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XS400 HIGH IDLE!!


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Hello Everybody!

I am new here and though this is my first post, I have frequently referred to these forums and found the member's and their advice to be extremely knowledgeable and helpful.

Anyway, I've had my 1980 xs400 for just shy of a year now. Generally it has been running very good, but I've noticed that if I go for a ride, it starts to suffer from HIGH IDLE SYNDROME after about 10 minutes when it really starts to heat up. After researching online and in my haynes manual, I decided to take the carburetors apart to clean them since the last owner had it sitting for a long time. I thought the cleaning went well; I noticed there was some gunk on the upper part of the needle, where it connects to the slide and diaphragm, as well as the pilot jet. From what I've read these can both affect idle performance. I also noticed there was some crud on the fuel/air mixture screw which is accessed from the outside (although the bike is 1980 the previous owner must have removed the brass plugs which I think are supposed to be there).

After cleaning I reassembled the carbs and reinstalled them on the bike. I connected the fuel line and it fired right up. Also, I made a homemade manometer using automatic transmission fluid, and got the carbs pretty close with that. Then I adjusted the mixture screws to about where they were when I took them out. The bike seemed to be running well, but alas! After I rode it for a while and it heated up the idle was again stuck at around 3500 to 4000 rpms. From what I've read this is a problem encountered by a lot of xs400 owners. Also, when the engine is under load, as in when i am going 30mph and above, it seems fine and is revving as it usually does, around 5500 to 6000 rpm. But as soon as I hit a stoplight, it continues to idle at 3000 to 4000. If I turn the choke on when it is idling high, it goes up even higher.

I've read that this high idle is indicative of a lean fuel mixture, but whenever I turn the bike off now (after cleaning the carbs), I get like a pop (I would'nt call it a backfire, not as loud as a gunshot) from the exhausts. Would'nt this be indicative of a fuel rich mixture??

Can anyone help me find a fix?? I think I may have screwed up the setting on the fuel air mixture screw! Can anyone suggest a position to put this at? I've read that the factory spec is 1 1/4 turns out from the needle being lightly seated, but that they were set up lean due to epa regulations.

Thank You!

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Go to 3-3.5 turns out and try it. Popping is more often lean than rich. In these old bikes, there is always some fuel in the exhaust. It will not ignite in the exhaust unless the exhaust is way too hot- and that's caused by lean.

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Interesting post. My 1980 XS400 does the same thing. I researched thoroughly in the forum and decided to check the idle mixture screw adjustment. The caps had also been removed on my bike by a P.O. The jets were set to about 2+ turns up. I increased them to 2.5 turns and the problem got even worse. I tried 3 turns with an even higher fully warmed up idle - now over 3K rpm. A couple days ago I set them back to 2 turns and now the warmed up idle is just under 2K rpm. Tomorrow, I'll drop it to 1 3/4 turns out and see what happens. I find these results curious! I don't get lean backfire and the bike runs very well otherwise. The factory setting was supposed to be 1.5 turns out and that may be a little too lean for some engines but maybe not for others. When you don't know the history of what has been done previously to these bikes, it makes corrections a little more challenging.

Now, I know the o-rings on the idle mixture screws can become tired and leak. I have a new set of screws, springs, etc from MikesXS and will replace the originals when I pull the carbs off. At the same time I will install new choke rod rubbers and butterfly shaft gaskets - when they are in stock again.

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Thanks for the input Ollie. But yea I agree, sometimes it does'nt seem to make sense when you adjust the pilot screws and the result isn't what you expected. It leads you to believe that the problem is originating from somewhere else. And I agree, it does make it a challenge when you don't know what the previous owner has "rebuilt" or possibly changed for the worse. I can tell that my carbs have definitely been opened several times by the stripped out float bowl screw heads.

Is this the screw, washer, and spring you ordered from mikes xs Ollie?

$10.00 USD Ea

Fuel Idle Mixture Screw with spring, washer and O-Ring

Part #48-5011

I notice that it looks to be made of steel or aluminum? I noticed the mixture screws in my carbs were made of brass. I am guessing the material isn't a big concern as the function is the same.

Should this be the first thing I should consider replacing if adjusting the idle mixture screws doesn't seem to help? I had both carbs apart yesterday, and the small o-ring looked alright, but it was so tiny that it is really hard to examine it properly.

Also, are there complete carb rebuild kits for the mikuni available anywhere? Like ones with new gaskets, jets and the whole nine yards? Or is it necessary to source the parts individually? I notice that mikes xs has rebuild kits, but I was wondering if a mikuni bs34 rebuild kit for one model of bike would be different from one for another bike because of the sizes of the jets and things like that.

I noticed with my carbs apart yesterday that my main jet said 135 on it. I also noticed on this jet that an R as well as a little square symbol were stamped. I am assuming that if I were to buy a new main jet I would get the same one, one that was size 135. Are there any other jets where size matters though, or are they universal? I did not see numbers stamped on some of the other jets such as the pilot jet.

Also Ollie, when you said, "The caps had also been removed on my bike by a P.O." what does P.O. stand for?

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you seem to be getting good help but i thought i would try to help aswell .

firstly the factory settings on a older bike are just a starting place and generally a fair bit out because of ware ect.

your symptoms are classic lean mixture or air leak.

try making the bike run over rich and see if the tick-over improves, then if it does turn the screws in until just before it starts to increase the revs.

it can be quite a balancing act on worn carbs to an extent where refurb kit is your only answer.

my yzf 750 runs poorly at 2, 1/2 out(factory) has similar promlems to you at 2 turns and is perfect at 2,1/4 so be patient.chris

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BVD,

P.O. = "Previous Owner"

The kit I received from MikesXS does have the brass idle mixture screw but it wouldn't really matter the metal type. Yep, that's the kit I got. I haven't replaced these on my carbs yet because you really need to take the carbs off the engine to get the screws out easily and I am waiting on the butterfly shaft seals to be back in stock at MikesXS before I do the complete job. Besides the o-ring, the idle mixture screw itself can be damaged if someone tightened it too hard and deformed or broke off the tip. This would cause inaccurate gas flow at what you would think to be a proper setting.

AFAIK, the only difference in a carb rebuild kit of the same model would be different jet sizes for different size engines. I really haven't searched out sources for a complete kit so can't help you there. Also haven't opened my carbs since this was done by the PO so I don't know jet sizes in them or any other XS400 carbs. Others here with more knowledge can help you there. On my bike, I am betting on vacuum leak(s) to be the cause on my bike. After I replace the various seals I will then do a valve clearance and timing check and balance the carbs and see what happens.

While the carbs are relatively simple, there are several areas which can cause problems: bad gaskets, leaking seals-butterfly shaft, choke rod, o-rings, improper float level setting, malfunctioning float valve, clogged jets-sometimes with almost microscopic deposits hard to see and settings-idle mixture screw, slide diaphragm & needle setting (if adjustable) and carb balance.

Hope this helps. :D

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Thanks for all the advice and suggestions guys. You have all been a great help and I really appreciate it!

HoughMade, I tried exactly what you suggested. I set the idle mixture screws to 3.25 turns out, started the bike, yet it still was idling very high. Then I looked at the throttle stop screw and realized that it was screwed all the way in, to the point that the spring under it was completely compressed, hindering it from moving any more. So I then backed it out considerably, yet the idle was still high. So I then shut off the bike, restarted it, and lo and behold it was idling at a reasonable speed. I then continued to fiddle with the throttle stop screw until the engine speed was about 1200 rpm. I then took it for a 15 minute ride and everything seemed great. Whenever I pulled in the clutch the engine happily settled to 1200 rpm, so I happily putted home and then set the idle mixture screw at 3 turns out, and it still seems to be running great.

I was really fooling with the throttle stop screw yesterday, and must have turned it all the way in without realizing it.

The only thing that seems strange now is the choke. Before I cleaned the carbs pulling the choke out all the way would rev the engine to about 4000 to 4500 rpm. On the middle setting it would rev at about 3500 rpm, and pushed all the way in the bike would idle normally. Even after I cleaned the carbs the choke worked in the same way. Now all of a sudden, when I got the bike to finally idle correctly, the choke doesn't seem to have any effect. I can't start the bike with the choke lever pulled all the way out. If I put the choke lever in the middle position the idle speed will increase slightly to about 1500 to 2000 rpm, and if I push it all the way in, the bike will idle normally at 1200 rpm. But now if I pull the choke all the way out the bike will simply shut off. This seems really strange to me. Anyone have any suggestions? Maybe my choke gaskets are bad??

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Just got back from a ride, and now the choke works when in the middle position, revving to 3500 rpm. It still has no effect in the fully out position though.

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Just got back from a ride, and now the choke works when in the middle position, revving to 3500 rpm. It still has no effect in the fully out position though.

On a cold start, when I pull the choke out to the 2nd notch and hit the starter button the engine will fire and die. I then push the choke in to the 1st notch and it fires right up (may need a few throttle tweaks) at a slightly elevated idle. Could be things are working fine and your bike just doesn't need the 2nd, richer setting. You can provide too much fuel on a cold start and flood out the engine.

One thing we must keep in mind is that these bikes are not fuel injected. I don't say that to make folks feel dumb but because many younger - even middle aged - folks may never have owned a carbureted vehicle. If an older bike is their first bike purchase, carbs are new to them. We get spoiled by the instant start and smooth warm up provided by fuel injection. Going back to a carburetor requires a learning curve - both in maintenance and performance.

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I use the first notch only almost all the time. I use the second notch only when the bike is cold and it's below about 55 or 60 degrees out. My routine is to pull the enrichener out to the first notch (it adds fuel, does not restrict air as a true choke), then start it. The idle is up around 2000 rpm usually. I sit still and let idle until the revs start to rise on their own. Then I push the plunger in and ride. The first mile or so, there is a small bit of stumbling at lower rpm (very slight), but when it warms up (shortly) everything is smooth.

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BVD,

The same thing happens with my bike, the first notch works really well on the choke. The 2nd, not so much. Hasn't really been a concern because the choke works well enough, and I have no issues after the bike warms up.

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Interesting post. My 1980 XS400 does the same thing. I researched thoroughly in the forum and decided to check the idle mixture screw adjustment. The caps had also been removed on my bike by a P.O. The jets were set to about 2+ turns up. I increased them to 2.5 turns and the problem got even worse. I tried 3 turns with an even higher fully warmed up idle - now over 3K rpm. A couple days ago I set them back to 2 turns and now the warmed up idle is just under 2K rpm. Tomorrow, I'll drop it to 1 3/4 turns out and see what happens. I find these results curious! I don't get lean backfire and the bike runs very well otherwise. The factory setting was supposed to be 1.5 turns out and that may be a little too lean for some engines but maybe not for others. When you don't know the history of what has been done previously to these bikes, it makes corrections a little more challenging.

Now, I know the o-rings on the idle mixture screws can become tired and leak. I have a new set of screws, springs, etc from MikesXS and will replace the originals when I pull the carbs off. At the same time I will install new choke rod rubbers and butterfly shaft gaskets - when they are in stock again.

when you adjust the mixture screws out and the bike revs up, its telling you it wants to be richer. Adjust the speed stop screw down, and keep richening up the idle mixture until you find the spot that making it richer, or leaner, makes the idle drop. The bike is very lean at the point you are now, so when you turn the screws out, it gets happier and revs up.

my xs is set at roughly 5 turns out. im actually going to put bigger pilot jets in it, and try to get the part throttle lean surge out, but still be able to idle nice.

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when you adjust the mixture screws out and the bike revs up, its telling you it wants to be richer. Adjust the speed stop screw down, and keep richening up the idle mixture until you find the spot that making it richer, or leaner, makes the idle drop. The bike is very lean at the point you are now, so when you turn the screws out, it gets happier and revs up.

my xs is set at roughly 5 turns out. im actually going to put bigger pilot jets in it, and try to get the part throttle lean surge out, but still be able to idle nice.

Thanks for the information!!

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  • 11 months later...

BVD... This is really all you had to do just idle screw adjustments and the mixture screws? I cant wait to get home and try this out. It has been giving me a headache lately.. Fingers crossed.

Thanks for all the advice and suggestions guys. You have all been a great help and I really appreciate it!

HoughMade, I tried exactly what you suggested. I set the idle mixture screws to 3.25 turns out, started the bike, yet it still was idling very high. Then I looked at the throttle stop screw and realized that it was screwed all the way in, to the point that the spring under it was completely compressed, hindering it from moving any more. So I then backed it out considerably, yet the idle was still high. So I then shut off the bike, restarted it, and lo and behold it was idling at a reasonable speed. I then continued to fiddle with the throttle stop screw until the engine speed was about 1200 rpm. I then took it for a 15 minute ride and everything seemed great. Whenever I pulled in the clutch the engine happily settled to 1200 rpm, so I happily putted home and then set the idle mixture screw at 3 turns out, and it still seems to be running great.

I was really fooling with the throttle stop screw yesterday, and must have turned it all the way in without realizing it.

The only thing that seems strange now is the choke. Before I cleaned the carbs pulling the choke out all the way would rev the engine to about 4000 to 4500 rpm. On the middle setting it would rev at about 3500 rpm, and pushed all the way in the bike would idle normally. Even after I cleaned the carbs the choke worked in the same way. Now all of a sudden, when I got the bike to finally idle correctly, the choke doesn't seem to have any effect. I can't start the bike with the choke lever pulled all the way out. If I put the choke lever in the middle position the idle speed will increase slightly to about 1500 to 2000 rpm, and if I push it all the way in, the bike will idle normally at 1200 rpm. But now if I pull the choke all the way out the bike will simply shut off. This seems really strange to me. Anyone have any suggestions? Maybe my choke gaskets are bad??

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