K75 starts well on high idle but stalls when warm and normal idle

[Rcgreaves]

I have been away from the asylum for a while nice to be back.

So my pal Bon Jovi or John Bobby or whenever my buddy Richard in New Mexico asked me to help him troubleshoot his brick. 

He will have to fill in some of the particulars as to mileage and the model year but he claims that after warmed up the engine stalls when the high idle lever is adjusted down he has made numerous efforts to Address what look like fuel system problems here is what I recall from our call:

Carburetor synchronized and airflow meter adjusted I honestly don’t know what that entails.

From the 2011 thread a link below the author did many of the same things Richard dead and found his exhaust valve lash too tight.  Question:  One of the posts refers to adjusting a screw for either fuel flow or mixture I’m a little rusty and need to crack a service manual to remind myself what’s involved. 

My takeaways are to run some Techron verify the valve lash and then like the writer in this previous thread look for components that might invoke a lean mixture. 

Here’s the thread I referenced

http://www.motobrick.com/index.php?topic=4586.0

Your ideas?

For those who know me here’s a great shot of me in my latest ride-

[The Mighty Gryphon]

Welcome back! 

First thought, has he checked/changed the infamous Z-tube? 

Second, if he leaves the "choke" on, will the engine rev smoothly?  An air leak at idle should have little effect on the ability to run at high rpm.  It might want to stall as the throttle is opened, but should improve as the rpm increase.

[Rcgreaves]

Hey Griff that is right on point Richard is probably going to read this in a couple hours great ideas

[Bon Jon Bovi]

HERE I AM TO WRECK THE DAY!!!
Hey all you Brickers, and thanks to RC with the thread start. So this has right at 16K on it. 1993 K75S. Sat in FL for 4 years before another Floridian bought it and got it running; 9 months later in January 2020 it came to me. It now lives and rides at 9000 feet up in the mountains; my garage is just 20 feet below 9 grand.

Immediate PO was a Honda guy, thought he'd check out Beemers but it didn't take. So when I got it I went through a complete refurbish including: OEM fuel pump and filter; Tills.de injectors; clutch; all cables; Spieglers and pads; radials; AGM battery; intake and outlet airbox boots; fuel lines; hoses; water temp sensor; exhaust seals all around; HES resto; fuel pump/level connector; z-tube; all fluids and filters; restored ABS. More stuff I can't remember at the mo.

Gryph, it revs smoothly with the choke half on, but if I move the lever all the way off I have to manually add throttle or it will die. I've already turned up the idle to compensate but of course when it's fully warm, I end up with high idle.

Valve clearances were within spec, TB sync spot on. Once it's good and hot it'll idle all day. I've got one of those altitude plugs but see no discernible difference with it installed or not. The bike hesitates a wee bit up here if you get on it, but otherwise pulls strong, even in 5th gear uphill at this altitude. Pretty impressive for an old 750.

That should about cover it, so suggestions are welcome. Cheers.
 

[Rcgreaves]

Strange $#%#$ happens at altitude.

Son #1 has MS.  Moved to Denver last year.  MS symptoms have stopped.  Blurry vision - clear etc.  Everything leans out at altitude.

Here's a BMW MOA thread on the topic, apropos 

https://forums.bmwmoa.org/showthread.php?28614-Altitude-plug-needed/page2&s=8e79517beb4936e9d2e3d0e90d2690a9

So if its going to behave rich at 9k Cloudcroft does that equate to dying when warm? ........Yeah.

Have you watched it warm up and die with the altitude plug installed?  What plugs do your run? 

[stokester]

Valve clearances were within spec, TB sync spot on. Once it's good and hot it'll idle all day. I've got one of those altitude plugs but see no discernible difference with it installed or not. If I remember right the BBSs are 1 1/2 and 3 turns, left and right. Both TB cables have the required free play and are seated in the ferrules. The bike hesitates a wee bit up here if you get on it, but otherwise pulls strong, even in 5th gear uphill at this altitude. Pretty impressive for an old 750.

That should about cover it, so suggestions are welcome. Cheers.
 

Looks like you've covered the most likely suspects.
I ran into an issue with my '93 where it was very cold-blooded and required an abnormal warm-up before it would run without "choke".  What I found was a couple of tight valves.  May need a second check.

[The Mighty Gryphon]

There's a couple possibilities I can think of.

That it runs at higher revs tells me the problem could be a lean mix at idle.  At 9000 ft it's a long shot, but it may be an intake leak.  You might have installed one of the throttle body bushings incorrectly. 

Have you been able to balance the throttle bodies?  If so, is there one that has turns out that just don't match the others?  I would expect all the screws to be between 1 1/2 to 2 1/2 turns out from lightly bottomed.  If one of them is out of that range I would look closely at that throttle body for an air leak. 

Do a propane test.  If you have a propane torch put a piece vinyl tubing on it and with the engine idling apply a slight flow of gas from the torch around the throttle bodies and the bushings while you listen for a change in speed.  That will indicate a vacuum leak that is causing that cylinder to run lean.

It's possible that at your altitude the idle mixture could be too rich.  In my experience, a rich mixture really doesn't affect idle as much as a lean mix.  It usually needs to be so rich that it causes stumbling and rough acceleration before it will mess up the idle.  A rich idle mixture usually won't idle better when the engine is warm. 

A test for a rich mixture would be to run the engine at the lowest "choke" setting possible until it's warm and the fan can cycle a couple times.  While the engine is running, check the headers for even temperature.  A cold one will identify a cylinder that needs a closer look.  After the engine has idled hot shut off the engine and pull the spark plugs.  what does the insulator nose look like?  If the mixture is rich, the nose will be covered in black carbon. 

A rich mixture is most commonly the result of a problem with the engine temperature sensor.  Have you checked it? 

About the high altitude plug.  I have taken my bike over 10,000 feet without it, and been down to sea level with it plugged in.  I've never noticed the slightest difference in engine performance.  I attribute that to the excellent job the air flow sensor does at measuring the air taken in the engine.  What I suspect the plug does is fine tune the mixture at altitudes above the "normal" operating range to reduce some of the exhaust emissions.  I seriously doubt that it has any bearing on your problem.  If it makes you feel better, plug it in.

Last, have you done a compression test?  If the bike hasn't been used for a long time the rings can get stuck in the lands and the engine will not get good cylinder pressure at cranking and idle speeds.  Poorly sealing rings may also fail to pull a good vacuum on the intake stroke, resulting in a richer idle mixture. 

[Bon Jon Bovi]

Thanks for all the suggestions, I'll get on this first thing tomorrow.