Mikuni Carb Guts Illustrated - 82 SECA 550

Mikuni carb guts for your viewing pleasure. 82 SECA 550. Just thought you might like to see the guts in one place.

I bought a whole spare rack of carbs as a teaching aide and as donor cadavers for anything that breaks as I rebuild my originals. I'm now familiar with the ways of the Mikuni and am getting ready to tackle my originals.



I bought these carbs used, history unknown, and hence they are of questionable lineage. Upon disassembly, it's clear that the butterflies had never been removed previously, but other than that, who knows? There were definitely signs of previous work elsewhere. Only thing that looks out of the ordinary is the idle mixture screw.... There weren't any O-rings or seats on any of the idle mixture screws, but all the carbs had plugs over the screws?? :?



Burnarr

 

Burnarr,

I don't mean any offense if you're a veteran at taking these carbs apart. But the O-ring and washer often get stuck in the carb body and need to be extracted with something like a paper clip or a safety needle.

C$

 

C$, Haha! No offense here. I'm no veteran.

Lets see... so far, I've disassembled:

one...
two...
three...
four...

That brings my total to, ummm, yeah, four. :lol:

On your advice, I went pokin around down in the idle mixture screw hole, and lo-n-behold, I got the spring seat and the O-ring. I was positive that there wasn't anything down there, but it sure was!! Thanks for the tip! I'll add them to my guts pic when I get the camera back to the shop.

Burnarr

 

great pics thanks. Nice catch Cmoney

 

I fixed the pics. Now they include the idle mixture washer and O-ring.

Here's a closeup of the idle mixture parts:

 

Burnarr,

Do you have a trick for getting the butterfly valves out? I couldn't get the screws to break free on mine. I was afraid of chewing them up so I punted the problem for later. Any advice you've got would be appreciated.

Thanks!
C$

 

First off, you need to have the correct size screwdrivers for this job, and japanese phillips-drive screws are different from all other phillips-screws in the world because they are slotted to "JIS" standards, and, you guessed it, what you really need is a JIS-standards screwdriver that will properly grab the screw and not "cam out" of the slots, thus destroying the slots, thus making you want to throw things about.....

By the way, you can recognize JIS-standard screws by a small punched dot in-between any two of the "+" slots on a phillips-drive head. Almost every last one of them on your entire bike are JIS-standard screws.


Now, once you have the correct tool for the job, open the throttle shafts WIDE OPEN and say "Ahhhhhh......"

Then take a small flat "jewelers file" and file off the backs of the screws, flush with the back of the throttle shafts.....the screws are pretty soft....and that removes the "peening" on the screws, which is what makes them so tight.

Be careful, when you are filing, not to gouge the walls of the carb throat.

And then lube, and go slowly anyway.

And you still might tear out a thread or two..........

 

As for the butterfly screws, a standard flathead that fit tight, did the trick for me. I have yet to need a single JIS driver.

Only things I have mashed up are my bowl drain screws (which I got out with extractors anyway) (and my new stainless ones should be here tomorrow from len!!!) and one air jet in one carb (and that was my own damn fault)

Seriously, my came right the heck out. No mashin, no kroil, no heat, not even impact wrench. Just a flathead.
I guess it's different with every set of carbs, but my drain screws were already mashed, as most are!

As for the idle mix screw o rings, until I saw you found them in the bottom my only suggestion was going to be maybe someone soaked the carb bodies before in a dip, and since you had caps they left that in, and it ate your orings.
But you found em, doh! haha

 

If you set-out upon rebuilding a set of Mikini or Hitachi Carbs; it is imperative that you have a set of Screwdrivers to FIT the Slots on the Fuel Jets, Pilot Mixture Screws and other Brass Fitting.

Why?

Because the Brass on a stuck Screw Head or Jet is going to Bend or Twist if the Screwdriver leaves space for it to do so.
It feels like the Jet is moving.
Well,... it is!
Just the Wings of the Slots until you snap them clean-off and wind-op needing to drill-out a Jet.

Trying to complete a complicated job without the right compliment of tools will make a complicated job a complete nightmare.

 

Well, you've certainly jinxed yourself now, haven't you?


The Hitachi butterfly screws come out much easier than the Mikuni screws, which are peened and thread-locked.

It's like the float pins on the Hitachis.....take your eyes off of them for even a moment, and they shoot themselves right out of their posts.

Mikuni float pins require viriginal blood sacrifices, buddhist chants and water blessings, and proper prayer position and recitations in order to extract without snapping off one of the posts. It also helps if the planets are properly aligned in friendly constellations during the significant equinoxes.......

 

Man, who-ever wanted Mikunis? lol.

 

Mikuni's are much more tunable than Hitachi's. Just the ticket for people who like pods......

 

Yeah, from what I've read, those butterfly screws can be a pain. Here's how I got mine out.



Step one - Break the screws loose. Not out, but just broken free. For this I used the seminal "anti-cam out tool", the impact screwdriver. Problem is, however, that you cannot expect the throttle shaft to support the force of the impact screwdriver. So what I did was clamp up a couple inches of a small diameter aluminum rod (3/8 inch maybe?) in a bench vice:



This aluminum rod was my backup mandrel to support the tip of the screw. I placed the carb over the mandrel, lined up the screw tip on top of the aluminum rod, and used the impact screwdriver on the screw head. Since the screw tips are already peened, you don't have to worry too much about damaging the screw tips, but I used aluminum anyway to minimize any additional damage.



It's not a Japanese standard bit, but it fit well, and with the force of the impact driver INTO the screw head, worked perfectly. You can get a quarter to a half turn out before you start running into the peened area of the threads. Way more than necessary just to break them free.

"Whack", reposition for the other screw... "Whack". Did all four carbs in less than a minute.

Step two - Tighten the screws up again and file the peened area off the back. For this, I used the ubiquitous Craftsman #1 Phillips screwdriver and a small flat jewelers file. Can't see it in this pic, but I filed a tiny bit off the tip of the screwdriver to get a better fit in the screw head. I'm sure an official JIS screwdriver would fit even better, but since the screws have already been broken free in step one, it doesn't take a lot of force to turn them anymore. So, please keep in mind that I don't want to cut into anyone's business but once the screws are loose, the US #1 with a couple thousandths off the tip worked great for me. I also rounded the tip of the flat file to minimize any potential damage inside the carb bore and again, can't be seen in the pic, but I also took the teeth off the edges of the file so both edges are safer if they accidently contact the carb bore.



I didn't have to file the tips completely flush with the throttle shaft. The peening was confined to the first thread or so. Here's what they looked like after the filing. The beauty of having the screws already broken free at this point is that you can try to take them out, take off a little more material, try again, take off a little more... Until they come out free.



Vacuum out the filings, take out the screws, pull the butterfly plate, take out the throttle shaft, and... The "money shot".



Cheapie Horror Fright impact screwdriver. Couple inches of aluminum rod. Ubiquitous Craftsman #1 Phillips screwdriver. Modified jewelers flat file.

Looking at the butterfly screw heads, there's almost no sign that they were even ever used. The Phillips head is still in great shape, no smearing, no torn threads in the throttle plate, no damage anywhere.

Burnarr

 

Yup.

I've got screwdrivers that I devoted to the cause years ago. I keep regrinding them to whatever I'm working on at the time.





You can get a good view of the Phillips I used on the butterfly screws. Just took the point off...

Today they are for SECA carbs. Last week they were for something completely different and some time in the future they'll be something different again, I'm sure. Sooner or later they'll be ground down to nubs and I'll have to buy replacements. :lol:

 

I must live right. I didn't have any troubles with mine... :?

I simply supported the ear by the pin head and tapped the pins out with a light hammer and a small diameter drift:





Tap, tap. Tap tap. Tap tap. Tap tap. All four done. No virgins, no chants, no blessings, no expletives? :?:

Those ears look pretty fragile, but how is that they snap off? Looking closely at the shape of the pins, the ear at the tip of the pin should be a loose slip fit and the ear at the head end is the only one that should need to be supported, right?



I mean, I'm not saying it doesn't happen. As a matter of fact, one of my donor carbs has an ear snapped off (not by me). So I know it happens, but why? Pilot error or something more complicated?

Burnarr

 

Yes, once a screw is loose, then a regular phillips-drive screwdriver is fine. It's the initial loosening force that normally chews up the head, though. Not a problem if you're using a impact hammer.

The posts snap off because the pins get seized within their post holes. The pins are an interference fit with the posts, and if time, varnish, etc. get added into the mix, they can become tight as a proverbial drum head. It doesn't take much deflection, at all, to get the "pop goes the weasel" result of a 2-piece post.

All I'm sayin' is that you're missing out on the sense of wonder, awe, and the high that then comes with a sense of great relief and of accomplishment if you're doing all this without chants, blessings, expletives.

And especially without the virgins.

Very nice photos!

 

The pins are only an interference fit with one of the posts, not both.

The pin is a larger diameter right under the head. The other end is supposed to be a slip fit. No interference. That was the point of this pic... You can see the area of larger diameter up by the head.



That's also why (in theory), you should only have to support the post closest to the head when removing or installing the pin.

I understand that's "by design" which doesn't account for time, varnish, corrosion, etc. It's supposed to be a slip fit on the end far from the head, but that's not to say that really works that way with all the gunk in the bowls. Not supposed to happen, but maybe both ends get glued in place? :?:

Heh. Yeah, I guess I gotta take the good with the bad.

And not to worry... All of this work so far has been on a set of donor cadavers. If something is going to break, I'm sure it will be on the set that I really want to use again. :lol: Murphy says so.

 

I just finished rebuilding my carbs. I used my dremel tool (carefully) and ground off the butterfly screws. I used a brand new #1 phillips screwdriver. I bought a #2 and 3 JIS from Chacal and didn't think I needed the #1. Any ways, I gave the end of my screwdriver a good whack with a hammer and got 7 of 8 screws out. #8 I drilled and used my smallest ez-out. I had new screws ordered so didn't care what they looked like in the end. I braced my float pillars like rice-burner did and I added a piece of flat metal between the pillars to support the top one. They can take a reasonable amount of force. All the pins came out fine.........and numerous times to set the float heights!