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  1. #1
    Millertime44's Avatar
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    SBN 44i - Broken Seat Hold-Down Screw - Ideas?

    BACKGROUND:
    Rebuilding my 3 SBN 44i carbs on my 2001 GP1200R. When checking pop-off pressure for my stock setup (1.2 N/S, stock airbox), I found pop-off pressure on carbs #1 & #3 to be at 85 psi. I used the only spring that came in the genuine Mikuni rebuild kit, but per my reading on stock settings HERE, those popoff values seem acceptable for my current setup. I recognize that's a crap ton of pressure compared to modded setups and will likely go with after market flame arrestors and 1.5 N/S with re-jetting in the future, but now am focused on getting this up and running stock.

    PROBLEM:
    For carb #2, I identified that the seat was leaking around the outside. I was struggling to get the N/S to ever cleanly pop off. It would fizz and bubble more and more as the pressure ramped up starting around 70 psi to over 95 psi before I stopped, but never truly pop. I tightened the seat hold-down screw a lot and wouldn't you know I broke the damn screw off.

    I need to get the broken bit of screw out first, then wondering if anyone else has run into this? I'd prefer to drill out to a larger size, tap, and use a larger screw. I suppose a helicoil may be another option but there's not a lot of room to work down there. Wondering how deep I can drill? What's immediately behind that screw hole?

    Hoping someone has advice or school of hard knocks knowledge to help me here? Thanks!

  2. #2
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    getting that particular screw out is something I would rarely promise to be able to do without serious compensation. It's extremely tricky wristwatch style work.

    given the pretty low cost of a replacement carb relative to the labor involved in getting out such a small screw in such a delicate spot...

    I'd go with a replacement carb, correction, carb prices seem pretty high even on ebay, I guess there has been a run on them due to restorations due to the covid

    you need a drill press a big ass magnifying glass a high quality drill bit and a teensy screw extractor to have a chance at getting this done.

    drilling out to a bigger size and tapping run a real risk of cracking the seat area

    I for one might be inclined to use some red thread locker around the seat to keep it in place. you could then utilize a screw extractor in the event you need to replace the seat in the future

    can we see a picture of the area so I can perhaps provide additional advice

    I've done this kind of microsurgey on antique motorcycle carbs, which were absolutely unobtanium as part of overall restorations I did back when dinosaurs roamed the earth and the BMW r/75 was a pretty hot bike

    I still have my r/75 by the way....don't ride it anymore..just a garage decoration these days. When I did ride it a bit I was besieged by other bikers as to who was working on them, I had a pretty good business ion that for several years when I lived in florida. Mostly on Pre-1960 models Those really old beemers are worth a small fortune these days. I regret never buying one. I certainly had the opportunity

    For reasons unknown, really old jetskis just don't hold the same value except in a few few cases.

    A crated wetbike, for example go some insane bids on ebay a few years ago.

  3. #3
    steve45's Avatar
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    Don't screw with it and make it worse. Take it to a machine shop that can do electro-discharge (EDM) machining. They can get it out.

  4. #4
    Millertime44's Avatar
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    Update: I got the broken part of the screw out very carefully using left hand drill bits. I used a center punch to put a divot as close to the center as possible, then the smallest LH bit in the set. As it started to bite, I was able to turn the screw piece right out. I was aided by the fact that the carb was already fully cleaned in chem dip and carb cleaner before I broke the screw off while tightening it. If it had been stuck/corroded in place and I broke it trying to loosen the screw it likely would have been harder.

    Threads still look great! Hard part over. Now I need a replacement seat hold-down screw. Anyone have one laying around from a Mikuni SBN?

  5. #5
    Millertime44's Avatar
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    Quote Originally Posted by steve45 View Post
    Don't screw with it and make it worse. Take it to a machine shop that can do electro-discharge (EDM) machining. They can get it out.
    Thanks for the tip - I didn't realize EDM could be used to remove broken bolts. I'll keep that in mind next time I break a rusted bolt off somewhere!

  6. #6
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    Quote Originally Posted by Millertime44 View Post
    Update: I got the broken part of the screw out very carefully using left hand drill bits. I used a center punch to put a divot as close to the center as possible, then the smallest LH bit in the set. As it started to bite, I was able to turn the screw piece right out. I was aided by the fact that the carb was already fully cleaned in chem dip and carb cleaner before I broke the screw off while tightening it. If it had been stuck/corroded in place and I broke it trying to loosen the screw it likely would have been harder.

    Threads still look great! Hard part over. Now I need a replacement seat hold-down screw. Anyone have one laying around from a Mikuni SBN?
    well you now qualify for the annual Captain Pete award, given when somebody ignores good advice and turns out successful, against all odds

    congratulations

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  8. #7
    Myself's Avatar
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    LOL!!! Good one Pete!

  9. #8
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    Quote Originally Posted by Myself View Post
    LOL!!! Good one Pete!
    Anybody who had to drill knows thatís is really a crapshoot if you are going to be successful

    It was 2 or 12 probability that the screw would back out give that the head snapped off

    Iíve gotten out a good share of problem bolts, also junked some rare stuff when things went poorly

    Carbs are dangerous territory but with the blessings of Poseidon he was ultimately successful

  10. #9
    Millertime44's Avatar
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    Quote Originally Posted by nmpeter View Post
    well you now qualify for the annual Captain Pete award, given when somebody ignores good advice and turns out successful, against all odds

    congratulations

    Thanks, Capt'n Pete, I am honored and will cherish my balls of steel award forever! Sometimes it's better to be lucky than good.

  11. #10
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    Quote Originally Posted by Millertime44 View Post
    Thanks, Capt'n Pete, I am honored and will cherish my balls of steel award forever! Sometimes it's better to be lucky than good.
    If you pm me your mailing address Iíll perhaps send you something commemorative for this event, assuming I can get off my ass long enough to make something

    Iíve started this new sideline business as with our lake closed for so long I needed something to take my mind off the crushing overhead of my business

    This is the first year ever I did not sell a single battery
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