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Info Mikuni BST29

Discussion in 'Maintenance' started by Frankster, Jun 11, 2021.

  1. Frankster

    Frankster "Play stupid games, win stupid prizes" Staff Member Premium Member Ride and Events Crew

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    God I hate Mikuni carbs (on 4-strokes). I swear, if it wasn't for Blair's (Litetek) knowledge & kits and the help of this forum, I'd never buy a mikuni-carbed suzuki or yamaha ever again.

    I have a set of Mikunis (BST29) off a 1999 Suzuki Bandit that I have to rebuild. They unfortunately leak like a sieve and, as I have discovered, there are quite a few issues needing resolution.

    Carbs.jpg

    Before I start, who designs these things? Do they hand out paper and crayons at the local pre-school centre and then take whatever's drawn and try to make a carby from it? Seriously, I've never seen so many orifices, jets and passages that not only do nothing, they're almost added on to confuse the person working on them! "Oh crap, we forgot the *** circuit." "Don't worry, we'll just run a copper pipe from the top to the bottom of the carb. No-one will notice."

    Give me a set of Keihins any day. Anyway, on with the story.

    I tend to break carbs down to their basic parts, so I can clean them properly as well as discover what else might be wrong with them and what needs to be replaced. The only thing I don't do is remove the throttle blade and replace the shaft washers etc. I do check how much the blades move from side to side. Too much and I'd have to think about pulling them all the way down and asking Litetek (https://litetek.co/Carb_Kit_Suzuki_GSF250-VY.html) if they have a kit.

    The bodies get a good clean in 3bond and the jets and other metal bits sit in a 3bond bath for a few hours.

    IMG_2146.JPG

    I noticed two of the diaphragms had been installed poorly. A little TLC should sort these out

    Squish.jpg

    The manual says these carbs should have a #30 pilot jet. They have #27.5, so I might order some. The main jets are in good condition and the correct size (#97.5).

    The bowl rubbers were flatter than (insert whatever is really flat), so they were replaced by some nice new Litetek units. Hmmm...blue.

    Bodies and bowls.JPG

    The Litetek kit(s) I bought had a full range of new O-rings and seals, so there's no point not replacing them all while you have the carbs apart. I almost always find a washer or O-ring that doesn't really belong where it has been used, so replacing everything is my best policy.

    IMG_2148.JPG

    So, what else did I find... one of the carbs has a manufacturing defect. These Mikunis (damn you all to hell!) have a weird rectangular piece in the middle that holds the emulsion tube and also the carb piston/slider. It has an O-ring at the bottom and a rectangular rubber seal at the top. The ones in the following image are sitting upside-down.

    Centre.JPG

    Anyhoot, one of them doesn't fit properly and it sticks out about 3-4mm into the venturi section.

    IMG_2149.JPG

    I think someone 'tuned' the carbs around the issue as best they could by changing the float height in the carb to compensate for the raised emulsion tube and subsequent needle position. After some time, I was able to 'modify' the carb so most of the piece sat properly. Unfortunately, doing any more would have caused damage, so it will stick out about 1mm into the oncoming air, but the fuel should be delivered closer to that in the other carbs.

    I have started reassembling the carbs, but the squished diaphragms will need a little longer to straighten out. By the way, the chokes on these carbs worked, but were very sticky, so I pulled them out (I do anyway) and replaced the rubber seals with new (Litetek) ones.

    Chokes.JPG

    What an absolute PIA these Mikuni chokes are. Seriously, who invents a plastic clip-on and puts holes into the carb body to install a choke? And don't get me started on those effing fuel floats and bowls . I'm going to have a cup of tea and a lay down.
     
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    Last edited: Jun 12, 2021
  2. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Never seen one that bad before. Probably the wrong emulsion tube. Looks like you have it all under control now. I am sure there will be loads of Mikuni owners banging on your door sooner or later. :)

    Here is someone else using blue stuff.
    https://www.csobeech.com/o-Rings.html
     
  3. Linkin

    Linkin The Apprentice Premium Member Contributing Member Dirty Wheel Club

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    The emulsion tubes have a slot in them to correctly orient them against the jet housing. I had my GSF250V (GJ77A) carbs apart and didn't have issues with the slide housings fitting back properly. They will go, just finnicky. Same on the GSXR250 and FZR250.
     
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  4. inchoate

    inchoate Member Premium Member

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    I'll take Mikunis over Amals or Dellortos any day.
     
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  5. Frankster

    Frankster "Play stupid games, win stupid prizes" Staff Member Premium Member Ride and Events Crew

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    They will go, just finnicky

    When I say "one of the carbs has a manufacturing defect", I mean one of them has a manufacturing defect. If you look closely at the picture, you will notice there is no Emulsion Tube being used.

    IMG_2149.JPG

    I tried all the other centres in that carb and they all stuck out that far. I've never seen a carb that's 'not right' from the factory, but now I have. I've heard people say 'no matter what I do, I can't get these carbs to work'. I used to think that was BS and that the carbs weren't being cleaned and prepped properly, but I won't be so quick to judge next time. The problem with this carb was not in the casting, but in the finishing off process (I think). There is an orifice at the bottom of the carb, that the plastic rectangular piece drops into. On the outside of that orifice, there is a secondary circular cut...except this carb didn't have the secondary cut. There was no way physically that the plastic part could seat properly. The O-ring at the bottom of the rectangular part would have stopped fuel leaking into the carb, but the main jet could only be screwed in a couple of turns. The carbs are now back on the bike and the bike is running fine. No leaks and no issues other than the carbs needing a balance, which I'm about to go and do. I may buy some #30 pilot jets to replace the #27.5 pilot jets that are in there, but it seems to be running okay with the smaller pilots.

    Mikuni QA didn't quite get this carb right.

    IMG_2153.JPG
     
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  6. Frankster

    Frankster "Play stupid games, win stupid prizes" Staff Member Premium Member Ride and Events Crew

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    Okay, to be fair... The Mikunis on this Bandit were easy to balance. No chance of burns or other injuries like when I'm doing Keihins on a ZXR.

    Balance Before.JPG
    Balance after.JPG
     
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  7. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    That's an really interesting find - obviously QC were having a flexi day that day - clearly someone knew, impossible not to have known with a visual indicator like that.
    Could it have been a rush job, end of a batch and just used because there we no others available

    Glad you got it sorted
     
  8. Frankster

    Frankster "Play stupid games, win stupid prizes" Staff Member Premium Member Ride and Events Crew

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    Could it have been a rush job, end of a batch and just used because there we no others available

    It's hard to believe that carb made it out of the factory on a bike like that. These carbs have been modified from their original settings, so maybe they were purchased to replace the originals at some stage or someone tried to 'tune' the issue out of the carbs. You have to wonder how many bikes have inherent issues from the factory and the dealer just does what they can to get it sellable. It's running pretty well now, but I'm going to order some #30 pilot jets to replace the #27.5 jobbies that are in there. Unless anyone has 4 x #30 jets they're happy to sell me.
     
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  9. Brandon Otte

    Brandon Otte Active Member Premium Member

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    @Frankster hey good work on the carbs - could you explain how it ran before you balanced the carbs please?
    I’ve had my mikinus rebuilt and only visually balanced, am having issues with wot and wondered if it may be something to do with balancing. Thanks
     
  10. Frankster

    Frankster "Play stupid games, win stupid prizes" Staff Member Premium Member Ride and Events Crew

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    I’ve had my mikunis rebuilt and only visually balanced, am having issues with WOT and wondered if it may be something to do with balancing.

    Brandon, I can't answer your question as when we went to fire this bike up, it started spewing fuel everywhere, so we shut it off and I've only recently got around to rebuilding the carbs. The bike isn't registered and I have to get new tyres for it to go for a Roadworthy Certificate (in Victoria). Once all that has been done, I will be able to tell you how it runs now, but I still won't be able to tell you how it ran before the carb rebuild. Balancing the carbs shouldn't have an affect on WOT, as it tends to be more about having a steady idle and the engine returning to idle in a uniform manner.

    What are you experiencing when you have the bike at WOT?
     
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  11. Brandon Otte

    Brandon Otte Active Member Premium Member

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    At wot it doesn’t rev smoothly, seems to miss fire and hesitate a lot. At part throttle even up to 3/4 throttle it Revs well.
     
  12. Frankster

    Frankster "Play stupid games, win stupid prizes" Staff Member Premium Member Ride and Events Crew

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    That doesn't sound like balance. The Mikunis (on this Bandit) were pretty easy to balance. Not sure about the FZR, but you might as well do them and eliminate that as a possible issue.
     

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