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low vacuum when idle on cylinder 4

Discussion in 'Yamaha 250cc In-Line 4's' started by NZdirtslinger, Jan 31, 2013.

  1. NZdirtslinger

    NZdirtslinger New Member

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    As the title says, when I rev it and put my hand over the intake theres a good amount of sucking going on, but when its at idle theres very little vacuum coming from cylinder 4.

    The other 3 cylinders are firing at idle. As for cylinder 4 the exhaust stays cold.

    I've done and checked valve timing. The float height seems right to me, about 16.5mm. This is a 2KR model.

    What are some of the possible causes for low or no vacuum at idle.

    This has been a big learning curve for me.
     
    NZdirtslinger, Jan 31, 2013
    #1
  2. GreyImport

    GreyImport Administrator Staff Member The Chief Contributing Member

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    Have u looked very closely at the intake manifold boots? .....they are prone to cracking from old age and leaking air.

    [​IMG]
     
    GreyImport, Jan 31, 2013
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  3. dontz125

    dontz125 Active Member

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    Have you synchronized your carbs? If that 4th butterfly is too far closed, then you get no vacuum and no mixture drawn in.
     
    dontz125, Feb 1, 2013
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  4. NZdirtslinger

    NZdirtslinger New Member

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    Thanks for the replies.

    What is the part labeled 32 on the diagram, does it play apart in vacuum? Is it only attached to the 3rd/4th carb overflow outlet?

    I've not synchronized since cleaning and changing some parts, i'll have to pay someone to do that.
     
    NZdirtslinger, Feb 1, 2013
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  5. dontz125

    dontz125 Active Member

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    That's the bowl breather filter, and is actually on the 1-2 connection as well. It stops dust, rain, and small animals from crawling in to build nests in the carb bowls ...

    Do some reading, here and on the wider web. Home-built sync rigs are cheap and easy to build, and syncing your carbs is easy and very satisfying, especially for the new mechanic - "I did that!" <!-- s:) --><img src="{SMILIES_PATH}/icon_smile.gif" alt=":)" title="Smile" /><!-- s:) -->
     
    dontz125, Feb 1, 2013
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  6. GreyImport

    GreyImport Administrator Staff Member The Chief Contributing Member

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    Lots of youtube vids on carb sync.

    I picked up these secondhand off ebay for about $50

    [​IMG]


    Even paying $100 would have to better and maybe cheaper than paying a shop to do it.....

    http://www.ebay.com.au/itm/CARB-CAR...orcycle_Parts_Accessories&hash=item484ed70753

    As Don says....the self satisfaction of doing it yourself is great....plus u learn to realise alot of things arnt that hard......and its WAY cheaper in the end <!-- s:D --><img src="{SMILIES_PATH}/icon_biggrin.gif" alt=":D" title="Very Happy" /><!-- s:D -->
     
    GreyImport, Feb 1, 2013
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