YAMAHA XS250 CARB TUNING HELP!

hey mate,
yeah the plugs are still black.
but it should still run though. it wont even run first gear.

I just did a compression test today. not sure if this means anything but i think im suppose to have 140.

Right: when just starting the engine with no revs was 70. then gave it revs to start it and was 125 max.

Left: when just starting the engine with no revs was 50. then gave it revs to start it and then was 110 max

what do you think this means. i know they are suppose to be even and suppose to be higher. how can i fix this issue.

also if the mix is too rich. how can i resolve this.

im trying to give as much information so i can ride this dam thing.i think there is no back pressure

That's low compression with a running engine, have you had top end apart?
Cam timing could be 'off' or something more serious?
First check valve adjustments.
Probably need smaller jets, 110 range if you have the large round jets
set floats 28mm, you don't need more fuel in float bowls
If needle is adjustable, DROP it one notch (so it's deeper into emulsion tube, move clip towards top)

-yes i have rebuilt the top end.
-c clip on the needle is on the last notch. but im confused, you say move towards the top? ive always been told because i have pods to move lower with the c clip. towards the point of the needle. are you saying now i should move it up? so perhaps on the second last notch. so away from the point of the needle?

-i have the smaller head jets. and i have size 117.5 size jet. the standard for my bike.
-floats are set at 25mm. but i find that if i change it to 28mm. no fuel gets in at all. and wont start. just runs out of fuel. i might try the float levels.

- i think the cam timing is ok. i re did all that. do you think it could be the valves?

thanks

People always say lift the needle, problem is, they have no idea how to work with CV carbs
I think you need to check cam timing, if cam is advanced a tooth on sprocket it could account for the low compression?

ok so i have changed the c clip position. now it is second from the bottom notch.
ive changed the floats to 28mm but the problem with that is that the left carb doesnt get any fuel. the needle shuts it off. i think 28mm is too much . its doesnt work at 28mm.

i think the cam timing is fine because i re did the whole top end. and re did the timing etc.

after moving the c clip up the compression on the left seems to be better. and now on the right seems to be worse. both still are not good.

is there anything else? ive done everything that everyone has asked. and done every step. im not sure if people are guessing or they are actually giving me the correct infomation.

Try second from the top
Thinking the cam timing is right doesn't cut it, it's either right or it isn't.
You did keep the 'front run' tight on the chain when doing cam timing, turn it over a few times and re-check?
If right carb gets fuel with float at 28mm, the left should as well.
If it isn't, there is something else wrong
Trying to fix someone's problems on the internet is less than easy
Of course everyone has to guess your skill level and ability, plus, your descriptions are vague at best.
I trained motorcycle techs, several couldn't follow instructions after a demonstration
Your at a disadvantage as we can only try and tell you what to do without being able to see what your actually doing

yeah i have set the cam timing. i kept it tight, and double checked it.

i dont believe the timing is off.

Also so again i change dthe float levels like you said.

Now the left wont get any fuel but the right does. they are the same height. i dont know why left doesnt get any and the right does. they are both set at 28mm. but its cant be correct because now it wont start. because lack of fuel. 28mm is ok for the right carb. but doesnt work for the left carb.

I feel like im going around in circles! and not getting anywhere

OK, cam timing fine.
Do you have stock gaskets in place?
Thicker or thinner gasket will change cam timing slightly
I haven't had an XS250 or 400 in pieces since early 80's, but, I just remembered I do have an XS400 cylinder head and cam to look at.
I'll look at it tomorrow, it's too late tonight
Do you have dual fuel lines or a 'T' piece between carbs?
Could be something restricting flow one side?
If both float levels are the same, fuel flow should be the same, if it isn't, something is wrong

yeah i put all new stock gaskets. and i replaced all the gaskets. i have taken the whole top end apart and put it back together with as many new parts and done it all very carefully.

the fuel line is not a t peice. only goes straight into the left carb and then flows into the right carb.

yeah not sure why the floats are the same but dont work the same.
i hold them upside down they are the same level. i sid them upright. they are the same.

makes no sense.

Dismantle fuel tap and clean out all the drillings.
Yamaha tank filters are easier to remove without damage than Honda ones.
Trek97 was having similar problem, cleaned out tap a couple of times and now bike is running rich ;D
I'm pretty sure XS has a vacuum tap?
Make sure you get springs and diaphragms in correct places

its has a brand new fuel tap. and everything is clean. i can promise you that. the fuel tap works great!

I have taken the pods off and put the original air box back on. i figure i will try and get it running and worry about the pods later.
Trying to get the bike back to stock. then work later on improvements.

So i tried taking it for a ride last night. didnt take off very well. an amount of smoke from the exhausts. but once again when taking off. the bike had no power. i had to give its a sh!t load of revs just to take off and start moving. when wouldnt go into second just conked out. then after all that. the bike wouldnt start again. and started to back fie. however the back fire wasnt from the exhaust. it was like a compression explosion in the engine. not sure what that could be. its just tuned really bailey.

I really think it has to be the carbies.

i hope im not annoying you. i really appreciate your help. but i really have tried everything. perhaps i just dont know what to look or hear for. i will try and give you as much info as i can.

also it is running rich. the spark plugs are black and sooty.

just another side note. there also could be an issue with it not taking off smoothly. i think it could also be the clutch. the gears are good , however it could also be the clutch is not releasing and thats why its not taking off.
just a though, im trying to think of all angels

while its ideling. it revs well. and revs high and it responsive when i pull the throttle. but as soon as i try and ride it. thats when runs like complete crap.

PLEASE HELP!!!!!!!!!!!!!!

It's got to be either cam or ignition timing, valves too tight or something broken internally.
Running too rich will give black smoke from exhaust, you may need to put something light below pipe to see it though
Blue/white smoke is oil burning.
Putting everything stock with carbs and airbox is a good move, if it still has same problem it can't be carbs

Here is my $0.02

My '81 xs250 has BS32 carbs. Completely stock except for k&n pods and runs a bit lean with needle c clip in the middle and main jets are 117.5. I haven't messed with any of the settings, just cleaned, cleaned and cleaned.

Your compression is pretty low - at least 125 would be good. When you did the rebuild did you install new piston rings? Did you lap the valves and check for leaks?

No offence meant with the next questions but it would not be the first time that this has happened.
Are you certain that you did not install the cam 180 degrees out.
Are you certain that the coils are firing the correct cylinders.

How does it run when on centre stand - is it ok? can you go thru the gears? No need to tell you to be very careful when doing this so you don't go thru your garage wall.

What colour is the smoke? Did you change the oil filter and how much oil did you put in?

Can you take lots of photos of all the things that you suspect or unsure about. Video would be better.

Was the backfire out the carbs?

On a side note, don't forget fuel goes stale - I always drain the tank and use fresh 98ron fuel if I haven't ridden for a few months.

If it was me, I wouldn't be riding it until I sorted this out as xs250 internals are rare and pricey and shipping from the US or UK is expensive.

Good luck.

One more thing, did you have much slack on the front side of the timing chain when you installed it? If there is slack then when you install the tensioner it can pull things out of whack.

You can't put the cam 180 degrees out :
It turns at half crank speed so is always 'off' every other revolution, turn crank 360 and it's back to being correct
The points cam is pinned in position so can't be miss-timed to the cam (unless advance mechanism is assembled wrong or pin is missing)
Already went through the incorrect cam timing, says it's good.
Maybe busted rings when being assembled?

hey guys thanks for all your input.

i dont know what to tell you all.

i have followed everyone's info for a long time now. i have done everything as per peoples direction and as per the manual.

The cam timing is fine. i put the cam chain on correct. its the carbs that dont seem to be working. thats what i think. the ike doesnt back fire anymore. it is a very inconsistent bike. sometimes it kinda works other times is does really odd things when i have changed anything.

I have taken the carbs off and on about 200 times now.

This is what is happening at the moment.
- no more back firing.
- jets 110. standard for this bike. stock air box ( removed the pods cos i thought it might be easier to tune without them) c clip on the second last notch
-the floats are set at 26mm. any higher and the floats shut off the fuel and nothing comes into the bowl. so the bike doesnt start.
- i dont understand why, when the floats are exactly the same in both carbs and are the same height. they work differently. the left carb isnt getting enough fuel. so therefore at the moment the left cylinder isnt firing properly.
- the bike has great spark for both sides.
-bike only runs on while choke is on.
-when i give the bike a little throttle, it slowly revs up. seems to be a flat spot as soon as i do it though.
-mixture screws are 1.5 turns out. ive tried more turns but doesnt seem to change anything.

ive checked the cam timing with the light. and it is spot on.

also guys i rebuilt the top end. and was very carefull while putting everything back together. cam chain, tension, pin on crank shaft is fine. i have a brand new points plate and it in correct position.

help would be great please!

please keep in mind i have been working on this bike for over 9 months. and have been given info by many people, some has been wrong. so please for my own sanity, the correct information is vital!

cheers!

Sorry mate but I have come to the end of my knowledge on this one.

How about asking on an XS specific forum?

Good luck.

I forgot to look at the 400 head/cam I have
I will do it later and get back to you.
I don't have a set of BS32 carbs but I do have later XS650 which I believe are the same, just bigger
There are two types used on 650, one has all the brass built into float bowl (BS38) and late ones are more conventional
I can understand how frustrating this is,I'll PM you for e-mail, then I won't have to upload pics o photobucket to post here

Let's stop at the carbs for a moment. How did you check float level? The carb has to tilted until the tang on the float just touches the spring loaded end of the float needle. You can't do them upside down.

You mentioned the clutch in that last post. If you stick the bike in gear when it's not running and pull in the clutch, can the bike be rolled? In other words, is the clutch separating?

Do you have the correct pilot jets in the carbs? There are two types that look the same but are very different. http://pinkpossum.com/GT750/carb/BS40carbs.htm I don't know which yours takes, but they must be the right ones or you will never get it right.

OK, now I want a left and right BS40 for my XS650 with 800 motor ;D
BTW, I posted pics on previous page showing angles for float setting