Downdraft Mikuni Carburetors

[River Rat]

I picked up a set of downdraft Mikunis a few days ago. It's a set of four inline that came off of a motorcycle. They had been removed some time ago, but the owner thought they came off of a 600cc engine.
I broke these things apart today (separated into individual carbs) and took two completely apart. So far I like what I see. These things do have changeable jets on 3 circuits... Good float, needle/seat design.
I plan to see if I can make my O84 run on these - one per cylinder. I'll post pics of the carbs after I get them cleaned up.

Rat

[Duane Scarborough]

Rat,

It definately is possible to do, and it would be different. However, synchronizing them will be a big challenge. If you've got experience synchronizing multiple carbs, then go for it.

If not, you may have your hands full.

Mikuni makes an excellent, very tunable carb.

Myself, I'd like to find a single 30 mm downdraft Mikuni. If I don't run across one of those, I'm planning to use a 30 mm Solex.

Why? I have some experience with multiple carbs. They can be a PITA.

Good luck with it. Keep us posted.

Duane

[Sandman]

Duane
I'll look around the hose and see what I can find for Mikuni carbs.
The are popular carbs on snowmobiles.

[airhead]

Rat,

I worked on Honda motorcycles in a prior life and have a little experience with this and also still have guages to do this. I agree they can be a PITA but if the jetting is correct it shouldn't be a biggie. The guages are just four vaccum guages and you just get them equalized. You just get the jetting correct and drag her down here if necessary and we will MAKE her GIT IT DONE :P

Jim

[Duane Scarborough]

Airhead,

Equalizing / balancing them at idle is just a small part of it.

Like I said, it CAN be done. Getting it to run properly, with all cylinders getting the correct mixture under full power may be a real challenge. Getting the mid-range correct will be a total crap shoot.

Good luck.

Duane

[Sandman]

well.. I guess what I thought was a downdraft carb is not a downdraft carb... these are carbs with the sliding valve. and I'm not sure on the diameter but they look large.

[Duane Scarborough]

Sandman,

That's O.K. Until Rat posted about downdraft Mikunis, I wasn't even sure they made them. And I'm still not so sure.

But, if they do make them, I bet they'd be good. Never saw a sidedraft Mikuni that couldn't be tweaked nicely.

I've got a line on a H30/31 Solex/Brosul that I'm chasing for now. I know it'll work with the 4A084, and I'm not inclined to do a lot of experimenting.

Thanks,

Duane

[airhead]

Duane,
I agree

[River Rat]

Actually, I don't necessarily see it being that big of a PITA. Multiple carb set-up isn't a first for me. 4 Mikunis on an O84 will be a first for me, but I don't percieve it as being rocket science. I do expect there to be some mechanical challenges, of course more complex than adapting a single Solex, but that's actually one of the reasons why I'm doing it.
If the carbs aren't junk, I believe I can make it work much better than any single carb set-up on the O84.
Gonna give it the ole redneck try anyway :P

Rat

[River Rat]

Until Rat posted about downdraft Mikunis, I wasn't even sure they made them. And I'm still not so sure. Duane

Are you saying I don't know what I have? Cause that sure sounds like what you are saying... Please splain what you mean? That sounds just like you're implying that I'm a dumb A$$ ??
Just because you don't know anything about them doesn't mean they don't exist. WOW!

[Sandman]

Wow.. well.. err.
Mikuni carbs are very easily rebuilt. I did look em up but really didn't find any online... Well I found one and I guess it was on a John Deer. But I didn't look far.. ebay.
I just modded the carb I already had and it seems to have made a difference.
Don't forget.. moods and tones are hard to decipher on forums...

[River Rat]

Sandman,
Most motorcycles have sidedraft carbs because there's not as much (not enough) space atop the engine for carbs and airbox. Because downdraft carburetors more efficiently flow air, over the years several bike manufacturers modified (tunnelled out) the gas tank to make room for the downdraft carbs in an attempt to gain some power that seemed to be low-hanging fruit.... The tunelled out gas tanks had to be built much wider to increase fuel volume. Of course, not everybody prefers function over form, so the bikes weren't as popular, so you aren't going to find near as many downdraft set-ups. I looked for quite a while. They're actually not that different from the sidedraft carbs. Biggest thing that gives them away is the float design, which of course is designed for the carb to "lay flat", versus "standing upright". If the carbs are off the engine, most folks don't know the carbs are downdraft because they look so similar to the sidedrft models.
If you google "Mikuni Downdraft Carburetor" or "Downdraft Mikuni" you might find some pictures... If you were serious about a multi carb set-up on one of the surplus engines, this would be the way to go. Keep watching ebay.

DT

[Sandman]

okay I did find a blow up of it but man these have got to be rare... cannot find them anywhere.

http://webservices.motorsportdealers.co ... URETOR.Gif

From this site http://www.powersedge.com/pages/parts/v ... 5QodHhUgKw
can buy em new but wow the only ones I've seen reference to are the bd34 and then I'm not sure if the blow up is of the bd34
I'll talk to a friend and see if he has any...

[Duane Scarborough]

[quote=River Rat]

Until Rat posted about downdraft Mikunis, I wasn't even sure they made them. And I'm still not so sure. Duane

Are you saying I don't know what I have? Cause that sure sounds like what you are saying... Please splain what you mean? That sounds just like you're implying that I'm a dumb A$$ ??
Just because you don't know anything about them doesn't mean they don't exist. WOW![/quote:rzrqj17w]

Rat,

I certainly didn't mean to impune you in any way. What I MEANT was that I hadn't seen one myself. Sorry for my poor choice of words.

Duane

[David]

Thats what happens in these forums one guy says something and it sounds like the other guy takes it the wrong way

Anyway back to the post I want to see that setup when you finish it

[Duane Scarborough]

David,

You're right. It's very easy to either say, or take things the wrong way on a forum.

I've P.O'd people when I didn't mean to, and vice-versa. Best you can do is appoligize, and try to move on. It sure ain't worth getting into a pissin' match over.

I'd also like to see how that setup turns out.

Duane

[Papee]

Ditto, I hate people who think they know more than me. Just to be safe

[River Rat]

No problem here, Duane... Yep. A lot is open to interpretation on these forum posts. Just making sure I was interpreting correctly. It's all good.

Like David said... Back to the post topic. I was afraid I might run into this, but having problems finding rebuild kits/parts for these things. I'm told there were thousands of OEM models of Mikunis and without knowing exactly the bike they came from, they can't get me the kits.
Anyway... Not giving up on those yet, but in the mean while, I have ordered a new single carb to see if it looks like something I would be interested in using for a multi carb set-up on the O84. If so, there may be three more of them to follow. It is a sidedraft model, though. We'll see what it looks like in a couple of days. If not a candidate for the multi-carb set up on the O84, it'll make a nifty replacement for the stock carb on the O32.

DT

[Papee]

Just to put myself out there.
The stock carbs deliver the fuel via the metering rod so I thought about stacking carbs hooked up with a progressive linkage figuring I could turn down the idle mix screw on one of the so it didn't deliver to much fuel at idle.

[River Rat]

Hmmm...