Intro to Carburetors

tommytwohands

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Hey there,
in advance, seriously thank you

Brand new motorcycle rider that could use an extra hand in learning my carburetor. Took on an older bike and I'm trying not to let it get ahead of me. This site has been a blessing since bringing home my 83' XS650, which came with a new carb pictured below. I've attached pics - it is a Mikuni, but I've struggled to figure the model. With some research, I believe it is a BS-34 linked - would appreciate a second opinion on that.

Furthermore, I'm wondering about the locations of the idle screw(s?) and air/fuel mix screws.

In the second picture, there is a screw with a coil touching up against the throttle body. Is that the single idle screw - I've turned it in and raised the idle and taken it a turn out and the idle drops. But I keep seeing people mention idle screw(S)!

The third picture shows a horizontal screw (direct to right of cable) that is on both sides on the top carb side. MikeXS identification pdf labels these are the multiple idle mix screws. But then where are the air/fuel mix screws? Also, whatever the hell they are, they are damned hard to reach from above - any advice on that front?

One more thing - in between the two carbs there is a screw I believe to be the idle sync screw from an identification guide by MikeXS. What is this?
Identification Guide: https://www.mikesxs.net/pdf/XS_Model_ID_Chart.pdf

carbside.jpeg

Is this the idle screw below, running diagonally ?
Idlescrew.jpeg

There is a horizontal opening on each carb - is this the air/fuel mix screw? Or are these the idle screws?
mixscrew.jpeg
 

650Skull

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1; Yes BS34 linked carbs

2; yes that is the idle screw.

3; yes that is called the pilot screw, commonly called the air mixture screw but in reality the air fuel has been mixed before it gets to the pilot screw and what it does is adjust the amount of the Air/fuel mixture flowing into the carb.

Back the screw off = more mixture, (richer)...........screw it in less mixture, (lean),
IMG_8187 resize 1200  crop copyright 2.jpg



crop IMG_8166 2 copyright.jpg
 
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5twins

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You look to have the original carb set on your bike and yes, they would be BS34 carbs. '82 and '83 sets had a cable operated choke, just like yours. This should answer the rest of your questions .....

AdjustScrews.jpg


The mix screws are the same thing as the fuel/air screws, just another name. But these BS carbs use a true mixture screw, it doesn't do any air adjusting. It adjusts about 25% of the fuel/air mix coming from the pilot jet.
 

grizld1

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www.amckayltd.com/carbguide.pdf. The mixture screw in Mikuni BS series carburetors controls engine vacuum admitted to the pilot jet. More vacuum=more fuel drawn through jet=richer low throttle air/fuel mixture. Rule of thumb: if the mixture screw is located on the motor side of the slide tower, the mixture richens as the screw is opened. If the mixture screw is located on the air intake side of the slide tower, the mixture richens as the screw is closed.
 
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tommytwohands

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1; Yes BS34 linked carbs

2; yes that is the idle screw.

3; yes that is called the pilot screw, commonly called the air mixture screw but in reality the air fuel has been mixed before it gets to the pilot screw and what it does is adjust the amount of the Air/fuel mixture flowing into the carb.

Back the screw off = more mixture, (richer)...........screw it in less mixture, (lean),
View attachment 208423


View attachment 208418
Wonderful, you made my day!
 

tommytwohands

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Tommy,

I encourage you to discover the Technical section of this forum.
There are many threads posted in the Carb domain authored by gents who spent a lot of time documenting the functioning of the BS34.
.
.
Didn't even think to check for a specific BS34 thread once I figure that's what I had, thanks a bunch
 
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