550 Seca Mikuni Carb Float Level 'Dry' Dimension

Does anyone have a dimension for dry-setting the float levels on a 1983 550 Seca? All my Clymer's manual will tell me is to check with tube on drain port with carbs, tank, etc. fully hooked up, ready to go. If level is off, totally dismantle the whole effing setup and tweak. Repeat...



*rant on*
On the dozens and dozens of other motorcycles I've worked on, a dimension is given from some fixed point (i.e. bowl gasket surface) to a point on the float just at the point where the needle valve is seated. The entire INDUSTRY accepts this as standard procedure. Why not Yamaha?

My Clymer manual even mentions that if you get a float level that's correct via the 'wet' method, then you can measure that one and use that dimension for subsequent 'dry' method efforts. Why can't they just give me the dimension to start with? Is there that much variation between these assembly-line-manufactured parts? Do you mean to tell me that Yamaha used to wet-check every single bike that came off the assembly line?

Further, the whole wet method requires shimming and/or jacking the whole bike around to meet some mythical 'level' condition. I mean, if the bike is off by a 1/2 inch or so left-to-right, that makes any individual carb off by, what, a fraction of a millimeter or so? What's the overall ratio here? And what about the side-to-side 'center' of an individual bowl? If the carb is not level, then one side will be high and one side will be low, but the side-to-side 'center' of the bowl will be fine.

Am I to believe that Yamahas, among ALL the motorcycles ever manufactured, are so damn sensitive to float level changes? I mean, does this mean that Yamaha designed a machine that will change it's essential richness/leanness condition just because my passenger happened to have a few extra burritos? I mean, I set the float levels with the carbs "leveled", then have a few extra doughnuts at coffee time, hop on the bike, and it's "...oh, so sorry, you're running lean now." Pshaww, I don't believe Yamaha built 'em so fussy...

Just seems a bit overkill. Surely, there has to be a sweet "range" that is very repeatable, not just some laboratory-perfect setpoint or you're screwed...

*rant off*


:roll:

Thanks for any help.

Kirk

 

Well, Chacal and I are working on tweaking an instruction guide for setting the floats and fuel levels.

Most of the manuals don't even list a float height; the Haynes does for the 750J, but there's no guarantee that's going to work. As for how they set the floats on the assembly line... I have no idea.

The suggested procedure is this: put the carb in a vise or rack or somesuch, and get one within spec. Then lay the rack down so that the tang on the float arm is just touching the needle, take a measurement, and tweak the other three tang arms to match. They should all fall into place. And no, adding a passenger with "a few extra burritos" shouldn't change the fuel levels in the bowls.

To quote a Minbari character on Babylon 5, "We know not WHY that it is. We know only that it IS."

Check this version of the file out and see what you think.

Fuel level draft 7

 

The number you seek is 21.5mm.

 

there's a big discussion on this everytime someone asks for that magical number.

i've never heard of the 3mm wet setting untill this sight. i've ALWAYS done dry setting. and those were the specs that was always listed. they are around but hard to come by looks like.

my vtwin motor manual DID come with dry height specs though. i don't see any way in heck your going to use the wet method on the front carb. which lays on it's back.

 

You use the wet method on that carb when it's off the bike.

Yes, wet checking seems overkill but I can tell you from experience that you can dry-set a rack of Mikuni floats to be as absolutely exactly the same as humanly possible and you know what?

One or two will still be wildly off when you wet-check them.

Which is why BOTH the Clymer and the factory book specify wet checking as the correct method. Because it works.

I think part of the reason is the geometry of the float bracket and tang. When doing adjustments, if you tweak the tang enough to know you bent it, you went too far. It's done a "nudge" at a time; the difference between right on and horribly off is so tiny. You simply can't see the difference between right on and really off doing it dry.

 

Schmuck, that's a good write-up. I like it.

I'll do the wet method on one carb and use it as a reference, then set the other three dry as a baseline. I'll double-check the other three using wet method.

I still can't believe the difference between dry and wet will be that variable. My experience base is primarily Keihin carbs, but other Mikunis thrown in there, too. I don't think I've ever had Hitachis apart, though. And I've never had any bike that was so very sensitive to float height. I mean, when you're running down the road, the bike is pitching, rolling, leaning, vibrating and generally moving around so much that I can't believe that a millimeter or three is going to make or break the way the bike runs...

Ah well. Thanks for the info.

Kirk

 

You're right. It won't. Provided the floats are set to the proper baseline level BEFORE all the rolling, leaning, vibrating, etc.

Getting them set to the correct static level is what makes it so it's NOT sensitive to the pitching, rolling, leaning and vibrating.

START OFF too low or too high, and then the aforementioned gyrations WILL matter, big time.

 

Yeah, you're right, of course. I guess I'm just ranting. Must be the varnish fumes...

I swear, it takes DAYS to get that varnish stink off my fingertips after carb work...

 

http://www.safecare-gloves.com/auto-mec ... loves.html

I buy the blue ones 100 to a box at the auto parts store for around $10.

 

LOL.

Funny you mention those. That's what I use as well. Have a box sitting right next to my toolbox, but the other day I was just, y'know, tinkering around at the bench, kind of tidying up from other projects and wasn't really planning to do anything on the carbs, and next thing I know, I'd been handling the varnish-y bits "unprotected" so to speak...

Damn. Stinks for days...

Kirk

 

Well, I've done the first carb.

"dry" measurement was about 21.75mm. Not perfect, but it was as I found it, and I'd already done the wet check with carbs on the bike, and felt they were good. So, I left 'em that way.

I mounted the carb in the vise which got 'em level front to rear, and I just eyeballed 'em level side to side.

Fuel hose stuffed into the drain port and into the supply port. This might get a bit messy on the inner two carbs since I'll need to block off the fuel supply cross-over, since fuel come in one side and passes right on thru to suply the outer carbs.

Hooked up a supply and opened the drain screw. It seemed to measure just about perfect. ~3mm below the carb body flange. So, I left it alone. I dumped the drain hose a number of times and let it fill back up again, just to check the repeatability. It came right back to the same spot each time. So, I'm pleased.

On to the next three...

Thanks, all, for the input.

Kirk

 

Actually, you're just a hair (~1-2mm) low. You measure to the meniscus (the bottom of the "U" formed by the fluid in the tube.) Spec is 2mm +/-1mm, or from 1mm-3mm below the flange. You're at about 4mm there, going by the bottom pic.

It's not necessary to break the rack to do the float levels (unless you had it apart already.)

 

Yeah, it's just the angle of the photo. It was bang on 3mm - bottom of the U, as you say. But, I had it in my mind that the spec was 3mm +/- 1mm. Dang, I'll have to go re-read the Chilton's.

No, I had the rack apart already for rebuilding. Much easier to manage a single carb vs. the whole assembled rack. Plus, I took the throttle butterflies out to replace shaft seals.

Kirk