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  1. #1
    Bushy260's Avatar
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    Mikuni BN38 carbie rebuild on 1994 SL750 - After market Vs OEM diaphragm variations

    Several thoughts and questions related to the use of aftermarket carbie rebuild kits versus genuine polaris OEM parts.

    As the available supply of OEM parts reduces, and more aftermarket parts are becoming available (and much cheaper than genuine parts), I thought I would raise this issue I currently have: -

    I purchased aftermarket carbie kits for a 1994 SL750. In the process of installing the new diaphragms, I noticed a big variation in the height of the centre contact point on the diaphragm that contacts the needle valve control arm that opens the needle when compared to the ones I had just taken out. Not having a micrometer, I used an adjustable wrench to gauge the original centre height against the aftermarket ones and estimate the variation is between 0.035" to 0.037". To assist in this, I have attached photos to show where I'm coming from with this variation.

    Based on the original diaphragm, which has the red centre, the level of the needle and seat control arm should be parallel with (or up to +0.015" above) the jet block surface. If the aftermarket diaphragm is up to 0.037" smaller, it seems logical that the needle and seat control arm will need to be increased 0.035" above block surface to cater for this variation? If not, then the needle may not open enough to allow sufficient fuel flow causing a lean burn. Could the carbies be tuned using high and/or low mixture screws or will the needle valve control arm need to be raised to cater for shortfall?

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    1. Original removed 2. Aftermarket 3. Both old and new 4-5 Makeshift Micrometer showing gap 0.037"
    Last edited by Bushy260; 10-14-2014 at 02:52 AM.


  2. #2
    Bushy260's Avatar
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    Taken from Service Manual dis-assembly illustrating position of control arm to casting.
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    Last edited by Bushy260; 10-14-2014 at 02:49 AM. Reason: Cut and paste didn't work for picture.

  3. #3
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    Im pretty sure you will always be able to buy Mikuni carb kits.

  4. #4
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    I have heard thats the biggest issue with aftermarket carb kits. The diaphragms suck.

  5. #5
    Bushy260's Avatar
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    Quote Originally Posted by BryanP View Post
    I have heard thats the biggest issue with aftermarket carb kits. The diaphragms suck.
    Thank you BryanP for your reply.

    That's what I am now realising, but a little too late for myself, and is another reason why I wanted to make this issue a separate post so visitors to the site may benefit from my mistake when considering buying aftermarket kits compared to genuine OEM Mikuni rebuild kits.

    Here in Australia, the cost of three aftermarket kits on eBay (with Postage from the USA) was around A$80 compared to three Mikuni kits at A$180. Working on two jet skis, the ability to rebuild six carbies for the price of three was a great saving but, upon receipt of the kits, finding the diaphragm valve control arm contact is so much smaller/lower was a surprise and disappointment, let alone working out a way now to correct this issue.

    Another thought I had overnight was that if I raise the control level arm up 0.035" above the casting, that it may change the tension on the spring, affect the Pop Off Pressure (test yet to be completed) and possibly the angle the spring sits into the dimple under the control arm causing the spring to slip out.

    Am hoping someone may be able to confirm my solution will work but, in the meantime, letting anyone who is considering purchasing an unbranded aftermarket kit know that they would be much better off getting the genuine OEM Mikuni kit. In this case, cheaper is not always better.

    Thank you for your assistance to date. Have a great day. Regards. Daryl
    Last edited by Bushy260; 10-14-2014 at 06:10 PM.

  6. #6
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by Bushy260 View Post
    ... Here in Australia, the cost of three aftermarket kits on eBay (with Postage from the USA) was around A$80 compared to three Mikuni kits at A$180.

    ... much better off getting the genuine OEM Mikuni kit. In this case, cheaper is not always better...
    John Zigler is an excellent source of genuine Mikuni carburetor rebuild kits and he sells a kit specifically for the Polaris which has just the pieces needed without the extra bits that are not needed. Needles and seats are sold separately from the rebuild kits, as I recall. He is also willing to ship internationally.

    Alos a good source for other OEMS Polaris parts and aftermarket, plus he has a related business selling good used parts.

  7. #7
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    John includes the needles and seats in his kits i believe.

  8. #8
    Bushy260's Avatar
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    Quote Originally Posted by BryanP View Post
    John includes the needles and seats in his kits i believe.
    Thank you K447 and BryanP.

    Just used K447's link for John Zigler and checked out https://www.watcon.com/fuel-system/c...ris-fuji-sbn38 and his kit ($150 for 3 carbies) includes needle and seats as BryanP indicated but photo (attached - WatCon Copyright acknowledged) does not show any gaskets. Would need to message to confirm if these are included prior to purchase. Benefit with his kit is that you do get the needle and seats compared to others that you need to purchase separately.
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  9. #9
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    What you see is what you get. Zigler's kit does not come with the fuel pump side plate o-ring or the valve body gasket. But it does come with 2.0 needle/seats and springs (for 750).

    With the exception of the fuel pump side plate o-ring and the valve body gasket... the gaskets you see with other kits are for the carbs with built-in fuel pumps on the backsides... so it's gaskets and a plastic diaphragm for that. Our carbs don't have these built-in fuel pumps... we have an external fuel pump... and thus don't need those other gaskets/etc.

    I've never had any issues reusing the old, fuel pump side plate o-ring or valve body gasket on the carbs I've rebuilt.

    Cheers!

  10. #10
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    Quote Originally Posted by Bushy260 View Post
    Quote Originally Posted by BryanP View Post
    John includes the needles and seats in his kits i believe.
    Thank you K447 and BryanP.

    Just used K447's link for John Zigler and checked out https://www.watcon.com/fuel-system/c...ris-fuji-sbn38 and his kit ($150 for 3 carbies) includes needle and seats as BryanP indicated but photo (attached - WatCon Copyright acknowledged) does not show any gaskets. Would need to message to confirm if these are included prior to purchase. Benefit with his kit is that you do get the needle and seats compared to others that you need to purchase separately.
    Click image for larger version. 

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    I don't think John minds the free advertising

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