YAMAHA XS250 CARB TUNING HELP!

Hey Guys,
I have been building a cafe racer from an old xs250 1979 i picked up.

I have re built the top end, put pods on etc. really put in a lot of blood sweat and tears.

I am almost done and nearly able to ride this bike. But i am really really stuck on the carburators.

Currently the bike starts great, runs for a bit and sounds great. 2 things then happen once i turn the bike off choke and let it run on its own. It revs at about 1500 rpm, then after a short while it will rev up on its own to 5000rpm. This is just what its doing currently. Before hand it was not able to run without the choke on. Also note the spark plugs are full of black soot. So its obviously too rich.
So I know the running of the bike is due to the carb setting not being correct. And i really need some help of what setting i should have for the carbies.

Current Carb Details:
-Running pods
-Bigger main jet. 122.5 (due to having pods)
-mixture screws about 1 1/2 turns out. (tried all differant turns didnt seem to fix it)
- float levels 29mm.
-pilot jet (standard)
-butterfly flaps set slightly open.
-c clip on second last notch on the pin.

-Static timing has been done.
-Tapets done etc

If anyone can help me with this guys i really would apreciate it. For the passed few weeks all i have been doing is playing with the carbs trying all differant things. but nothing seems to fix it and make the bike run and idel smooth. If anyone needs for info. let me know.

Thanks so much.

Hi

I can't help with the baseline settings for an XS250 but the following links to some guides that were put together by one of the guys on XS650.com and 650garage.com might help you as the carbs are similar being CV carbs and a couple of good generic guides.

Carbs - a basic guide (sorry don't mean to offend if your knowledge is above this level)
http://star.walagata.com/w/sundialmotosport/C._Carb_repair_helpfull_hints.pdf

Guide to CV carbs as fitted to 650's
http://www.amckayltd.com/carbguide.pdf

Info on tuning CV carbs for both low reving engines (up to 8,000 rpm) and high reving engines
http://www.factorypro.com/tech/carbtun.html

Hope you get sorted soon, for info I had an XS250 new in 1979 and it was always a bitch to start from cold if left for more than a couple of days and I used to stuff a rag into the air intake to restrict air flow which used to do the trick (even when it was completely stock). I ran that bike on open reverse cone mega's and standard jetting with no problems.

Might be worth swapping back to the standard settings to see if it runs better on them.

What exhaust are you running?

Oh thanks guys.
-Cornish Pixie i will have a read through those links you sent, thanks so much.

-VonYinzer i just have the standard exhaust that are original on the bike.

Thanks for your replies guys.

With stock exhaust 122.5 is way too big a main jet.
You need to recheck the pilot jets are clean plus ALL of the drillings/passageways to discharge ports.
If you didn't remove every piece of brass you didn't clean carbs properly
I haven't done XS3250 carbs since about 1978~79 so I'm not sure exactly how they compare to XS 650 carbs, I think they are 'simpler' as they don't have jets in float bowl?

Yes i have thoroughly cleaned the carbs. a couple of times, i have removed all the brass.
Used carb cleaner and compressed air. New gaskets etc.

The xs250 carb has the main jet and pilot jet in the float bowl. I am using bigger main jets (122.5) because i am running pod air filters and needed to use bigger jets, the standard size is 110.

Sounds lean or like it has an intake leak...

OK, they are pretty much smaller version of 650 carbs.
What is the bore size? (34mm CV?)
Are diaphragms 'soft' (they harden up and stop slide moving properly)
You need to use a x10 jewelers loup to check cross drillings in emuslion tubes and check jet holes in pilot system.
You probably need a bigger pilot and smaller main jet, the main doesn't operate below about 4 grand (slide doesn't bottom out, the stop is about 1/4~1/3 up bore)
Are there any external brass tubes?
On 650 I had to clean carbs 4 times because stuff was moving around in drillings, even though air and/or carb cleaner would blow through real easy
Check ignition coils and leads, it may be ignition/misfire causing black plugs and not over rich mixture?
You may be able to use stock jetting with stock exhaust

-Eric123: There is no intake leak because i have new manifolds and everything is tight, i have checked. Also the spark plugs are black, which means the mixture is too rich.

-crazypj: they are 32mm. Mikuni bs32. Through all my research before doing this bike up. everything told me i need to have a bigger main jet because i have pod filters on there. Ignition coils are fine and leads are great. ive checked that.
Current pilot jets are #20 with not holes.
I really have cleaned these carbs well.

Also i put the stock main jets on last week to see if it would change it, and it didnt really make any chance. the bike still did not idel very well at all.

How would having bigger pilot jets help?

crazypj said:

OK, they are pretty much smaller version of 650 carbs.
What is the bore size? (34mm CV?)
Are diaphragms 'soft' (they harden up and stop slide moving properly)
You need to use a x10 jewelers loup to check cross drillings in emuslion tubes and check jet holes in pilot system.
You probably need a bigger pilot and smaller main jet, the main doesn't operate below about 4 grand (slide doesn't bottom out, the stop is about 1/4~1/3 up bore)
Are there any external brass tubes?
On 650 I had to clean carbs 4 times because stuff was moving around in drillings, even though air and/or carb cleaner would blow through real easy
Check ignition coils and leads, it may be ignition/misfire causing black plugs and not over rich mixture?
You may be able to use stock jetting with stock exhaust

Click to expand...

Hey PJ Hey Liam Hi all! *waves*
PJ... the XS250 carbs are BS32's... 32mm bore I'm thinking about this and like you and Pixie and a couple others have said... perhaps putting it back to the orginal settings might help?... IDK.. I've been helping Liam with this.. and he's practically running the same intake setup i was running on my first XS250 cafe... it was a bitch to tune.. but eventually it tuned good.... but.. I'm thinking... I just had an epithany... I was running shorty pipes.. straight thru jobs... Liam is running standard exhaust.. perhaps he's better off going back to the factory main jet size and one size up in the pilot? I'm kinda lost myself hahaha.... but I am all for just giving to the local wrench and throwing some coin at him

Liam.. have you run a 120 in the main yet? You should try that if you haven't... I still think it's getting too much fuel in the Air/fuel ratio... you could try pushing the tabs on the floats back to 37mm setting too ^^ try the 120 with the 39mm float.. if not work.. perhaps try the 120 with the 37mm float setting.. it'll slow the fuel rate a little... you said you got new rubber intake manifolds ay? :s

Yo hey
ok so i have pulled the carbs apart again and going to try and start again.
Im going to get 120 main jet. and try and see if that helps at all.
Steve how can i set the floats at 37mm. are you sure thats correct. they are currently at 29mm.
They wont fit in the bowl if i set them at 37mm.

I dont understand that part. measuring them upside down from the base of the carb to the top of the float right??
37mm?? not sure how that is possible

MCKAY34 said:

Yo hey
ok so i have pulled the carbs apart again and going to try and start again.
Im going to get 120 main jet. and try and see if that helps at all.
Steve how can i set the floats at 37mm. are you sure thats correct. they are currently at 29mm.
They wont fit in the bowl if i set them at 37mm.

I dont understand that part. measuring them upside down from the base of the carb to the top of the float right??
37mm?? not sure how that is possible

Click to expand...

Sorry dude.. typo.. meant 27mm LOL my bad oops ;D
Just leave the float installed in the carb... flip the carb up-side-down... and measure from the lip of where the float bowl meets the carb to the very top of the float... remember.. we did this a while ago... you just need to bend the little tab on the float in a out a little so the float sits lower on the float needle I'll email you if you get to that point... in the meantime.. the 120 jet may do the trick without needing to adjust float. If the 120 works.. you will want to leave flaot as it is anyway probably.. as the fuel will be going thru faster than the factory jet anyway.. and factory float is 26.5 - 29mm approximately... anyway.. see how the 120 goes.. then we go from there... who knows.. over weekend a carb genius who knows the BS32 (I thought PJ would know.. or be close LOL) might just turn up some info... but try the 120 first. As long as your bike is not on fire.. you have no reason to panic LOL ;D

Don't have carb inverted
Tilt it so float tang just touches shut off valve tip without compressing damper spring
Stock XS exhaust is pretty restrictive (and quiet)
'Modifying'it for more noise doesn't usually work (it just gets louder without any performance increase)
You really won't need a large main jet increase as syock airboxes are not too restrictive, and give virtually no restriction at low/medium rpm
You didn't shim needled?
They need to move down not up to lean things out a bit at transition (except they may not be adjustable?)

Hey So you think i should get the 120 main jet?
Maybe having the 122.5 main jet could be the reason why the plugs are black. and the mix is too rich.
I might give 120 a go. i know i need to have a bigger main jet because of the pods.

No i didnt shim the needle.

Thanks for the tips on the floats, i will measure it at 29mm. the manual say about 26mm.
Just to confirm making the floats higher or lower which direction makes them lean or rich? did that make sense.

MCKAY34 said:

Also i put the stock main jets on last week to see if it would change it, and it didnt really make any chance. the bike still did not idel very well at all.

Click to expand...

If you put the stock MJ back in last week and it didn't make any difference, I would say your problem is not with the MJ size but somewhere else. The XS250 is notorious for being difficult to set up right and even when new some XS250's had variable idle speed in the same way that yours is behaving.

The carbs on these have to be absolutely spotless and all components in the carb need to be in spot on condition or new, my guess is that although you have already spent a lot of time cleaning the carbs, they may need to be ultrasonic cleaned or you have a component either failing or not working properly due to wear or slightly incorrect seating on reassembly.

Some good content that might help on this thread from another forum

http://yamahaclub.com/forums/topic/27374-i-think-xs-250-has-beaten-me/

yeah you make a good point mate.
It has been really weird. for ages it would run without the choke on. and then afetr playing around thats when the bike started reving up really high.

Im going to put it back together once again. and see how i go. I might try having the butterfly flaps closed when throttle is shut.

Everything else on the bike is spot on. and this dam thing should finally ride!
a nice new year treat for myself ey boys!!!!!!!!!!!!!!!!!!

Check enricheners are closing all the way and the rubber washers in them are in reasonable condition.(don't need to be perfect to seal)
If they are even slightly open or leak it will be real rich
Also check return springs on enrichener plungers are intact

Well thats a new one for me. Sorry if i sound naive. but i havnt heard or know anything about enrichener plungers.
Maybe i call them somthing else.

Can you explain what and where they are?

I think i have an idea. but i just want to confirm.

Thanks Mate

Most people call them the chokes, but, Kei-Hin use choke, Mikuni use enricheners ( really it's a mini fixed jet carb with huge jet compared to 'throttle bore')
What made me think of it was neighbors ATV 8 stroking away after carb rebuild, he put enrichener plunger in wrong and it was running '1/2 choke' all the time ;D

oh yes yes i know this, i will have a closer look and get back to you.
Im going to put the carbs back together tomorrow and try and start it up again.
i will keep you posted on how it goes, and no doubt i will put some info up on this post needing more help. so stay tunned all.
thank again for you help!