'99 115 Yamaha choke butterfly plates are different

[brodmann]

(1998 S115TLRX)I'm sure I've just never noticed this before, but rebuilding my carbs and while scrubbing and cleaning I just noticed that the holes in the choke butterfly plates have different sizes. On the top carburetor, the holes in the choke plate are abut 1/8" in diameter, but the bottom carburetor has holes that are about 1/2" in diameter. Is this common, and if so, why? I'm just not sure why we restrict the airflow to 2 cylinders so much more than the other 2.

 

[boscoe99]

Depending upon their position I can see one set of carburetors allowing more or less fuel to flow. Which is why some carburetors use different idle and main jets. The carburetors with the smaller choke plate holes must need a bit more help drawing fuel into the carburetor when it is cold.

 

[brodmann]

It still doesn't make sense to me that the top 2 cylinders get 5 times as much air when choked than the lower 2 cylinders do. We're not talking about a slight difference here. I'm just guessing that basically only 2 of the cylinders are actually "being choked" and that should be enough to get the motor started. It's not a problem, just a curiosity.

 

[boscoe99]

Formula for the area of a circle is pi X r squared.

Area of a 1/8" diameter circle is 0.012272

Area of a 1/2" diameter circle is 0.019635

The area of the larger hole is not even twice the area of the smaller hole. Not 5 times as large.

Your first post said the choke plates with the smaller holes are on the top carburetor pair. Your second post indicates the larger holes are a on top.

If indeed the smaller holes are on the top pair of carburetors I can see the top pair of carburetors as causing a bit more fuel flow with the choke plates closed. Or perhaps attempting to equalize the flow of fuel through both pairs of carburetors since one is higher than the other.

 

[99yam40]

It is hard to keep up with if they keep changing things around

but I think your area of 1/2" is one decimal place off 0.19635

 

[boscoe99]

It is hard to keep up with if they keep changing things around

but I think your area of 1/2" is one decimal place off 0.19635

Oops. You are correct.

 

[boscoe99]

Choke plates restrict the flow of air so as to increase the flow of fuel. Some have holes and some don't. I can't imagine the two different sized holes being a mistake. I am just guessing as to why they are different. But if if it is a manufacturing mistake then I will not argue against it.

 

[racerone]

???-----The inline 6 Mercury 2 stroke only had choke plates on the top 2 of 3 carburetors !-----The idea on the YAMAHA here is that a motor in good condition will start with full choke on 2 cylinders.-----And will not flood and stall because bottom 2 cylinders are getting plenty of air.-----I do not believe there is anything wrong here.

 

[brodmann]

I was just curious and asking for an explanation. I guess choking just 2 of the 4 cylinders is just enough to get it started when it's cold, as I mentioned above. I don't remember actually if the smaller holes are on top or bottom, and not sure that that would change my curiosity or the answer to the question. I also never thought that it was a manufacturing error.
And everyone here failed the math test!!
Yes area is PI X Radius squared. An 1/8" diameter hole would have a 1/16" radius or .0625" Radius .0625 squared is .00390625 X 3.44 is .0134375
The radius of a 1/2" hole would be .25. .25 squared is .0625 then multiplied by 3.44 is .215. .215 is is exactly 16X .0134375!! (.215/.0134375). Do you know why that is? Because 1/2" is 4x the size of 1/8" When you square 4, you get 16!! Common sense would tell you that an 1/2 of anything is much more that 1/8 of something. Like a pizza!! With 8 slices!!

 

[99yam40]

why do you think PI is 3.44 instead of 3.141592654?

 

[brodmann]

Yep. You're right. I usually use 22/7 for PI, but I meant to use 3.14. That's usually enough digits to get a good calculation. Sorry for my arrogance!!