Greenerict Posted May 13, 2011 Posted May 13, 2011 Hi, I have a 1980 Yamaha XS400 Special that only fires on one cylinder. Recently, I cleaned the carbs, changed the fuel filter (the old filter was black and really dirty, and debris was floating around after I disconnected it.), and put new fuel in it then it ran fine. I then put some oil in it and it ran even better. Yesterday, I put more oil in it (since I didn't put a lot in it originally) and tried to start it up and it did so reluctantly. It idled at around 750 rpm, and stalled out a few times. It also stalled when I shifted into first gear. What do you guys think is a possible problem? I'm looking around these forums as much as I can for a solution. Thanks, Eric
Moderator drewpy Posted May 13, 2011 Moderator Posted May 13, 2011 the tickover is 1200 rpm so at 750, it won't run
Greenerict Posted May 13, 2011 Author Posted May 13, 2011 Well it was sitting there not moving, and the tachometer was between the 0 and the 1. I'll try it again later today and see what happens. If it happens again, I'll try and take a picture.
HoughMade Posted May 13, 2011 Posted May 13, 2011 Adjust the idle speed to begin with. That's the knob between the carbs shown below. as Drewpy said, 1200rpm
Greenerict Posted May 22, 2011 Author Posted May 22, 2011 New problem. It's not running.I cleaned the carbs and changed the oil today. It started and ran for 2 seconds at the right rpm a little while ago, but except for that it won't start. Before it started, it wouldn't run; so I opened the gas cap then tried to start it and it worked, but maybe that was just luck, because after that it didn't start.
Moderator drewpy Posted May 22, 2011 Moderator Posted May 22, 2011 New problem. It's not running.I cleaned the carbs and changed the oil today. It started and ran for 2 seconds at the right rpm a little while ago, but except for that it won't start. Before it started, it wouldn't run; so I opened the gas cap then tried to start it and it worked, but maybe that was just luck, because after that it didn't start. what exactly did you do to the carbs? maybe gas cap air hole blockage? rust in the fuel tank, running an inline filter?
Greenerict Posted May 22, 2011 Author Posted May 22, 2011 I blew out the jets in the carbs. the diaphragm seemed kind of stiff (just a little) and the gaskets were pretty stiff too. I am using an inline filter. Actually, while I was writing this I went out to take a pic of the filter. And it ran on full choke at 1200, when I put it to half choke it dropped to 1000, and once I put it on no choke it dropped to 500-750 then stalled. It was burning oil, but not anymore (that's my fault). Attached is a picture of the filter. It was clean when I put it in (like peach colored). Uploaded with ImageShack.us
kindest Posted May 22, 2011 Posted May 22, 2011 I blew out the jets in the carbs. the diaphragm seemed kind of stiff (just a little) and the gaskets were pretty stiff too. I am using an inline filter. Actually, while I was writing this I went out to take a pic of the filter. And it ran on full choke at 1200, when I put it to half choke it dropped to 1000, and once I put it on no choke it dropped to 500-750 then stalled. It was burning oil, but not anymore (that's my fault). Attached is a picture of the filter. It was clean when I put it in (like peach colored). Uploaded with ImageShack.us well the petcock isn't switched to on? did you try that. also the fuel filter should have arrow on it to make sure you align it right so the fuel can flow.
Garygazza Posted May 22, 2011 Posted May 22, 2011 well the petcock isn't switched to on? did you try that. also the fuel filter should have arrow on it to make sure you align it right so the fuel can flow. Looking at the picture the petcock is set to prime,which means fuel is gravity feed to the carbs.
kindest Posted May 22, 2011 Posted May 22, 2011 Looking at the picture the petcock is set to prime,which means fuel is gravity feed to the carbs. isn't this bad to leave on it will flood? i've never used the pri feature on my carb ever.
Moderator drewpy Posted May 22, 2011 Moderator Posted May 22, 2011 the xs400 carbs need a real good clean as they are old and finicky buggers. try unscewing the pilot mix screw 3.5 turns from bottom, this seems to offset the shite liquid which passes for fuel today! by carb clean I mean stripped down, everything spotless. does look like it has rust in the carbs before the filter fit?, also the float hight needs checking (26mm top of float to under bowl gasket)
Greenerict Posted May 22, 2011 Author Posted May 22, 2011 well the petcock isn't switched to on? did you try that. also the fuel filter should have arrow on it to make sure you align it right so the fuel can flow. That position is "prime," and it ran in that position
Greenerict Posted May 23, 2011 Author Posted May 23, 2011 the xs400 carbs need a real good clean as they are old and finicky buggers. try unscewing the pilot mix screw 3.5 turns from bottom, this seems to offset the shite liquid which passes for fuel today! by carb clean I mean stripped down, everything spotless. does look like it has rust in the carbs before the filter fit?, also the float hight needs checking (26mm top of float to under bowl gasket) The carbs looked fine before I changed the filter (there was one before). I'll soak the stuff in cleaner tomorrow, and do what you said about the pilot screws, if I can find them. Thanks.
HoughMade Posted May 23, 2011 Posted May 23, 2011 I'm on board with another cleaning of the carbs. How they look means squat unless you have the ability to see inside all the passages. My bet is that the idle circuit is clogged. However, if the bike idled at 1200rpm on full choke (enrichener) why did you take it off choke? It's supposed to idle at 1200rpm. Until the engine speed starts to increase from 1200rpm on its own to close to 200rpm, it's not time for half choke. Until it rises to close to 2000rpm on half choke, it's not time for no choke. Then, when fully warm, it should be set to idle at 1200rpm with no choke. Unless its very warm out or the engine is warm, it should not idle very well (or at all) from a cold start with no choke.
Greenerict Posted May 24, 2011 Author Posted May 24, 2011 I took the carbs apart, and am soaking the parts now. One of the "buttons" on the float bowl needles are stuck, so I should order one of those? Also there was crud on the seat(not sure if that's what it is, but it's what the needle rests in). I have no idea where the pilot mix screw is, the diagram in the book I have is different from my carbs, I'm pretty sure. Also in the book it says the float bowl height should be 32mm.. Is this plug where the pilot mix screw is? And attached is a picture of what type the carb is...just to be sure it's the right one...
Moderator drewpy Posted May 24, 2011 Moderator Posted May 24, 2011 that is a brass plug, take it out by self tapping screw and yank on it. under neath that's where the pilot screw is. That version will have an O ring which will almost certainly have perished. Mikes XS have spares. know the manual says 32mm but if you have brass floats, they are best @ 26mm
Greenerict Posted May 24, 2011 Author Posted May 24, 2011 that is a brass plug, take it out by self tapping screw and yank on it. under neath that's where the pilot screw is. That version will have an O ring which will almost certainly have perished. Mikes XS have spares. know the manual says 32mm but if you have brass floats, they are best @ 26mm Will Part #48-5012 on Mikesxs work? And do I need a new float needle? Will Part #48-5014 for that?
Moderator drewpy Posted May 25, 2011 Moderator Posted May 25, 2011 Will Part #48-5012 on Mikesxs work? And do I need a new float needle? Will Part #48-5014 for that? if you have sight of your parts and they look the same, i would get them. on part #48-5012 I would look at a new needle kit instead/as well, as I'm sure it wil be worn.
Greenerict Posted May 28, 2011 Author Posted May 28, 2011 if you have sight of your parts and they look the same, i would get them. on part #48-5012 I would look at a new needle kit instead/as well, as I'm sure it wil be worn. I drilled the plugs out, and only the screw and a spring came out on both sides. No o ring or washer did. And they seemed to be screwed in pretty tight... Oh and mikesxs is out of plugs...Sigh..
vonnieglen Posted May 29, 2011 Posted May 29, 2011 Check your compression. If it is low... hook an air supply to an adapter in your spark plug hole. Listen or feel where the air is going when the piston is at near top dead center when both valves are closed. From your symptoms... it is likely that you have a sticky exhaust valve. If you do you will be able to hear air escaping through the exhaust system. The adapter that I use to hook up to the air supply came with my cheap compression tester ($10 at Harbor Freight). If you take your oil fill plug out and air comes out there you have stuck or broken rings. If air is coming out back through your carburetor your intake valve is sticking or broken. If air is escaping around the top of your cylinder... your head gasket is probably blown. Everyone always thinks the carbs are at fault when their bike starts to run bad, but sometimes it can be other things.
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