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Mikuni butterfly valve screw size for JIS screwdriver

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Hi folks,

 

I need to order a JIS screwdriver to undo the butterfly throttle valves, but have no idea what size JIS Screwdriver I require, no one locally stocks JIS screwdrivers and I will have to buy mail order and hence would like to know the JIS size I need.

 

Also, is there a source for these screws new as I used a normal Philips on them and you can tell on some of them.

 

Any help will be most appreciated.

 

Thanks.

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I damaged a few of those screws on the last set of Mikuni 44's I rebuilt.  The 44's throttle plate screws are now NLA, so I struggled to source replacements - ended up winning an auction on Yahoo Auctions Japan for 10 used ones that were all in good condition.  There are sellers on that auction site that sell reproduction ones too, and on tiny items like those, the shipping isn't so bad.  I use a bidding service called Zenmarket for buying from the site, here is a link to an auction for said screw:  http://zenmarket.jp/auction.aspx?itemCode=u101899483

As far as screwdrivers though, can't be of any help there I'm afraid.  Good idea though, reckon I will try and source some myself.  

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Hi Ryan,

 

Thanks for your input, my throttle plates are for 40mm but I hope the screws are the same. Incidentally how well did yours seal when they were closed? Are you supposed to aim for even light all around them or no light/hardly any light? Can't seem to find any reference shots online.

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A tiny bit of light might creep through on used throttle plates with an old throttle rod. What's really important is being sure that both sides of the carb are pulling equal air. It sounds silly, but I check this by holding a vacuum cleaner up to the back side of the carb, and measuring the pull with my synchrometer. It works well for me that way.

 

When installing the throttle plates onto the shaft, I lube them up with some heavy oil too. Then once they are both in and close, I'll open and close the plates by turning the shaft a few times sharply and they'll move into place and seal themself. Then it's just a matter of threadlocking the screws and installing them, then degreasing the plates. There is a knack to this sort of work, but practice helps.

 

I've seen some people use molybdenum disulfide on the back of the plates, which is common practice on throttle bodies for getting a perfect seal. I've never personally found it necessary though.

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Thanks for the advice guys.

 

Ryan - I had thought of this idea myself a few days ago and had dismissed it as too crazy.... I already have a synchronometer and a vacuum cleaner (not mine) and gave it a go today - after you giving me the confidence that its not such a mad idea... Amazingly these tools work perfectly and proves to show though that even a tiny bit of light makes the figures vastly different from a butterfly that is properly seated...

 

Any other tips on getting them perfectly seated is appreciated.

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Be careful, the threads on those throttle plate screws are peened (usually) and will strip the threads in the throttle shaft as you attempt to remove them. Hope that's not why your screws are damaged.

Ask me how I know

Recommend you tighten them back down and file/grind off the threaded end of the screws. Then they come out easy.

 

 

Upgrade motoring is a good source for parts.

Edited by Samurai7one

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It's not molybdenum disulfide, which is an oil based anti-seize, but another similar product available from Nissan and BMW for sealing EFI Throttle Plates. In EFI it is critical that all the air pass through the proper metering orifices and control devices. On ITB's this can be critical. If you can't get the plates sealed well enough, you end up with a minimum idle of like 1700 rpms!

 

Carbs are not sealed with this goop, mainly because you will likely goop over the idle and transition ports in the body (though stopping short of that area may be a way to use it. Throttle  Bodies are gooped up, almost mandatory on them if you want a 'soft seat' and a plate that doesn't stick.

 

Beware letting plates 'slam shut to seat' as well, that's how they stick, and consequently on re-opening how the throttle shafts get bent in the first place!

 

They will settle closed by light nudging of the shaft and letting them jiggle into place. Then they are prevented from EVER reaching that point again by the idle speed stop screw. Guys who back the ISSS out fully then snap the throttles from the accelerator pedal usually end up with synch problems as the throttle shafts bend torsionally from the throttle plates sticking in the bores when snapped fully shut!

 

Precision is the key here, not my Avtar.

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Nope,

Maybe, if it's MOS in some sort of adhesive or paint-based binder...that would allow it wearing-in but without sticking.

What's expensive? Speed costs money, how fast you want to go? Somewhere I had found the BMW Sealant....Darn me not writing it down!!!

 

These guys say they use the same stuff BMW Uses, maybe you can get what it is out of them:

 

http://www.westcoastriots.com/htmlfiles/obd1.html

 

Another who noticed it was sealed with stuff:

 

http://www.bimmerforums.com/forum/showthread.php?1049580-Need-help-with-throttle-body-plate-housing

 

Not a fan of this, but it's being done out there:
 

http://forum.miata.net/vb/showthread.php?t=521573

 

This guy used Enamel Model Paint! I think in TB's the  choice of what the sealer is will be less critical than something with fuel passing over it all the time:

 

https://youtu.be/DmBKLefwvV4

 

The GTR Guys in Canada apparently like the Testors Model Paint as well...

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