Carb Fails Pressure And Pop Off Test After Full Rebuild

All gaskets went on dry. Didn’t have a gasket between the flame arrestor and the carbs so I used a small amount of three bond there only.
That airbox to carb uses 518 but any sealant is fine as it is not a critical seal.
 
ALWAYS USE GENUINE AUTHENTIC MADE IN JAPAN MIKUNI SBN KITS.

ONLY USE GENUINE AUTHENTIC MADE IN JAPAN MIKUNI SBN KITS.

ONLY USE GENUINE AUTHENTIC MADE IN JAPAN MIKUNI SBN NEEDLE VALVE KITS.
Ok, dealing with this issue myself recently.

100% agree. authentic Mikuni kits/parts because of this crap.

SBN seats, are CRITICAL to function of the carb. As you know pop-off pressure is critical to carb function for starting, idling, low speed, and acceleration.

Genuine Mikuni seats are machined with an inner FLAT face, no taper or deburring or chamfer to them.
Why? this results in a tight seal with the needle, and results in a sharp, clean consistent pop off pressure.

Aftermarket seats all for some reason are tapered here or chamfered in some way.
Results? inconsistent pop off pressure, leakers, pop off pressures that are far below specification and unreliable.

I'm currently rebuilding a carb rack and noticed no matter what spring combo I chose, or what needle I chose, or what I did. I could not get a pop off pressure that made sense in relation to the SBN manual charts.
Why? oooh my idea of getting new "1.5 seats" from eBay. bore diameter is certainly 1.5mm so not an issue. but the inner seat where the needle seals against is rounded/chamfered in a way that when compared to the Mikuni Genuine seats, is definitely wrong.

Glad I caught this early on, and glad I owned a lathe.
I Took another messed up old WSM 1.5 seat I had and it had the same issue. (this is the one with the gnarly chewed up outside diameter that I would never actually use in a carb......
Tossed it in the lathe, used an end mill to clean it up on the inner bottom face, and poof, it began acting like a real Mikuni SBN 1.5 Seat.


Before I continued with my carb rack rebuild. Got out my microscope cam, took all these images.

Enjoy.
 

Attachments

  • Aftermarket ebay 1.5 seat inner angled shot.png
    Aftermarket ebay 1.5 seat inner angled shot.png
    1.9 MB · Views: 1
  • Aftermarket ebay 1.5 seat inner.png
    Aftermarket ebay 1.5 seat inner.png
    1.9 MB · Views: 2
  • Aftermarket ebay 1.5 seat outer.png
    Aftermarket ebay 1.5 seat outer.png
    2.6 MB · Views: 2
  • Aftermarket WSM 1.5 seat inner angled shot.png
    Aftermarket WSM 1.5 seat inner angled shot.png
    2 MB · Views: 2
  • Aftermarket WSM 1.5 seat inner outer machined.png
    Aftermarket WSM 1.5 seat inner outer machined.png
    2.6 MB · Views: 2
  • Aftermarket WSM 1.5 seat inner repaired machined angle shot.png
    Aftermarket WSM 1.5 seat inner repaired machined angle shot.png
    2 MB · Views: 2
  • Aftermarket WSM 1.5 seat inner repaired machined.png
    Aftermarket WSM 1.5 seat inner repaired machined.png
    2 MB · Views: 2
  • Aftermarket WSM 1.5 seat inner.png
    Aftermarket WSM 1.5 seat inner.png
    1.9 MB · Views: 2
  • Genuine Mikuni 1.2 seat inner.png
    Genuine Mikuni 1.2 seat inner.png
    1.8 MB · Views: 1
  • Genuine Mikuni 1.2 seat outer.png
    Genuine Mikuni 1.2 seat outer.png
    2.1 MB · Views: 1
Adding the Genuine Mikuni 1.5 seats, since I can only do 10 files per post.
 

Attachments

  • Genuine Mikuni 1.5 seat inner angled shot.png
    Genuine Mikuni 1.5 seat inner angled shot.png
    2 MB · Views: 1
  • Genuine Mikuni 1.5 seat inner.png
    Genuine Mikuni 1.5 seat inner.png
    1.9 MB · Views: 1
  • Genuine Mikuni 1.5 seat outer.png
    Genuine Mikuni 1.5 seat outer.png
    2.4 MB · Views: 1
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