The mysterious Mikuni throttle plate numbers

TwoManyXS1Bs

TwoManyXS1Bs

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Measuring my XS1B's #125 throttle plates, plus #120 and #135 throttle plates submitted by DogBunny.
Thought I'd share some of this trivia.

Throttle plates are an oval shape, as though they were sliced (sausage style) at an angle through a cylinder.
The width of a throttle plate is the diameter of the cylinder from which the plate is patterned.

Placed each plate on my digital compass, leaving a tiny sliver of light between the bevel edge and the compass arm.

Measuring the plate height is total height, including opposite bevels.
Viewed from the edge, the throttle plate resembles a parallelogram.

Parallelogram.jpg

To get the true plate height, a bevel protrusion from one end must be subtracted.

All 3 plates are 0.060" thick.

Depending on bevel angle, must subtract 0.013" (for 12.0° & 12.5° bevels), or 0.014" (for 135.0°bevel)
The bevel edges are very slightly rounded, so will add 0.001" adjustment to total height to recapture the edge.

The plate's base cylinder diameter (its width) divided by the true plate height will be the cosine of the bevel angle.
 
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#120 Throttle plate

Butterfly-#120Plate01.jpg Butterfly-#120Plate02.jpg

The true height = 1.540" + 0.001" (adjustment) - 0.013" (excess bevel) = 1.528"
The ArcCosine of (1.495" / 1.528) = 11.93° (rounded to 12°)
 
#125 Throttle plate

71XS1B-#125PlateAngle.jpg Butterfly-#125Plate.jpg

The true height = 1.543" + 0.001" (adjustment) - 0.013" (excess bevel) = 1.531"
The ArcCosine of (1.495" / 1.531) = 12.45° (rounded to 12.5°)
 
#135 Throttle plate

Butterfly-#135Plate01.jpg Butterfly-#135Plate02.jpg

The true height = 1.388" + 0.001" (adjustment) - 0.014" (excess bevel) = 1.375"
The ArcCosine of (1.337" / 1.375) = 13.50° (rounded to 13.5°)
 
In summary, it appears that:

Mikuni #120 throttle plates have a 12.0° bevel angle.

Mikuni #125 throttle plates have a 12.5° bevel angle.

Mikuni #135 throttle plates have a 13.5° bevel angle.
 
Interesting,but what does all this mean? And what effect will each have on running performance if used on same carb.My 77D has #120 or #125 if i recall.
 
Thanx for your interest, XS650D. This was mostly a mystery chase, just wanting to know if the numbers had a useful meaning. I wouldn't recommend swapping plate types unless you're real knowledgeable on carburetor design and tuning. The write-up on this Weber performance site does a good job of explaining the critical nature of throttle plates, idle and transfer (bypass, progressive) ports, and their relative positioning:

http://www.performanceoriented.com/special-procedures

The very early XS1-XS1B-XS2 carburetors apparently went thru some teething pains, and numerous subtle changes occurred during then, for example:

- Changes to throttle plate
- Changes to fuel jetting
- Changes to sizes and positioning of idle/transfer ports
- Changes to size of bowl vents
- Changes to sizes of air jets

Plus, those early carbs were Solex/Mikuni, with the "Solex" dropped after 1974.

Then, there's the disturbing discovery that my early carbs don't have perfectly round intake throats.
Revealed in these album pics:

full

full


From this album: http://www.xs650.com/media/albums/2684/

A 0.004" difference in throat diameter will cause the edge of the fully closed throttle plate to relocate by as much as 0.009", which would influence the idle and transition tuning.

Maybe those early carbs were mismanufactured.
Maybe Mikuni changed throttle plates in an attempt to fix the warped bores.
Maybe the later carbs have better/rounder bores, ........or not.

I've thought about boring my carbs to the precise 38mm dimension, just to see what happens.
But, they're kinda rare, and I've got them tuned to where I've got no problems with off-idle response.

I'll let others experiment with this...
 
For the record,my 77 has throttle bores that are totally different from each other and far from round,in fact they are so out that i had to sand some aluminum off the inside off the bore just to get the throttle plates to close properly and not bind.The throttle shaft is fine as well as the bushings and seals are new.Just bad casting i guess.I rigged up a piece off clear tubing with some oil in it on a piece off wood and adjusted the carbs via the vac lines to balance Lol,no vac gage!
 
XS650D said:
...my 77 has throttle bores that are totally different from each other and far from round...
Click to expand...

Very interesting! DogBunny and I were discussing this several days ago, and we were wondering if this non-round/non-matched bore issue continued on after these early carbs. Would be great if others would post their findings.

Maybe: IF the BS38 carb throats were precision bored to the proper 38mm, and with good condition throttle plates, off-idle response problems could be resolved.

Or, get worse.

It's possible that the selection of throttle plates, combined with the positioning and size of the idle/transfer ports were made to accomodate faulty carb body castings. And, to fix the throttle bore may introduce an undoing of any manufacturing bandaids.

An experiment for someone with a lot of carb bodies, ......and time...
 
Im sure that reboreing the throttle bores to perfect round dia would help in the dialing in off the carbs for smoother idle and smoother off idle performance.I was actually kinda surprised (after looking at my carbs) that Yamaha, known for quality and precision would allow shoddy workmanship on such a vital and precision part to pass.I guess its a sign off the times back then!
 
I think a lot of the rough low speed running on the earlier BS38s can be attributed to the sloppy way the needle is mounted in the slide. It's pretty loose in there and can bounce up and down near half a MM. That doesn't make for very precise metering. With the '78 carbs, the needle mounting method was changed. It was spring loaded and that helps keep it in a constant and more precise position.

I ran a set of '76-'77 carbs for a bit on my '78 because I had read they were considered one of the best BS38 sets. They do run well in the midrange and upper end but the low speed was noticeably rougher than my original '78 set.
 
5twins said:
...rough low speed running on the earlier BS38s can be attributed to the sloppy way the needle is mounted in the slide. It's pretty loose in there and can bounce up and down near half a MM. That doesn't make for very precise metering...
Click to expand...

Hey, 5twins! Now that's a good idea for a future experiment.

I can easily make shim washers that would fit atop the needles' circlips (probably bond them to the bottom of the nylon retainers), and close-up that up/down gap...
 
Standard M3 washers will work for this ......

iEVv8Mf.jpg


I bought a bag to use for shimming needles so I could get half step settings. If used on the '77 and earlier carbs, they also shim that slop out.
 
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Great info here.I have a bud with a totally stock and mostly original and mint 78 that i have ridden and could compare to my 77.I definately noticed a few things in running performance and will second 5twins observations.His 78 definately ran smoother and slightly stronger from idle and off idle, but once underway my 77 seemed to have more mid and top end.I also noticed that his shifter was more direct but coarse in shifting characteristics.I guess that could be attributed to tuning but we both noticed it right away.Twomany,if and when u try this shim please advise how well it worked and ill give it a go also.Thx
 
From experience you need to be careful about the needle slop, get it too snug and it will bind in the orifice. Found that out on BS34's, they use a spring to keep the needle down, shimming with one washer gets rid of an annoying off idle flat spot on those carbs with US jetting, but adding a second washer binds the spring and needle, then the slide may not return fully, this is not a good thing.
 
I can see the BS34 needle binding up if you shim too much. It's held in the slide by a plate screwed down over the top of it. Eliminate all the play in the little spring with too many shim washers and the needle will get real tight with no "give". On the '77 and older BS38s, a large plastic disc holds the needle in the slide and the slide return spring holds the disc in. The whole set-up is, in effect, spring loaded. It should give a little if need be. But all you're adding here is one washer to eliminate the half mm of slop between the e-clip and the plastic disc.
 
Yes, you can't buy them new anywhere, I don't think you ever could, so scrounging a used one is your only option.
 
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