Unleashing Inazuma 1200 Full Power | Dramatically Transform Performance with GSF1200 BST36 Carburetor Swap

Year-end rush! We’ve arranged most of the parts and items we wanted and needed.
Delivery dates are already coming in, like January 〇th of next year. That was close…

We received the bike on 10/17. It feels like ages since we ate hamburgers together…
The exhaust has already been shipped.
We’ll keep things moving smoothly!

I’m parting with my precious Mikuni BST36 (*´ω｀*)

This Mikuni BST36 is in pretty good condition if I do say so myself.
I wonder why it fetches decent prices on Yahoo Auctions and Mercari?
What vehicles are people using these on? I’m curious.

So I’m going to clean this up and do a full overhaul.
Some replacement parts are ridiculously expensive, so I’m reusing anything that doesn’t matter.
Hopefully it’ll turn out looking beautiful.

13439-17C00 Holder
This part handles the choke plate’s sliding and locking…
By the way, this costs 1,100 yen before tax.

I spray it with silicone, warm it up, remove it, and reuse it.

Finishing with wet blasting and gun coating.

I used to do this painstakingly with baking soda media, but with the new wet blasting system, I’ve upgraded the media too.
It’s very fine and powdery, so I can process it with confidence.

Of course, after wet blasting, I rinse it thoroughly with the high-pressure washer in the garage.
I used to use a warm ultrasonic cleaner, but gravity limits media removal.
How smoothly and reliably you handle post-wet-blasting cleanup is also crucial.

I’ve applied gun coating to the plates.

The plastic parts came out beautifully too, since the blasting is gentle on them.

Hit the key replacement parts and make them work!

For GSF1200 BST-series carburetors, parts delivery is surprisingly stable, so no worries.
I replace all O-rings and seals.

The Suzuki OEM float valve assembly is 13370-46E21 7,650 yen.
Multiply that by 4… 30,600 yen before tax. That’s seriously scary.
I’d advise against going with aftermarket overhaul kits just because they’re expensive.
Once you throw away the OEM parts, there’s no going back.

I won’t say which one, but I’m reusing just the float valve body from a certain brand.
Even buying four Mikuni OEM ones doesn’t cost as much as one Suzuki OEM unit.
I’m reusing the receiving side, so I need to wet blast it
and polish the contact surfaces with compound, etc.

A clean carburetor really feels great.

Aluminum naturally forms an oxide film, so applying some anti-rust oil will keep it looking pristine longer.

The actual fuel level on Mikuni BST-series needs verification.

In any case, this is a carburetor prone to going out of adjustment.
The float assembly and arm are separate, and everything’s plastic held by O-rings, which is part of the problem.
Individual variations, compatibility with the body, wear—all sorts of factors can cause issues.

When checking the fuel level, I plug the passages like this.
No point sending fuel to all the carbs unnecessarily—that just adds work.

I think about efficiency with every job, and that makes the work more enjoyable (*´ω｀*)

I’ve forced the fuel inlet to the right layout.
This gives plenty of hose length to the petcock, making tank removal easier.
Plus, fuel supply when on the sidestand should flow smoothly and quickly to everything.

The float assembly is untouched—it was in great condition.

And that’s it—all done! (*’ω’*)

Of course, #1 and #4 are the face of the carburetor, so I’ve finished them really cleanly.
I’d say they’re cleaner than new.

All bolts are new, and the carburetor prep is complete.

To be continued~