running rich, but that's a progressive thing and doesn't occur in the time it takes to change gears.
    EDIT - I see you've put keyster kits through, so it's not emulsion tubes

    Time to check that fuel height/level as well I think - the carbs are finicky buggers

    https://www.2fiftycc.com/index.php?threads/first-bike-the-fzr250-3ln.10816/page-3#post-130275

    Also, in the video it sounds flat as in throaty which is usually the sound I get when I forget to turn on the fuel tap - how is your fuel filter?
     
    Last edited: Apr 4, 2020
  5. sumthingdarkside

    sumthingdarkside Member Premium Member

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    right so i pulled of the exhaust and had a look at the exhaust valves, all are in place no wiggling or anything, is there something else i should be looking for for symptoms of a dropped valve?

    Checked the exup while i was at it, and it has full range of movement and is calibrated spot on, wee motor moves perfectly in time with engine revs. also threaded some wire through the exhaust header and there were no blockages so can rule that out.

    fuel filter is new as part of trouble shooting.

    Would fuel levels have this much of an impact, especially on just one cylinder? Is it worth waiting until lockdown is over and checking them before i compression test it?
     
  6. jmw76

    jmw76 Well-Known Member Premium Member

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    I think you are up to the compression test stage now. You have just about ruled everything else out.
     
  7. sumthingdarkside

    sumthingdarkside Member Premium Member

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    Yea im thinking so too, hopefully that can give me a definite answer.

    Thanks everyone for all the help and suggestions. Once lockdown is over ill get a compression tester and ill be back with some answers.
     
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  8. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    I think you have two things - Cylinder 3 isn't playing nice - unknown reason, you also have fouled plugs because it's running rich

    Checking the fuel levels is quick and easy.

    BTW carbon buildup on a plug can cause misfiring, try swapping 2 & 3 and see if the issue travels with the plug.

    Spraying aerostart, start ya bastard or one of those on the intake boot of cylinder 3 will also let you know if there's a leak at that point letting in air - it will rev up if there is a leak

    For the clearance they have at TDC a dropped valve would have broken off at the stem and hammered it's way into the piston crown by this stage - an ever so slightly bent one would possibly give you the issue you have.

    I recall one of the cars doing Moto Time Attack last year at Eastern Creek changing plugs every run chasing a misfire, they still won the thing and set a new record. It ended up being a bent valve the entire time.
     
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  9. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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    We've been suggesting to you to check the fuel level's for a while now, yes if the fuel level for carb 3 is too low it wont run properly, will struggle to rev etc

    Read post #31 again, my second question, and check the fuel height's in each carby, then let us know what they are in each carby
     
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  10. jmw76

    jmw76 Well-Known Member Premium Member

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    It certainly will not hurt to check the float levels first. Particularly since you have not got a compression gauge readily available.
    All the diagnostic information you can gather at this point will help.
     
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  11. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    I was thinking that if carb 3 fuel level is high it will also have issues of not running properly - essentially the chugging at idle that we all know and love so much
     
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  12. sumthingdarkside

    sumthingdarkside Member Premium Member

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    Hi there everyone.

    For the sake of some closure and just to say thanks again for everyones great suggestions I thought I'd let you guys know what happened to my bike.
    I've attached some pictures of the cause. Basically, snapped a valve revving the bike up resulting in no compression on cylinder 3. Hence the poor response on the carb diaphragm and the low exhaust temperature. Luckily it didn't do too much damage to the piston and cylinder, a bit of a dent in one but if I can't find a new engine i think i can just run that piston. Bore is still fine on that cylinder.


    IMG_1846.jpg IMG_1864.jpg IMG_1865.jpg IMG_1883.jpg
     
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  13. jmw76

    jmw76 Well-Known Member Premium Member

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    You were very lucky. I have destroyed engines when over reving on a race track.

    Big bore GSXR1100 engines do not like >15k rpm.
    The valves float and you end up with a mess.

    head-640-480.jpg piston.jpg
     
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  14. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Well at least you know the cause, trying to see the glass half full

    Suggest you get in contact with @Mike Green from his thread, he's got experience with the valve float on the exhausts causing precisely that outcome - he may have some suitable pistons as he's only using engine halves in his turbo build

    Suggest you read this thread
    https://www.2fiftycc.com/index.php?threads/fuel-injected-turbo-fzr250-half.10957/page-2#post-133561

    Also @gregt may be able to assist with parts also as he's got his FZR150 bucket race motor project
    https://www.2fiftycc.com/index.php?threads/fzr150-bucket-race-motor.8949/
     
  15. Mike Green

    Mike Green Member

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    I think I only have the one early model piston but that seems all you need. I'd be more worried about the head. You don't jam a valve down the port and not cause a bit of grief. It looks like an early head with the larger valve stems. I have a mandrel to fit the guides for a valve cutting tool you are welcome to borrow if the head is OK and you need to freshen it up. After testing a number of heads I would be looking seriously at checking the valve sealing. A simple test is to place the head flat on the bench, combustion chamber up, and pour a bit of petrol in each combustion chamber.
     
  16. Mike Green

    Mike Green Member

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    A couple of cylinders have serious problems. It wouldn't surprise me if that motor was junk. I'd have to have another look but I might have a spare useful late model head
     
  17. sumthingdarkside

    sumthingdarkside Member Premium Member

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    Engine only has a small amount of damage to the piston in the one cylinder where the one valve broke. Local machine shops say they’d struggle to cut a new seat so at the moment I’m just looking for a whole replacement engine. May try to rebuild this one if I struggle to find one. But it’s winter anyhow so not much riding to be done, as such it can wait a while before it all needs to be back together.

    Did get very lucky that only one valve broke and the damage to the bottom end was limited to practically nothing. Piston is probably still useable if it came down to it.
    Is there an agreed upon point where these engines start to float valves? This occurred before engine even got to redline, just before 18k.
    But obviously old worn out valve springs don’t exactly lend themselves to extreme high rpm. An expensive lesson to keep in mind when all is said and done and back together.

    No point scrapping it. Too few of these bikes left must keep this reasonably nice one alive to enjoy for years to come.
     
  18. Linkin

    Linkin The Apprentice Premium Member Dirty Wheel Club Contributing Member

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    You can get standard and oversize pistons / rings from aliexpress (AHL brand specifically)
     
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  19. Mike Green

    Mike Green Member

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    My mistake. All the photos got blended in the view I had. Good to hear you are going to fix it. You are welcome to anything I have spare which is quite a bit. Change the piston. I don't care how good it seems and whether you can dress it up. From my own investigations I think the valve springs will be fine. The problem will be the installed height which will be too great. This will reduce the seat pressure, and the overall spring pressure keeping the valve train following the cam profile. I've measured a couple of different engines worth of springs and their free lengths have all been near the maximum length. Bores and pistons don't seem to wear and cranks are good unless oil isn't changed and you are a dick when the engine is cold. I have a stack of OK valves. Valve guide seals will be needed
     
  20. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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    Mine has done 62,000km's, i get valve float at 18,500rpm's, so i tend to keep the rev's under 17,500 or slightly lower when i do rev it that high.
     
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