RPM's will randomly skyrocket/Bike dies while at an idle.

[Radish]

Hello everyone, first off I would like to give a big thank you to everyone on this forum. I recently bought my 1980 XS400 special as my first bike, and this entire forum has been a godsend with helping me learn how to work on my bike. But now i've run into some issues that I would really like advice on, and i'm hoping that I can get some help to fix these problems.
First off, every once in a while, the RPMS of the bike will skyrocket. Either while i'm coming down to a stop or if I rev the throttle open a bit. Usually it goes up around 4000 RPMS. Now, once I am in first and release the clutch a bit, the RPMS will go back down to the normal idle, which is ~1200-1300 RPMs. Would this be the result of lean fuel? I've recently replaced the carb boots and made sure that they were secured quite well, some I wouldn't think it to be them.
My second issue is that my bike will die randomly while at an idle. This isn't something that always happens though, but the few times it has happened has left me paranoid, so now while stopped i'll slowly open and close the throttle, which seems to keep it from dying. This isn't an instant thing though, and will usually just happen a little after i come to a stop.
Hopefully this is enough information to get a bit of help, but if I need to clarify something or provide pictures I will try and do so to the best of my ability.

 

[arfstrom]

Hi Radish, Welcome to the club!

First off, every once in a while, the RPMS of the bike will skyrocket. Either while i'm coming down to a stop or if I rev the throttle open a bit. Usually it goes up around 4000 RPMS. Now, once I am in first and release the clutch a bit, the RPMS will go back down to the normal idle, which is ~1200-1300 RPMs. Would this be the result of lean fuel? I've recently replaced the carb boots and made sure that they were secured quite well, some I wouldn't think it to be them.

It seems to me that is symptom matic of an air leak. Perhaps the last person whom cleaned it, left the rubber gaskets in place, (I am guilty of that) I counter that by downshifting all the way down. and occaasionally I will let the clutch grab a little bit to bring the RPM's down. There is a trick of spraying WD40 or starter fluid near the carb's. that will tell you have a air leak. I just never learned where to spay it.

My second issue is that my bike will die randomly while at an idle. This isn't something that always happens though, but the few times it has happened has left me paranoid, so now while stopped i'll slowly open and close the throttle, which seems to keep it from dying. This isn't an instant thing though, and will usually just happen a little after i come to a stop.

So how do I not feed your paranoia? randomly dying coming to a stop? That seems like your charging system needs help. Or you idle is set too low.with glove fingers, and a warmed up engine. between your carbs is big screw head. that is your idle screw. Back it out, or in, till you get your idle to 1200 RPM.
Start here at this thread, and see where that takes you. don't forget we have the Tech section on top, as well as the google search.
www.xs400.com/forum/showthread.php?t=10715
Do the inline filter, Later Welcome

 

[Rambo814]

First off, welcome to the forum!!

My bike was doing this just recently after I had come back to it after not touching it for about a year. What I had found was that the right carb boot did not have the vacuum nipple plugged. That helped it idle better but I was still having the erratic and hanging rpm's. Upon searching this forum I found that the idle air adjust screw should be around 3 turns out rather then the 1 1/2 turns like the Haynes manual says. That seems to have cured my problems, but I have not actually gone for a ride, yet.

 

[jvdavidson]

I've got EXACTLY the same problems - and I mean EXACTLY! I adjusted the idle air screws to 2-3/4 or so from the Haynes manual - (by the way, that is the single most useless manual I've ever seen. I also have a Harley and a '85 Honda - their respective manuals are fantastic but man this Haynes is crap). Anyway - 'nuf of my rant. I had a helluva time getting the thing to run right and it was a nightmare to start - I'd seriously spend a solid 10min trying to get going in the morning (at that point the screws were 1-1/2 turns out - Thanks Haynes). Finally, in my parking garage at work, I was NOT able to get it started AT ALL. So I logged on here and looked through threads and found the 2-1/2 to 3 turn setting. Having no tools I had to use my fingers on those dumb knurled screws (man did that hurt) but I finally got it. At that point I had to screw out the big idle speed adjustment screw then while kicking the starter lever, I'd slowly screw it down (increase idle) until the bike fired up (with choke obviously).

Prior to dealing with this aggravating mess, idle would skyrocket at darn near every light (I live in Chicago ... we have many lights). Sometimes dragging it down with clutch in 1st gear would not fix the problem and it would rev as soon as I put it in neutral.

After setting the air adjustment screws and idle screw, the rev crap happens only rarely.

I've rebuilt my carbs and I'm darn sure the boots are all on as air-tight as possible but at idle, every once in a while, it sounds like the bike has a sporadic backfire and it dies leaving me at a stoplight trying to kick the darn thing - quite embarrassing.

So if you're looking for a fix ... I do not have one, my idle is set high (1600-1700RPM) as I feel it dies more frequently at idle speeds below 1500RPM. Just know that you're not alone in your struggles ... I ride a 1978 XS400 ... the thing is old - that's what you gotta deal with.

As far as other comments blaming the charging system ... I'm inclined to think that's not accurate and here's why. I bought this bike with a broken motor (I concluded that a previous owner dropped it and it landed on the kick starter (maybe against a curb - I dunno) that drop busted a chunk out of the boss on the inside of the motor case which serves as a bearing-surface for the kick starter shaft. When I bought it, the bike ran, the kick starter wouldn't engage every once in a while as the boss was broken and things weren't perfectly lined up anymore but you could still get it started. Finally, a significant oil leak developed. Each time I kicked the motor, it put strain on the remaining fragment of the boss which is part of the engine case and finally it completely broke off and left a pencil-sized hole in the motor - good motor but the case is done for so I had to craigslist another motor. I got a good engine, installed it, set timing, checked valves etc all golden). Well folks, the exact same idle rev and random dying problem existed identically with both motors - this leads me to believe that it's just part of the game and something you have to deal with. I apologize for not being more helpful but that's all I got.

Cheers

- J

 

[xschris]

A lean condition/dirty carbs, hole in one of the diaphragms, crappy pod filters or out of sync carbs ( use a manometer to fix this). These are just some of the more common things to cause a high idle.

 

[arfstrom]

2-3/4 or so from the Haynes manual - (by the way, that is the single most useless manual I've ever seen. I also have a Harley and a '85 Honda - their respective manuals are fantastic but man this Haynes is crap)

Thats why we are here!

 

[jvdavidson]

XSChris, by "crappy pod filters", do you mean to say that any pod filter is crap (shouldn't use them at all)???. or are you implying the use of pod filters of poor quality? I'm currently using the stock filters - I blew them out with compressed air but otherwise, that's it.

Thanks for your reply,

J

 

[xschris]

With these cv type carbs, pleated style pods cause turbulence which makes for hard tuning. These carbs like "dead air". Some of them (emgo brand) even have lips inside that block off the vents on the carb. Oil-able foam type uni's work better. But of course stock is best There are a lot of threads on different "pod" filters.

 

[jvdavidson]

With these cv type carbs, pleated style pods cause turbulence which makes for hard tuning. These carbs like "dead air". Some of them (emgo brand) even have lips inside that block off the vents on the carb. Oil-able foam type uni's work better. But of course stock is best There are a lot of threads on different "pod" filters.

Thanks for the input XSChris ... I've thought about going pod on both this xs and my '85 Honda Nighthawk 700S (both are presently stock in this regard) ... if for no other reason but that in both cases pods would tremendously simplify removal and installation of carbs ... especially so in the Honda with 4 carbs and very little clearance. I've seen a lot of XS builds using pods and leaving the frame virtually empty which is a very attractive look in my opinion but I'll probably take your advice ... save $ and keep stock system.

Cheers,

- J

 

[Faniman57]

I'm having the same high idle issues. I'll adjust the screws and report back if it fixed anything

 

[Faniman57]

Nope, nothing.

 

[Wolfe_11B]

Have you checked for leaks?

 

[arfstrom]

I used vinyl magic tape from Menards. about 10 yards for less than 4 bucks (USD) It seals to itself, and when I had crappy carb holders, those worked great. It was easy to wrap on there tight, and forget it. (takes over 500 degrees F.) as well as a ton of other things that make it worth while to us bike repair-ing peoples.

http://www.menards.com/main/electri...-self-fusing-repair-tape/p-1954678-c-6482.htm
currently 4.42 USD
its been a year since I bought some, but I did get different colors.

 

[Radish]

First off, thanks for all the helpful replies! I've slowly been going through and trying the suggestions, and the bike still seems to have the issues, but thankfully, not as much as it used to.
What i've done so far is spray for leaks, which turned out to not cause anything, so the boots look to be good. After that, I went through and adjusted the screws, turning them all the way in and pulling them out to 3 1/2 turns. Once I finished that up, I built myself the four dollar manometer that I saw on the fourm and got my carbs synched. One side was pulling really hard, so it is good to know that I ended up fixing something with it.
My bike certainly sounds alot better now, but it still will sometimes hang or rocket up. Not as much as before, but still every once in a while. But, I want to try my best to keep it from ever doing it, so next time I have a lazy weekend i'll finally learn the joys of carb cleaning.
Also one thing I have noticed though, is that I need to adjust my idle screw to get my bike going. I'll get it to where during warm up it will linger around 1200, but once fully warmed up it will rest around 1600. But with a glove I can just adjust it and get it lower, so it isn't really an issue.
And I found that sometimes my blinkers won't flash, and just stay solid and kind of dim. I'm not sure if that would indicate charging system or if the switch is just going bad. I'm running the stock lights.

 

[xschris]

A weak battery or charging system for the lights and a lean /dirty carbs for the lingering idle.

 

[jvdavidson]

So I'll have to report in the near future but I believe I found my own reason for the high idle business. Last night, I balanced my carbs (not much as they were pretty darn close anyhow), but in doing so, I noticed that one of the brass pipes on the carburetor-to-head boot (for connecting a vacuum apparatus) was plugged - as it should be to prevent vacuum leaks and a very lean mixture. However, the other pipe had connected to it a black tube which was intended for the fuel petcock vacuum diaphragm (which is broke on mine but I don't care, I installed a second valve). Well, this black tube was not connected to anything and in the multitude of times I've had the gas tank off for various reasons, I completely failed to notice this ... so I had a significant vacuum leak this whole time - frankly, even when it was hooked up to the petcock as it is supposed to, with the petcock diaphragm likely shot, it was probably sucking air anyhow. As such, I plugged that hole (certainly for the first time since I bought the bike in August), and MAN I've got a completely different machine - acceleration, power, idle, everything is gorgeous - I fell in love with my bike all over again ... but the process of getting here made me feel like a total moron for not seeing that vacuum leak a long long time ago.

As far as the idle business goes, historically speaking, my idle issues often followed a lot of city riding (I live in Chicago during the week and suburbs on weekends) and when I have periods of riding 1 stinking block and catching a light over and over and over again, this was generally what caused my idle to skyrocket. Since I just fixed this vacuum leak last night but didn't do very much city riding, I didn't notice any idle problems. This afternoon, I should be able to get slammed with enough lights to test the idle issues but I remain quite confident that I'll be good to go. The only issue that remains is a periodic backfire which will kill the bike when I'm idling ... this leaves me trying to kick start my bike with a green light in front of me and pissed off cars behind me.... not fun. I'm guessing that between rechecking the timing and adjusting the valves, I should have it.... hopefully.

Hope this helps!

- J