christoph21 Posted June 15, 2008 Share Posted June 15, 2008 Hello all. I have recently restored my third bike, a 1981 Yamaha xs400, and I just got it started up today for the first time. Currently, the bike starts right up with the push of the ignition switch, but does not idle on it's own. Also, when holding the throttle at one point, the bike can sometimes rev up higher and higher on it's own. I just cleaned all the jets in the carbs, floats and everything else EXCEPT the venturi slide as the screws were in rough shape. The venturi seem good though because the carb was already pretty clean when i got it and they slide pretty smooth. Any specific methods to remedy this? I know I probably need to adjust the idle screw but i'm not familiar with yamahas quite yet so a walkthough would be great. thanks Link to comment Share on other sites More sharing options...
christoph21 Posted June 15, 2008 Author Share Posted June 15, 2008 Nevermind. I fixed everything just fine. Damn vacuum petcock... Link to comment Share on other sites More sharing options...
Cy Welch Posted June 16, 2008 Share Posted June 16, 2008 Nevermind. I fixed everything just fine. Damn vacuum petcock... Leaking vacuum? There are several kits available, and frankly when they work right, I love not having to worry about turning them off and on all the time. When I shut it off fuel flow stops, when I start it fuel runs. I have two of them and one leaks and the other doesn't. Haven't really dug into the one that leaks yet since I have a working one, the other is just a spare. Link to comment Share on other sites More sharing options...
christoph21 Posted June 17, 2008 Author Share Posted June 17, 2008 Yeah I forgot to connect the hose to the carb boot. I was anxious after cleaning the carbs out so when it didn't start i panicked and jumped on here. thankyfully it was a quick fix Link to comment Share on other sites More sharing options...
christoph21 Posted June 18, 2008 Author Share Posted June 18, 2008 Well, the bike DID run great. I'm have idling and revving issues again. There are no cracks in the carb boots now after I siliconed them. The jets are clear and floats are set accordingly. I have NOT looked at the venturi slides. I suspect one has a bad vacuum seal. This would lead to a high idle/rev problem yes? Link to comment Share on other sites More sharing options...
Moderator drewpy Posted June 18, 2008 Moderator Share Posted June 18, 2008 Well, the bike DID run great. I'm have idling and revving issues again. There are no cracks in the carb boots now after I siliconed them. The jets are clear and floats are set accordingly. I have NOT looked at the venturi slides. I suspect one has a bad vacuum seal. This would lead to a high idle/rev problem yes? it means a weak mix, pilot jets up to 4k then main jets take over after that. idling issues are nothing to do with the diaphram, the idle sytem are notoriuos for clogging up especiallly if you don't run with an after market fuel filter inline twixt the tank and carb. try anti clockwise unscrew the idle on top front of carb 2.5 turns from bottoming out Link to comment Share on other sites More sharing options...
christoph21 Posted June 18, 2008 Author Share Posted June 18, 2008 Unscrew the idle mixture screw? I take it that's what you meant. Any tips on removing the cap? Link to comment Share on other sites More sharing options...
christoph21 Posted June 19, 2008 Author Share Posted June 19, 2008 it means a weak mix, pilot jets up to 4k then main jets take over after that. idling issues are nothing to do with the diaphram, the idle sytem are notoriuos for clogging up especiallly if you don't run with an after market fuel filter inline twixt the tank and carb. try anti clockwise unscrew the idle on top front of carb 2.5 turns from bottoming out All right. I took the idle mix screws out 2.5 turns like you said. I started it up. It was a good low idle then i gave it some throttle and it went back up and hung at 3500, 4000 RPM or so. What should I do? Should I keep turning the screws out by a half turn and trying again? Link to comment Share on other sites More sharing options...
Cy Welch Posted June 19, 2008 Share Posted June 19, 2008 All right. I took the idle mix screws out 2.5 turns like you said. I started it up. It was a good low idle then i gave it some throttle and it went back up and hung at 3500, 4000 RPM or so. What should I do? Should I keep turning the screws out by a half turn and trying again? My experience with exactly the same symptoms was that I was still way lean. I setup so I had gas with the tank lifted enough to get to the idle mix screws and then adjusted them out till highest idle and starting back down and then came back to the highest idle coming down. I did this to both carbs and had to turn the idle speed WAY down in the process as it went from 1200 RPM to almost 4000 just by adjusting the mixture. I ended up about 5 and 5 1/2 turns out on the mix screws, I know that is way out from spec, but what works works. If you haven't replaced the o-rings on the idle mixture screws you probably need to, but try adjusting it first just in case. Link to comment Share on other sites More sharing options...
christoph21 Posted June 20, 2008 Author Share Posted June 20, 2008 My experience with exactly the same symptoms was that I was still way lean. I setup so I had gas with the tank lifted enough to get to the idle mix screws and then adjusted them out till highest idle and starting back down and then came back to the highest idle coming down. I did this to both carbs and had to turn the idle speed WAY down in the process as it went from 1200 RPM to almost 4000 just by adjusting the mixture. I ended up about 5 and 5 1/2 turns out on the mix screws, I know that is way out from spec, but what works works. If you haven't replaced the o-rings on the idle mixture screws you probably need to, but try adjusting it first just in case. Yeah i figured i needed to keep turning. I have the screws about 3.5 turns out from bottom and i still noticed a little sticking at the 3k line but it comes back down. I think i'll take it out another half turn and that should do it because it's running pretty good right now aside from the 3k thing. Link to comment Share on other sites More sharing options...
Cy Welch Posted June 20, 2008 Share Posted June 20, 2008 Yeah i figured i needed to keep turning. I have the screws about 3.5 turns out from bottom and i still noticed a little sticking at the 3k line but it comes back down. I think i'll take it out another half turn and that should do it because it's running pretty good right now aside from the 3k thing. I would still recommend adjusting them with it running if you can at all get it set up to. Then you can actually find the best setting. When the idle is highest that is about where you want it. You might go back just a little in from that point if you want it slightly lean (slightly lean is not really a bad thing at idle) but running gets it best. When it's right it will drop right back to idle when you let off, no hanging at all. The idle hanging high is a prime sign of it being lean. Link to comment Share on other sites More sharing options...
christoph21 Posted June 23, 2008 Author Share Posted June 23, 2008 I would still recommend adjusting them with it running if you can at all get it set up to. Then you can actually find the best setting. When the idle is highest that is about where you want it. You might go back just a little in from that point if you want it slightly lean (slightly lean is not really a bad thing at idle) but running gets it best. When it's right it will drop right back to idle when you let off, no hanging at all. The idle hanging high is a prime sign of it being lean. I was going to tune up the carbs today but as i was letting the bike idle it just kinda fixed itself. I dunno, i kept the screws out 3.5 turns and everything was great. I put on 60 miles and when i got back it was revving clean and idle perfectly. Link to comment Share on other sites More sharing options...
xtonyx Posted June 23, 2008 Share Posted June 23, 2008 hrrmm.. vacuum leak? are your vacuum ports capped on your intake boots, intake boots snug, etc, etc.. clean the carbs and run it around the block a few times? is your throttle cable getting hung up and pulling the butteryfly open a little? if it's routed through the fork wrong it will do it. Link to comment Share on other sites More sharing options...
christoph21 Posted June 24, 2008 Author Share Posted June 24, 2008 hrrmm.. vacuum leak? are your vacuum ports capped on your intake boots, intake boots snug, etc, etc.. clean the carbs and run it around the block a few times? is your throttle cable getting hung up and pulling the butteryfly open a little? if it's routed through the fork wrong it will do it. What? Nothing of that. I've already repaired that. The bike runs swell now and it's consistent. Link to comment Share on other sites More sharing options...
theoben22 Posted June 24, 2008 Share Posted June 24, 2008 What? Nothing of that. I've already repaired that. The bike runs swell now and it's consistent. Sounds like the carb fairy has been round to your spot and chased them pesky gremlins away Link to comment Share on other sites More sharing options...
christoph21 Posted June 24, 2008 Author Share Posted June 24, 2008 Sounds like the carb fairy has been round to your spot and chased them pesky gremlins away I'd say. It finally came for this one and not the first Honda i had. . . Link to comment Share on other sites More sharing options...
Gnash Posted June 25, 2008 Share Posted June 25, 2008 So my 400 is hanging high upon start up, and in between shifts. I have an 83. I think my carbs are a little different. Does anyone know where my mix adjustment is on my carbs? Do I need to back out on the pilot air jet? I don't see any other adjustments on my carbs other than these and the idle screw. Maybe the carb fairy will visit me. Is she hot? Link to comment Share on other sites More sharing options...
Cy Welch Posted June 26, 2008 Share Posted June 26, 2008 So my 400 is hanging high upon start up, and in between shifts. I have an 83. I think my carbs are a little different. Does anyone know where my mix adjustment is on my carbs? Do I need to back out on the pilot air jet? I don't see any other adjustments on my carbs other than these and the idle screw. Maybe the carb fairy will visit me. Is she hot? I believe through at least 83 Yamaha used Mikuni BS34 III's which have the idle adjust screw under a plug on top of the carb between the main body and the carb mount. You have to drill and pull the plug to get to them and often the o-rings on the screws are pretty much gone and it is almost impossible to get a good idle without them. If they are the same carbs, Mikes XS has kits with the mixture screw spring, washer and o-ring and the plug for when your done adjusting. They were pretty cheap too. High idle usually is lean mix though, at least if it's the hanging kind that comes down after a while. Link to comment Share on other sites More sharing options...
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