Mikuni BS36 (FJ1200) carbs

[XrockerS]

Hello XS lovers! When I bought my bike a 3 months ago It came with a set of FJ1200 carbs and I was wondering if any of you have successfully ran these carbs on your bikes? If you have how did you work on the Vacuum advance? Ive read on the Australian forums that you can plug up to your manifold and place some sort of restrictor on the tube running down to vacuum advance unit. is this correct would that work? I've also read that you can drill/bore your own port on carb number 2 but that sounds like unwanted work. Heres a pic of my tops I have looked at them over nd over. I'm almost positive that they are BS36's

They used to be gold tops before I painted them to match the paint job I gave my bike.

[TopCatGr58]

Hey Marc,

The vac. adv. is essentially just for providing better fuel economy by providing the ability to increase the timing during cruising rpms and leaner mixtures. When the throttle is opened up considerably, vac. levels drop and the vac. adv. releases allowing the timing to actually rotate/return to the lesser amount of around 36 degrees BTDC which is the maximum adv. under LOAD at ~5krpm+ for maximum power!!

SO...you can run the bike without the vac. adv. pot connected, the bike will have all the power it can have, it's just that it won't have as much fuel economy when cruising at low load/lean throttle level because it won't be able to create the additional maximum 16 degrees more advance above the max cent. 36 degrees to be able to take advantage of the leaner cruising fuel level/rpms.

Connecting it to the Synch Port, even with a restrictor can still cause it to generate TOO MUCH advance too soon or at the wrong throttle input/fuel level vs. the cruising mode. The synch port will provide vac. ALL the time, the port on the Carb#2 port is both metered/restricted, but also not active when the butterfly is shut or barely open.

I haven't read any of the postings on the AUSSIE site, but more real world testing/monitoring of the amount of vac., adv. the vac. adv. pot is actually seeing at the various rpm and load levels would be needed/documented before we will be able to believe that their technique properly mimics the carb port location.

T.C.

[XrockerS]

TopCat that's a great explanation. Thank you. So the best thing to do would be to just cap the tubing going down to the advance unit. The way the P.O. had it rigged was into carb boot #1 but Ill cap that off as well. I need to check what Jets I have so I can determine a good float height for these carbs. I figure that the reason my XS had these carbs was to make it run "better" with the 4 into 1 open exhaust.

As far as fuel economy goes i'm not to worried about it quite yet. I figure I should get this bike running with these carbs while I find the perfect set of 78 carbs and jet them according to what I find with these. Who knows if it runs "right" I might just leave these on.