High Idle after initial warm up.

goutes2006

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Hi everyone. I just bought my first motorcycle, a 78 xs400, and I'm having some issues with high idling. When I first start it, it idles at about 1500 rpm. However after the bike is warmed up it idles at around 3000 to 4000 rpm. It seems like the longer it's on the road the higher it idles. If anyone has any information on this problem I'd greatly appreciate it.
 
Hi goutes, it sounds like your bike is running lean. First check for air leaks (cracked carb boots, cracked air intake boots, missing vacuum plugs). If you cant see anything spray some wd40 in that area and if the engine noise changes there is an air leak. If no leaks adjust your carb mix screws to 3.5 turns out. See if that fixes it and let us know. Good Luck
 
The other big possibility is that you have a vacuum leak, either between the engine and the carb boots, or the carb boots and the carb. WD-40 sprayed on the boots while the engine is running will tell you if there's a leak (the rpms will dip from suddenly going from lean to rich.)

But honestly, if it's a new bike to you, then get the XS650 carb guide (look for it in google), and read all the stuff about BS34s (your carbs). Do all the cleaning, balance by looking first, and with a home made tool second, and try again.
 
Hey Guys I'm having this same issue, i replaced the air intake boots. My carb holders are a bit cracked. I just had the carbs synched tonight so the bike is running really great. if i get new carb holders and replace them will that mess up the work that was done to synchronize the carbs? Also does it hurt to run the bike with this idle?
 
First question- what are you doing with the enrichener (choke)? When you first do a cold start, you should almost always have to use it. if you don't have to, it could be too rich and have the throttle stop too far in (opening the throttle) to compensate which will cause high idle as it warms.

Start with the enrichener all the way out and when RPM rises above 2000, move it to the middle notch. Start riding then (and pop it in at the first stop) or wait until it rises above 2000 again, then push all the way in. Then it should idle at around 1200. If not, adjust the throttle stop to set it at 1200 rpm. It could then rise to 1500 rpm or so as it warms, but should not go higher. If it does, then look at a possible air leak.
 
I guess i should clarify. Not at start up or cold start. After the bike has been driven a couple miles after start this begins. Nothing to do with choke or mixture. There is an air leak I am sure, I just don't know what else to check to fix it. Or if there is a way I can temporarily fix it to see if/what part needs to be replaced. I replaced the air intake boots. It idles fine until I ride for a while then come to a stop again. I can get the engine to settle down by easing out on the clutch slightly and pulling it back in. The bike runs great until that idle. Basically every stop light or stop i make i have to ease the clutch out to get it to quiet down. Air filters look fine, air intake boots were just replaced. I am not sure what other places i should look that would be causing this issue.
 
it certainly sounds like a lean condition to me. check the plugs to confirm. you can check for air leaks by getting the bike running and spray some carb cleaner around the intakes, if the engine noise changes, you have an air leak. How far out are your mix screws set?
 
I did spray carb cleaner on the carb holders, no change, and on the air intake boots there was, however, I replaced the boots and it is still doing it. Any ideas? Mix screws, I am not sure they were set to about 3 turns out, however, i had the carbs synchronized by a shop and they said they were way off so i don't know if this has changed now.
 
The other thing that may be causing this, and which is a quick check, is that your valves are set to the right clearance. Remove your cam cover, check your clearances with a feeler gauge, and make sure they're right. I had all sorts of demons until I fixed my valve clearances, and then it's amazing how everything smoothed out, and the idle dropped to normal.
 
If the engine noise changes when you spray the air intake boots, air is coming in where it shouldnt be. Make sure all the clamps in there are tight and seated properly. Is your bike stock or has it been modded? I also agree with bentwrench, check the valve clearances, it's really simple and rules out another possibility. Good luck.
 
What I am saying is that things are not adjusted right at cold start, when warm, it will idle too high. The bike may start and easily, but if it is not adjusted right, as it warms, the idle will go too high.

Do you have to use the enrichener to start the bike from cold? If not, the carb is not adjusted right. Not having to use the enrichener at cold start is a sign of a problem. It is not a good thing. If you have to use it, fine, this isn't the issue.

Honestly, true air leaks are kind of rare. Adjustments being out of line are more common.
 
I have to use the enricher to start the bike from cold. When it is warmed up it idles fine. If i pull the throttle and put the bike above 4000rpm it will stick until I let the clutch out slowly to settle it down. I sprayed carb cleaner on the inside of the carb holders tonight and it revved up. However, when i do the same to the air intake boots on the back of the carbs (which i just replaced) it does the same. Do the air intake boots leak some air on their own? Do I replace the carb boots and see if that fixes it? Do I need to re-synch the carbs after that?
 
The intake boots shouldn't leak any air. From the air filter back should be a pretty much closed system (my intake boots to the filter are a little loose, but besides that, it's good.

Make sure that all your bolts are tightened to spec. The clamps around the carbs should be as tight as you can make them - on mine, the stock clamps actually touch ends.

Have you checked your valve clearances? Takes a little time, but if they're out of spec, then your bike will never run right.
 
I have not checked that yet. This is my first bike so i'm not too mechanically inclined. However, I am willing to try anything. What do I need to do to check that? Does anyone have pictures of what I should look for? I really appreciate all this help.
 
another question. How do I go about removing the carb holder boots I want to try to temporarily patch them up and see what happens before I order new boots...? Will this mess up the synchronization?
 
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