Idle increases after 30 minutes

[nwforester]

Help!

My 79 XS11 runs great. Good start up and idle (just under 1000rpm). However, after riding for 30-60 minutes, the idle starts to increase until it settles at over 3500.

Sitting at a stop light like this is way too hard on the engine, not to mention loud and embarassing. I usually just feather the clutch in 1st to keep the revs down. In itself not a good solution, either.

Like I said, it only occurs after at least 30 minutes. With such a long span in between startup and the problem, I can't think of a particular reason for this increase.

Anyone?

[81xsproject]

I had this exact issue. Turned out to be leaky/poorly adjusted float valves. Have you checked the float heights? Have you synchronized the carbs? Does gas drip from the carbs/airbox if the fuel vavle is left in the on/prime position?

[TopCatGr58]

Hey there Forester,

Okay, we see you have a 79SF! What if any mods have been done to the bike? OEM pipes and airbox or aftermarket pipes and Indy filters/pods?

Are you new to the bike, or have you had it a long time? What's the history of the bike, daily ridden all year, or just gotten, rebuilt, or sat a long time?? What service have you done to it?

The more info we have, the better we can "guess" what you should try to do next!

And engine gets leaner as it warms up, and so requires less fuel to run. But if it gets too lean, it can run up the idle all by itself!

Have you tried turning back the idle adjuster knob once it gets warm? You say it starts good and idles cold....is that WITH the choke/fuel enrichener or without? If without...then it may be jetted incorrectly, or just adjusted incorrectly. It should require the choke to get it started AND to idle...until it warms up. As it's warming up, the idle will slowly creep higher, but that's when you push the choke in to the next detent, etc., so that by the time you are running at normal operating temps, you are not using the choke, and it's warm idle setting will be correct!
T.C.

[Dragon Tamer]

Hey there forester,
Just my two cents worth here, with all due respect to 81xsproject,(who knows his stuff) I would (carefully) not worry about the float levels at this point, any problem with them usually will show up before this amount of running time. Go with Top Cat and get us some more info so we can try to work it out for you.
Having spent some many years now with '79 SF's my experience points to carb synchronization as the problem. For whatever reason the two SF's that I have owned do NOT want to stay in sync for any significant amount of time. On these bikes even one carb slightly out of sync will give you very erratic idle speeds and these may not show up until the bike has run for a while. You should also check to make sure that the "choke" linkage is working on all four carbs. Visually check that the movement of the lever makes all four carb levers move at the same time, a loose set screw on one carb can leave that carb in the "choked" position with the same result as an unsynced carb. In this case, exactly like an unsynced carb, the problem will not surface until the engine has attained comfortable operating temperatures. There may be a small variation in even a well tuned engine. The Dragon tends to idle at about 750 to 800 rpm until it has run for over twenty minutes then will idle at about 1000 to 1100 rpm. If I have any significant variation from that range, I check the carb sync and about 90% of the time that will be the problem!
Please keep us posted on what you find!

[Dragon Tamer]

Also

just noticed the last part of TC's reply, make sure that you have both detents available on the "choke" lever. I have seen a bent choke lever keep the system from going all the way to the warm position! There should be TWO noticable notch positions in the lever!!!!

[malber]

After reading the first post I was thinking, "This sounds exactly what happens to me at the first stoplight on my morning commute when I've got the enricher still on the middle notch." Then I read TC's reply and I'd say he sounds about right.

[nwforester]

Thanks everyone for jumping in.

Here's a little history: I bought the bike three months ago. Great condition. At the time it wouldn't idle without the enricher in the middle position. It would backfire constantly. I finally found the main idle adjustment screw and adjusted it so that the engine only needs the enricher open to start. And when I say open, I mean the first position, not the full Cold Start. Of course, now that the temps are dropping in KC I will probably need it. Anyways, after a few minutes I can confidently close the choke completely and it idles great.

The bike is not my daily driver, but I'm riding as often as possible, say 2-3 times a week. The ride to work is only about 10-15 minutes through neighborhood streets and school zones. Rarely touch 5th gear. For these rides, the idle problem never appears. It's on the weekends when I go cruising.

Gas does not leak from the airbox, although it definitely needs a new filter. Could this have something to do with it? The large, plastic tubes leading into the carbs are a little cracked. From what I've read on the forum this is pretty common. I'm sure the carbs themselves could use a synch job just for good measure, but the bike runs super smooth and strong. The gas tank has a little rust in it. I've bought the 2-part treatment for it but I'm waiting for the weather to turn sour before I pull the tank. Was planning on completing breaking down the carbs, too, this winter to clean them real good. That process would require synching.

I've read about the floats sticking and thought that may be related. Sometimes the bike is slow to idle back down after cutting the throttle in neutral. But this issue doesn't explain the idle creaping up, does it?

[nwforester]

Oh, and thanks again for jumping in. This is my first bike. I could've bought something a lot newer that wouldn't have required much maitenance. But I love tinkering with engines, especially the old ones when things were "simpler."

Based on the responses thus far, I'll definitely check the linkage on the choke. It sounds like I might just need to deal with it for another month or two until the average temperature drops below 40 and then just break everything down.

Thanks everybody.

[malber]

You'll probably need to replace the plugs at that time if not before then.

[Erik the Red]

hmm...

Something else you might want to check is the throttle cable. I'm just throwing a guess out here, but I'd think that a throttle cable that is dry in its sheath could potentially be binding some. Lube the cable (make sure it's routed properly too) and see if that helps. If not, at least it's an easy procedure, and you now know you have a good responsive throttle.

[79XS11F]

I also had the same issue even after sync and color turning my carbs. I cured it by re-syncing and color tuning them while the engine was at full operating temp.
So take her for a spin and get her up to the temp where the idle increases and sync the carbs while shes HOT . Baring no other issues that may fix her up for you. It worked well for me. Keep a fan in front of the engine blowing air across the head area while working on her cause she will already be HOT..
Rob

[fredintoon]

First bike?

Hi NW,
your first bike? Not "I have been a moto-cross racer for years but this is my first street bike" but "This is my first bike ever"?
Be careful out there man! The XS11 has the potential to take a newbie at least 60mph faster than his riding capability is good for.