2V L-Jet bikes. How well are they supposed to idle?

[k75novice]

I have a 1994 K75. 2 valve, with L-Jetronic fuel injection, as all the K75's are. Previously, I owned an e24 BMW, which also had L-Jetronic fuel injection. Neither vehicle ever held an idle the way a modern BMW will (staying, unwavering at one idle speed when warm regardless of weather); they fluctuate ever so slightly and hold slightly different speeds at different times.

L-Jet is primitive: it may be Bosch's first mass-market EFI system. Also, the K75 does not have an O2 sensor, meaning that the FI computer essentially guesses how much fuel to add to the incoming air with no mechanism for feedback/correction.

Since I have yet to see an L-Jet vehicle idle perfectly in all weather conditions (with engine up to temp) I will detail my experience below. Please let me know if you have a perfect-idling L-Jet bike, or if yours idles like what I describe.

e24: the slightest vacuum leak will cause massive issues, like hunting idle or failure to hold idle. With all new vacuum system rubber, idle fluctuated slightly, as indicated by slight movement of the tach needle. Can't remember whether temp played a role, but I wouldn't be surprised.

K75: similarly intolerant of vac leaks. I have replaced ALL vac system rubber parts within the last year. Also replaced spark plugs and wires, fuel filter, techron, replaced injectors with the 4-hole Ford ones, reshimmed valves, & synchronized TBs. Still, the bike usually idles at 1400RPM or lower (with slight tach needle movement in any case). Though I don't have hard data, it seems to idle higher when cold. Idle set screw is fully out, any further and it won't contact the stop unless I push it down (it's possible that the throttle return spring was mis-installed by a PO)

Possibly of note is that my cylinder 1 TB adjustment screw is almost fully "out" when synched, whereas the other 2 are more in the middle/bottom of their travel.

[Chaos]

something is out of whack. Mine only idles that high when the "not a choke lever" is engaged.  It stays around 900rpm hot, cold, never been an issue. 

[k75novice]

I guess I'll take another look at the throttle mechanism return spring (preventing my idle set screw from working) when I get the chance. It's just mind boggling that a PO would have messed with that, but stranger things have happened on 64k mile, 24 year old stuff.


Laitch, thanks for reassuring me that my pile of scrap metal is not unique

[CNRED]

1990 K75rt, 32K miles.  Idles at 1100 rpm when warmed up. ambient temp makes no difference.

[Laitch]

Laitch, thanks for reassuring me that my pile of scrap metal is not unique

It looked like you did all the writing and I just quoted you. I've deleted that post.

Anyway, the L-Jet is primitive but so is motorcycling. :giggles

My bike idles placidly at around 1000rpm unless a turn indicator is blinking then it quietly pulsates rhythmically. As Chaos observed, it seems like somebody might have exercised misplaced intuition in adjusting your bike's performance to one degree or another.

In this thread where I'll drop you in the middle and you can work backward, there is method that attempts to bring the throttle bodies back to balance after incorrect technique has been applied.

[Martin]

Remove the black cover held by a large Philips head screw next to the twist grip. Make sure it is lubricated with a very light grease and align the two marks on the gears.
Regards Martin.

[rbm]

Is there any evidence that the PO fooled with the sacred inter-body linkage screws on the throttle bodies?  If the blue enamel paint on the silver screws is disturbed (i.e. chipped or missing), it's possible a now badly out of sync throttle body is your problem.  Read this post.


Alternatively, the MAF may be acting up.  It is common for the barn door to become sticky, or the carbon track for the variable resistor inside to become worn or the wiper to lose contact.  Many threads on this topic. http://skylands.ibmwr.org/tom/tech/l-jet-cal.html

[The Mighty Gryphon]

I have two '94 K75's.  Depending on how I have the throttle bypass screws set they will idle rock solid when warm someplace between 900 and 1050 rpm.  One bike has 39,000 miles and the other 83,000.  One thing I have noticed, and it may be placebo effect, but it seems like the fuel has some effect on idle speed.  Pure gasoline adds about a needle width to the idle speed compared to ethanol mixed gas.

Like RBM, I suspect that the blue screws on your bike are misdjusted.  That is about the only thing that would cause a balance screw to be that far out if there are no air leaks.  Those screws are set up at the factory on a flow bench.   

Yes, the L-Jetronic is first generation late 70's technology, but I would rather have it on my bikes than a four carburetor setup.  Been there, done that, and don't want to go back.  I really like the idea of my bike always starting with a quick stab at the start button regardless of the weather. 

[beemuker]

I have two '94 K75's.  Depending on how I have the throttle bypass screws set they will idle rock solid when warm someplace between 900 and 1050 rpm.  One bike has 39,000 miles and the other 83,000.  One thing I have noticed, and it may be placebo effect, but it seems like the fuel has some effect on idle speed.  Pure gasoline adds about a needle width to the idle speed compared to ethanol mixed gas.

Like RBM, I suspect that the blue screws on your bike are misdjusted.  That is about the only thing that would cause a balance screw to be that far out if there are no air leaks.  Those screws are set up at the factory on a flow bench.   

Yes, the L-Jetronic is first generation late 70's technology, but I would rather have it on my bikes than a four carburetor setup.  Been there, done that, and don't want to go back.  I really like the idea of my bike always starting with a quick stab at the start button regardless of the weather.

hmm. my K75 idles at 1400rpm. I thought that was a bit high and MG seems to confirm this. Is there an easy way to lower it a bit without me messing something else up?

[beemuker]

found where the screw is located in another post

[K1300S]

I have two '94 K75's.  Depending on how I have the throttle bypass screws set they will idle rock solid when warm someplace between 900 and 1050 rpm.  One bike has 39,000 miles and the other 83,000. 

yup.  the L-Jet on a K75 when adjusted correctly yields a rock solid 950rpm idle.  both of mine do.  one at 40k and one at 60k miles.  if your K75 is not doing that, something is wrong.


oh, and the L-Jet on my old bmw cars also produced a perfectly smooth idle.....