YAMAHA XS250 CARB TUNING HELP!
- Thread starter MCKAY34
- Start date
hey guys. this is the conclusion that i have come too. this bike is extremely hard to tune with pods on. they simply will not work with pods on. some people may have got lucky and done it for this specific model. however i am not having the same luck.
So i am just going to run the stock air box for now. so just waiting on some parts for the air box. so i can get this bike back to stock and tune it and run it this way. i have had the float levels, c clip , jets and mixture screws at all different types of levels and settings. and it still doesn't run.
ive been working on the bike with pods for over 9 months now and have not really fixed the problem. i believe that its the pods. the bike is simply not tuned correctly to what it should be with pods. every man and his dog has a different opinion on how all the settings should be. So who is correct?
Im taking the bike back to stock. and seeing if it runs and can be tuned. i will get back on and let you guys know the outcome.
any thoughts?
So i am just going to run the stock air box for now. so just waiting on some parts for the air box. so i can get this bike back to stock and tune it and run it this way. i have had the float levels, c clip , jets and mixture screws at all different types of levels and settings. and it still doesn't run.
ive been working on the bike with pods for over 9 months now and have not really fixed the problem. i believe that its the pods. the bike is simply not tuned correctly to what it should be with pods. every man and his dog has a different opinion on how all the settings should be. So who is correct?
Im taking the bike back to stock. and seeing if it runs and can be tuned. i will get back on and let you guys know the outcome.
any thoughts?
porterhouse
New Member
K&N foam pods work fine on my '81 xs250 - a tad bit lean with stock jets but did not cause any issues.
fyi - the guys on the xs400 forum recommend if you are using cheap emgo chrome pods then put the pods on the end of the stock air filter hose where you would put the stock air filter box.
fyi - the guys on the xs400 forum recommend if you are using cheap emgo chrome pods then put the pods on the end of the stock air filter hose where you would put the stock air filter box.
porterhouse
New Member
Well spotted Crazy. I do have uni filters & a mushy brain. 
)
Sent from my GT-I9000 using Tapatalk
)Sent from my GT-I9000 using Tapatalk
I know pods do work on the xs250 I have seen it. But at the moment they are not working on mine. I see the best option is to go back to stock just to get it going and rideable . This way I know it will run and nothing different is trying to be changed on the bike ie pods.
Because I've been trying to tune it for over 8 months with pods and took everyone's advice and still didn't get a disable bike.
I'll keep you all updated
Because I've been trying to tune it for over 8 months with pods and took everyone's advice and still didn't get a disable bike.
I'll keep you all updated
So here is where im at.
I put the stock air boxes on the bike. so now everything is at factory settings.
Problems.
The bike wont run without the choke on, when giving the bike throttle it has small backfires in the air box. and also i think there might be an air leak somewhere, because after a short while of running. its revs itself up to 5000 revs. so that say to me an air leak. either way no real change with the air boxes.
but i will say when i first started it. its sounded more solid.
But still no happy
I put the stock air boxes on the bike. so now everything is at factory settings.
Problems.
The bike wont run without the choke on, when giving the bike throttle it has small backfires in the air box. and also i think there might be an air leak somewhere, because after a short while of running. its revs itself up to 5000 revs. so that say to me an air leak. either way no real change with the air boxes.
but i will say when i first started it. its sounded more solid.
But still no happy
ok so after more playing , not much playing. just with the mixture screws.
Everything is now stock on the bike.
Mixture screw is 2 full turns out, the manual says 1.5 turns.
Problems.
- doesnt run without the choke on.
- after a short while the bike revs itself up to 5-6000 revs. on its own. (thats says to me air leak. but manifold etc are all new and done up tight) could somthing else cause this? maybe lack of petrol.
thoughts anyone?
Everything is now stock on the bike.
Mixture screw is 2 full turns out, the manual says 1.5 turns.
Problems.
- doesnt run without the choke on.
- after a short while the bike revs itself up to 5-6000 revs. on its own. (thats says to me air leak. but manifold etc are all new and done up tight) could somthing else cause this? maybe lack of petrol.
thoughts anyone?
porterhouse
New Member
I think most people do 3-3.5 turns out.
Out of interest, how do the plugs look?
Out of interest, how do the plugs look?
If EVERYTHING is stock those carbs should not need to run on choke. That and the2 turns out comment suggests that the pilot jets are still partly blocked or the air passages to them are blocked. It's also possible that there's an air leak somewhere - usual culprit is the rubber carb holder where it bolts to the head or has tiny cracks in it. You mention that the rubbers are new and done up tight. That suggests maybe too tight and they have bowed.
Let's try something different. Does that bike have stubs for vacuum synching the carbs? If it does, are they capped off ?
OK, so remove the carbs and get an expandable pipe plug from your local hardware store. They look like a fat rubber doughnut with washers either side and a bolt through it. Insert that into the intake rubber and expand it enough to get it to seal. You can slather it with petroleum jelly to help it seal if necessary.
Next grab a length of tube and slip it over the synch pipe/connector/adapter and add air pressure - you can blow into it. That's all it needs. make sure the valve is closed though.
With your spare hand paint a mix of soapy water around the rubber where it meets the head and as you blow into that tube, watch for air bubbles in the soapy water.
It's just much easier to find a leak that way. That's how you find leaks in gas pipes at home too.
Another tool you could make up is a plug the same size as the spigot on your carb. Drill and tap that to take a tire valve and use a pump to apply pressure. If you have a Motion Pro leak down tester, you already have both of those tools plus a small hand pump.
Let's try something different. Does that bike have stubs for vacuum synching the carbs? If it does, are they capped off ?
OK, so remove the carbs and get an expandable pipe plug from your local hardware store. They look like a fat rubber doughnut with washers either side and a bolt through it. Insert that into the intake rubber and expand it enough to get it to seal. You can slather it with petroleum jelly to help it seal if necessary.
Next grab a length of tube and slip it over the synch pipe/connector/adapter and add air pressure - you can blow into it. That's all it needs. make sure the valve is closed though.
With your spare hand paint a mix of soapy water around the rubber where it meets the head and as you blow into that tube, watch for air bubbles in the soapy water.
It's just much easier to find a leak that way. That's how you find leaks in gas pipes at home too.
Another tool you could make up is a plug the same size as the spigot on your carb. Drill and tap that to take a tire valve and use a pump to apply pressure. If you have a Motion Pro leak down tester, you already have both of those tools plus a small hand pump.
I have read through this whole thread and it seems that the problem is not with your carbs at all. Through different adjustments and settings it has not changed a thing. I believe the problem is with either the ignition timing of your engine or your cam is not timed properly. Another reader also suggested the same as you mentioned you had rebuilt your top end.
Symptoms you have mentioned:
Black sooty plugs. Adjusting your carb has not changed this. Which means this is not causing the problem. If your engine timing is out then the combustion cycle will be shortened to not allow the fuel to fully burn and soot up your plugs.
Engine backfires through carb. This means that your inlet valve is opening too soon. ie. engine timing is out.
Engine revs to 5000 with no input. You can make an engine rev up and down by advancing and retarding the timing of the spark with no throttle input. ie. engine timing is out and the engine is running is an advanced or retarded spark state.
Engine only runs on full choke. Tough one. The choke normally causes a higher negative pressure in the inlet side of your engine. If the timing of your engine is out your inlet valve may be opening too soon. If this is the case then you will get positive pressure into your inlet side. This will then cause the choke to have the opposite effect and restrict the outflow of gasses. Once the cylinder has moved on to suction the choke will more easily allow gasses to start flowing in the right direction. Once you open the choke the gas will flow all the way out your inlet making it harder for the fuel air mix to go back in for the next stroke. I hope that makes sense.
No power. Again ignition and engine timing.
Symptoms you have mentioned:
Black sooty plugs. Adjusting your carb has not changed this. Which means this is not causing the problem. If your engine timing is out then the combustion cycle will be shortened to not allow the fuel to fully burn and soot up your plugs.
Engine backfires through carb. This means that your inlet valve is opening too soon. ie. engine timing is out.
Engine revs to 5000 with no input. You can make an engine rev up and down by advancing and retarding the timing of the spark with no throttle input. ie. engine timing is out and the engine is running is an advanced or retarded spark state.
Engine only runs on full choke. Tough one. The choke normally causes a higher negative pressure in the inlet side of your engine. If the timing of your engine is out your inlet valve may be opening too soon. If this is the case then you will get positive pressure into your inlet side. This will then cause the choke to have the opposite effect and restrict the outflow of gasses. Once the cylinder has moved on to suction the choke will more easily allow gasses to start flowing in the right direction. Once you open the choke the gas will flow all the way out your inlet making it harder for the fuel air mix to go back in for the next stroke. I hope that makes sense.
No power. Again ignition and engine timing.
ok thanks for that.
i agree somthing is not right because i have done so many different setting with the carbs, but still does not make any difference. i do understand that carbs are difficult to tune. i get that. but i must have gotten close to tuning them correct to at least the bike running and rideable. but still haven't got to that stage.
Ok so you say it could be the timing.
i have done the static timing. and done it with the little light, hooked up to the points plate etc. i did the LF and the RF. and it looked perfect to me. no slack.
i also did the running timing with the strobe light. it was a tiny bit off while it was idleing. so i am going to fix that. it may be because of the idle turned up too much. however when i did rev it. it moved to full advance with no problem.
the gap of the points seems to be ok. i dont know dude im running out of ideas to be honest. it really shouldn't be this difficult. i get its not easy doing this. but it shouldn't be this hard. which makes me belive perhaps i still have not found the issue.
the right mixture screw is not in great shape either. i dont see how it would be the issue. but im just being picky now. it just arrived, its the correct type. but the thread is different so i cant use it and also i cant get the one i need. so it will have to do.
any other thoughts would be great. i'll fix the running timing with the strobe light. but im not hopefull that would be the problem.
i agree somthing is not right because i have done so many different setting with the carbs, but still does not make any difference. i do understand that carbs are difficult to tune. i get that. but i must have gotten close to tuning them correct to at least the bike running and rideable. but still haven't got to that stage.
Ok so you say it could be the timing.
i have done the static timing. and done it with the little light, hooked up to the points plate etc. i did the LF and the RF. and it looked perfect to me. no slack.
i also did the running timing with the strobe light. it was a tiny bit off while it was idleing. so i am going to fix that. it may be because of the idle turned up too much. however when i did rev it. it moved to full advance with no problem.
the gap of the points seems to be ok. i dont know dude im running out of ideas to be honest. it really shouldn't be this difficult. i get its not easy doing this. but it shouldn't be this hard. which makes me belive perhaps i still have not found the issue.
the right mixture screw is not in great shape either. i dont see how it would be the issue. but im just being picky now. it just arrived, its the correct type. but the thread is different so i cant use it and also i cant get the one i need. so it will have to do.
any other thoughts would be great. i'll fix the running timing with the strobe light. but im not hopefull that would be the problem.
10 years ago I had a 1971 Holden LC Torana (An Aussie car). I used a strobe gun to set the timing perfectly on the old girl and it ran like crap, with much the same symptoms as you are describing. My advice is to get the bike to run on the choke as I guess you can. Then put the strobe gun down and just play with the timing by ear. If you don't have any luck then you can just set the timing with the strobe gun again. Also clean up the spark plugs if there is too much build up on them as this can effect the spark.
porterhouse
New Member
Hey McKay, bummer that this problem is still giving you troubles. I suggested you post on an xs specific forum but haven't seen your name show up. I can tell you that the guys on xs400.com have been invaluable help to me fixing my bike. Troubleshooting an xs400 is basically the same as trouble shooting an xs250. Note that the xs400 runs a bs34 carb but from experience, info relating to that carb can be applied to the bs32.
These bikes are not rocketships but can be fun for bombing around the city - so once running ok you should be happy with it. I run a 39 tooth rear sprocket so I do have to use the gears a bit but that is half the fun.
Good luck mate and don't give up.
These bikes are not rocketships but can be fun for bombing around the city - so once running ok you should be happy with it. I run a 39 tooth rear sprocket so I do have to use the gears a bit but that is half the fun.
Good luck mate and don't give up.
Monaro had some good ideas in checking things like valve timing and ignition timing, but the choke issue is a non starter if the bike has BS mikuni crabs. They use an enricher circuit and do not "choke" off air.
Sooty plugs mean rich mixture - at some point in the rev/throttle range - maybe not at idle though.
Revving up on its own suggests either timing changing on its own or an air leak. Did you check for an air leak around the intake manifolds?
Did you check that there are not vacuum synch connectors or that they are capped off?
Are the slides lifting normally?
Sooty plugs mean rich mixture - at some point in the rev/throttle range - maybe not at idle though.
Revving up on its own suggests either timing changing on its own or an air leak. Did you check for an air leak around the intake manifolds?
Did you check that there are not vacuum synch connectors or that they are capped off?
Are the slides lifting normally?
porterhouse
New Member
porterhouse
New Member
Thanks Crazy, I will check those valve springs (gotta stop working on this when I am half asleep). According to Haynes, you set the crank at LT and the cam pip at 12o'clock.
Sorry for the thread hijack.
Sorry for the thread hijack.
