Idle is 950RPM, but goes up to 1400RPM when warm - normal?

[Gubro30]

The Hayne's manual states a 950 RPM idle speed, which is roughly what I get after starting the bike, but after 10-15mins riding, it will stick at 1400RPM. Is that normal?

Should I set the idle speed to 950RPM once fully warmed (ie after riding) ?

Thanks!

[mac]

I was wondering about this as well. My '86 K75 will warm idle at maybe 1200 RPM.

[ReneZ]

The idle speed should be set with a warm engine.

[wmax351]

Set the idle speed while hot (after a brisk ride), and then use the "choke" lever to increase the idle speed for starting and warm up. When I set the idle speed, I actually carry around a small screwdriver in my tankbag, and fidget with it when I see the idle at a wrong speed.

[mac]

Know of anything that could cause idle speed to drift up? Cracked vac caps perhaps?

[Gubro30]

Set the idle speed while hot (after a brisk ride), and then use the "choke" lever to increase the idle speed for starting and warm up. When I set the idle speed, I actually carry around a small screwdriver in my tankbag, and fidget with it when I see the idle at a wrong speed.

Thanks Max;

How exactly however can you set the idle speed without synching the throttle bodies? You have to touch all three screws to adjust it right??

[Gubro30]

Know of anything that could cause idle speed to drift up? Cracked vac caps perhaps?

That was my main question; is it normal the idle speed drifts way up like that once it's warmed?

[wmax351]

Could be the Throttle position switch pushing the butterflies open.


There is the idle adjustment screw. It sticks out from the others. It is between the first and second throttle bodies.

[Gubro30]

Could be the Throttle position switch pushing the butterflies open.


There is the idle adjustment screw. It sticks out from the others. It is between the first and second throttle bodies.

Thanks!!

How do I test if it's the TPS?

[kennybobby]

The throttle butterfly valve plates on fuel injected vehicles are factory set and the threads on the rack are locked with serious glue to let you know not to mess with them.  The brass air bypass screws are used to synch the throttle bodies to each other.

You should normally need to use the choke to start a cold engine.  If the choke is turned off before the temperature has come up to spec, then the engine will run at a rich mixture until the engine temperature sensor (NTC) indicates that it has reached the operating temp.  As the temperature rises the rich mixture will be reduced.  On Motronics the idle speed is controlled by the engine temperature sensor, but on L-Jetronic that is not the case.

The idle set point is made using the adjustable screw that controls the entire rack.  The TPS is located on the end of the rack and it must be adjusted such that the microswitch toggles when the throttle is closed.  Make sure that you have a slight amount of free play in the throttle cable at the handlebar.  If the cable is too tight it will hold the throttle plates slightly open or keep the TPS microswitch from toggling, both of these will make the bike run faster than normal idle.

Check you throttle cable free play first, then check the TPS toggle (you can hear the microswitch or check it with a voltmeter), then check the engine temperature sensor (resistance vs temperature).  Hopefully the rack screws have not been effed with, or you will be chasing your tail on this one...good luck

[Gubro30]

That's a very pertinent write-up Kenny, thanks!

I don't think the factory-set screws have been messed with.

Your theory makes a lot of sense.

I do have free play at the handlebar.

Wouldn't a bad TPS toggle make the engine run always higher than supposed to?

I also never need the choke to start the engine, always fires up right away running smooth at 950. It's only once it's warmed up that it will idle around 1400...

[Scott_]

I also never need the choke to start the engine, always fires up right away running smooth at 950. It's only once it's warmed up that it will idle around 1400...

I'd check your vac caps, and other vac lines as well.
I had this issue with my '97 last week. The idle had been creeping up past 1200, which told me something wasn't right.
My vac caps were shot. They were all crumbly from the heat.

[Gubro30]

Scott : by vac caps, you mean the small black "pinched" plastic cap obturating the vac tubes by every throttle bodies?

I'll def check that out.

However, I'm still wondering why it wouldn't fast idle right away from the start instead of always beggining at 950 and going to a stable 1400, every day I rode?

[Gubro30]

Oh and, being relatively new to this engine, what/where are all the vac lines related to intake and idle?

[Scott_]

Since I don't have 1st hand experience with a k75, or a brick older than '95, I cant tell you what you should have on your specific bike.
But, I'll bet you have a vac line that goes to your fuel regulator that sits behind the throttle bodies.
Being an '86 you may be experiencing other vac leaks at the "tubes" and O-rings that set UNDER the throttle bodies.
Mystic Red did a decent write up on checking for vac leaks and removing the TB assy. Though his bike is also an 1100, it will give you the idea.

[mac]

My vac caps are definitely crusty. The picture in the fiche leaves a bit to be desired here. Can someone confirm that the vacuum caps are #2 in the drawing (13 54 7 694 924 "PROTECTION CAP")?

[Inge K.]

[frankenduck]

Or for a quick fix you can stick a piece of vacuum hose with a screw in it on them.  (Haven't done it myself since I live close to a dealer but it will work to seal them.)

By the way, if those are leaking then it will lower, not raise, the RPM.

[mac]

Inge- That pic is way better than the fiche. Any chance you have the p/n for the floral print tablecloth/bedspread?

Duck- don't those vac ports go straight into the intake runners? So wouldn't a leak be like a partly-open throttle?

[kennybobby]

If i try to start my engine cold without the choke, the idle speed is a rough 550, the low battery charging light comes on, and it is barely running.  If i let it warm up this way it will eventually reach 950 and run normal.

The fact that you can start from cold without the choke is not normal, and then it speeds up to 1400, tells me that your idle adjustment screw is turned in too far.  Back the screw out when it is warm until the idle drops to 950.  Then you will need to use the choke to start.  The throttle plates should be closed at idle and the speed is adjusted and set by using the air bypass screws, the mixture screw on the Luftmeter--the air flow meter in the air filter housing, a set of vacuum gauges on the throttle ports, and an exhaust gas analyzer.

Any air leaks unmetered by the Luftmeter will lean the mixture and cause idle speed to fall.  The vacuum ports with the black caps open into the manifold below the throttle plates.

[mjydrafter]

You can get the vac caps at any decent auto parts store.  A really good one will have a little boxed assortment of colored vinyl caps, I used the blue ones. :yes

[frankenduck]

Duck- don't those vac ports go straight into the intake runners? So wouldn't a leak be like a partly-open throttle?

You have a T.  The TBs are exposed.  Pull those little caps off and see how well/poorly it idles.

As a prior poster mentioned, a poorly positioned TPS can hold the throttle open.

[Gubro30]

If i try to start my engine cold without the choke, the idle speed is a rough 550, the low battery charging light comes on, and it is barely running.  If i let it warm up this way it will eventually reach 950 and run normal.

The fact that you can start from cold without the choke is not normal, and then it speeds up to 1400, tells me that your idle adjustment screw is turned in too far.  Back the screw out when it is warm until the idle drops to 950.  Then you will need to use the choke to start.

I guess that's pretty much the problem; the shop must have adjusted the idle at cold engine temperature.

I'll try it and see how it works!

Thanks!

PS Do I need to re-synch throttle bodies after adjusting the idle or will the adjustment remain valid?

Thanks

[frankenduck]

FYI: The OEM idle spec is 950+/-50.  I set mine at 1,000 since K bikes like higher RPMs.

If you have the tool handy then it's probably not a bad idea to check the TB synch. (Engine fully warmed up.)

[Gubro30]

I just had a look at it today after doing the splines.

The idle adjust screw is not even touching the plate when throttle is closed, meaning it's not that.

I slackened the choke cable adjuster, didn't make a difference.

Could it be the throttle bodies are synced together too opened out to start with?

I have slack at the throttle too.