Carbs - Carburetors

yes I checked ...most of the kits are rubbish to be honest.

I'll find the part numbers and see if I can order the parts I need from a Yamaha dealership. Some I could probably get someone to make like pt #37
 
i have a set of bs34's (stock for my 1980xs) so it is a bit of an assumption they are 34's . stock air box, vacuum petcock. stock exhaust and mufflers.

i turned the mix screw in to find out the setting. and the left was at five. and right was at 3ish . when bottomed out the left turns in farther . ??? normal.?

screws are on the engine side, top centre. i turned them both 2 out and im running rich . . . this will be my first carb tuning. i want to instal pods and short drag pipes, but i want to get it all running good before i start messing with factors .
 
:thumbsup::thumbsup:
- cutaway schematic for a CV carb...eg. BS34..BS38..Keihin
CV_Cutaway.jpg
Simply awesome diagram and schematic for thinking through problems thanks great job.
 
Hi,
Just 've got 1982 xs650 heritage.
Does anybody know how to adjust the carbs? It was hard to get the mixture screws out, but it happened.
Now I need to set them correctly.
I am pretty sure you guys know how.
Thank you
 
Page 1........first post...............first link..........

Read through it thoroughly...............If you post questions without reading the "carb guide" then you will keep getting referred back to it.

The carbs on an 82/83 should be BS34's
 
Why does the carb. guide say that the mixture works in reverse? Is this true on carbs from a 75 all stock no mods. original carbs and does anyone have a good way to check if I'm running to lean I'm about to take a long trip and don't want to damage my engine any help would be much appreciated thanks!!!!!
 
Why does the carb. guide say that the mixture works in reverse? Is this true on carbs from a 75 all stock no mods. original carbs and does anyone have a good way to check if I'm running to lean I'm about to take a long trip and don't want to damage my engine any help would be much appreciated thanks!!!!!

On stock Mikuni carbs the mixture screws do not work in reverse. Screw in CW and the mixture is lessened, screw out CCW and the mixture is increased.

Tell everyone what exhaust you have and do you still use the stock airbox and stock type air filter? If you have open exhausts and pod filters, you will run lean unless you have rejetted the carbs.
 
schlad, there are two different types of pilot mixture screw controls in common motorcycle carburetors. The type found in Mikuni BS series carburetors increases engine vacuum over the pilot jet as the screw is turned out, so that more fuel is picked up through the pilot jet and the pilot mixture is richened. As a general rule, the mixture screw will be located forward of the slide tower on carbs with this type of system. Older designs like Mikuni VM series, mechanical slide Bings, and most Amals, use a control system that adds air to the pilot mixture as the screw is turned out, leaning the mixture. As a general rule, carbs with this system have the fuel screw located behind the slide tower.
 
how do you clean the oem stock air filters that come on a bs38 mikuni carburetor on a late 70s xs650? as you know ,they have a wire cage support inside so you cant soak it in kerosene and squeeze it out.
 
I have a 76 650. I'm brand new at this site. Thanks for all the carb links. How do l know if I have CV or VM carbs ? My carb problem is this; on startup & @ idle the left cylinder runs fine but the right doesn't fire til about 2k rpm.pops a nice flame out the right pipe ! Plugs look fine. At idle with air cleaners off a mist wafts from rt cylinder. Any thoughts gratefully accepted.
 
Easy. Click the link to the Carb Guide, and read it. Many versions of the BS-series vacuum carbs were used on different models of the XS650, and often carb sets from one year find their way onto a bike from another year; read the identification material carefully, as well as the carb ID material at Mike's XS. VM carbs are aftermarket items, easily identified by cables passing through carb tops to mechanically lift slides.
 
I've read theough many of the files in the opening post, and these are my lingering q's:

xs650se bs38 carbs

Air mix screw: why do they say go all the way down and then turn out more? Wont that do damage to the screw tip?

Needle jet (z2): mine fits snug but is it supposed to face any particular direction?

Idle screw: After bench synching butterflies, I wasnt sure what setting to leave idle screw at. I read that the left screw should be screwed all the way down. But That leaves the butterfly traps slightly open... I cant see this being right as I'm sure they need to close when rider is off the throttle.
 
Air Mix screws: turning them in lightly won't hurt any thing. Just bottom them out so you know your at zero. And then you know how many turns out from zero you are. Count and document how many turns they are before and after for reference, they don't always match up exatly. "Every carb is its own animal"

Needle jet points down. If it doesn't slide nice clean it better. Maybe lightly sand with <600 and polish.

Idle screw: is ajusted when you are idling . turn it out to lower idle. I adjust mine all the time at red lights when I notice I'm idling high.

Disclamer . I am by no means an expert . so yeah. Just a fellow bike fiddler .
 
I've read theough many of the files in the opening post, and these are my lingering q's:

xs650se bs38 carbs

Air mix screw: why do they say go all the way down and then turn out more? Wont that do damage to the screw tip?
No the tip doesn't touch anything at any setting ,the pilot air flow restriction is made by the stepped part further up the screw.

Needle jet (z2): mine fits snug but is it supposed to face any particular direction?

No .

Idle screw: After bench synching butterflies, I wasnt sure what setting to leave idle screw at. I read that the left screw should be screwed all the way down. But That leaves the butterfly traps slightly open... I cant see this being right as I'm sure they need to close when rider is off the throttle.

If you initially bench sync the butterflys a good method is to blow into the carb at the butterfly end like a trumpet. You can adjust the sync screw until you feel an equal pressure when blowing into both the carbs then set both the throttles with the idle set screw.
A good starting point is to have the butterflies almost closed so that blowing into the carbs requires quite a bit of pressure and your cheeks fill up with the back pressure. Butterflys often do not fit precisely in the venturi so using drill bits and pieces of paper etc might not necessarily give you equal air flow when bench syncing

When the carbs are fitted you should adjust the throttle shaft idle screw to get a reasonable initial idle speed (don't touch the central sync screw setting) so that you can run the bike warm the engine up and then set the idle mixture using the air screws
 
Butterflys often do not fit precisely in the venturi
[QUOTE 650Skul] lThis is bad advice.................Butterfly Plates have to fit precisely, if they do not then the carbs will never be in sync.

If the plates will not seal then look for reasons that could be causing the problem.
I will post my answer to yesterdays accusations of being, a troll, a raciest, not here to help and to ignore me, #120, here today

This is a "Tech" thread and the answers will be used for solving problems. If someone posts an incorrect statement that will/could lead to ongoing problems for someone looking for help, and i see it and know it to be misleading/wrong, i will correct or dispute the information.

If there is a difference of opinion then i would advise anyone reading the dispute to do some research though the Google search, above left, or through members handles for posts and threads created. This will help determine the experience of the member. Post counts count for nothing

I know enough about carbs to suggest that a trumpet player who hasn't hit a bad note in.............. 10 years.............. may..........and i stress...........MAY........... be able detect a slight variation in wind velocity when bench sync-ing carbs by putting the throat of the carb into your mouth and blowing.

An easier and accurate way is to ensure each butterfly is centered and seated properly with a visual check against a background light.
Place a piece of paper, same thickness, under each closed butterfly. Using the throttle lever the paper should be able to slide out as soon as there is any pressure applied to the throttle. Do the same with the other carb. For linked carbs this process can be done when the carbs are separated but the adjustment screw will have to be backed off before the carbs are linked again.

If this is done when the carbs are linked, back off the sync/throttle adjustment screw, and do the left carb first. Tighten the screw till it beds without opening the butterfly, again use the paper to check that the butterflies open together and the paper slides out at he same time.

The other 2 accusations i do not need to address
 
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ignore Skull .:D
He won't post anything helpful to you.....I've put him on IGNORE :laugh:
He's just here to Troll me cos he's got this thing about Pomeys :laugh::laugh:

The advice I gave you about bench synching the carbs is good ...hope you get somewhere today.:thumbsup:
 
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