Hi, Guys:

I am restoring an '86 K75T. I am down to one problem with the idle setting and could use some ideas. I am unable to drop the idle below 1300 rpm. Here's what I've done so far:

- Valve Adjustment (3 exhaust valves shimmed to 0,30mm, 3 intake valves shimmed to 0,15mm)
- Replace all intake manifold boots and air plenum to throttle body boots with new Oetiker clamps (everything verified not to leak)
- While throttle bodies were on the bench, I did a thorough cleaning of all idle bypass channels and cleaned the brass needle valves
- I shot carb cleaner through all the passages and blew all the ports out with compressed air
- Confirmed that the intake manifold clamp is not interfering with the throttle shaft attachments between #2 and #3 cylinder
- Slackened the fast idle cable and removed the throttle cable
- Backed out the idle screw so there is about 1 to 2 mm clearance between the screw end and the stop plate
- Loosened the screws holding the throttle position sensor so that it rotates freely
- Warmed up the bike until the cooling fan cycles on and off
- Installed Morgan Carbtune sync gauge and set all 3 cylinders to draw a balanced flow at idle

So here's where I'm confused. The idle screw is not even touching the strike plate, but the idle won't go below 1300 rpm. If I press down on the throttle pulley, the idle drops slightly, and when I release, it goes back up. The first thing that comes to mind is the throttle mechanism is binding on something, like the #3 intake boot clamp. I've confirmed this is completely clear of the throttle mechanism.

The other thing that is noteworthy is, after cleaning the throttle bodies and changing all the rubber boots, I do, indeed, have a very smooth idle. I noticed, however, that the #1 air bleed needle valve had to be turned out 1.75 turns compared to #2 and #3 which were only 0.75 turns. The first thing this made me think is the throttle bodies are out of sync. Perhaps the previous owners turned the wrong screw trying to set the idle at some point. It "SEEMS" to be that the #1 throttle body is closed more than #2 and #3. I am very tempted (although very anxious) to turn the sync screw between #1 and #2 and try to open throttle plate #1 a little more to perhaps equal #2 and #3 better. I can count the turns and if it doesn't do anything, I can return the screw to the exact position it's in now.

Does it make sense to anyone else that this could be the case? If #1 was closed more than #2 and #3, it would explain the binding problem and why the vacuum was relatively higher in #1 when all the air bleeds were at the same initial setting.

I do have a spare set of throttle bodies I could try, but I would prefer to keep the current bodies installed, if at all possible. Any ideas on what I'm missing or what else I could try?

Thanks a lot,

Ty

UPDATE: I also forgot to mention the compression is 10,7 bar (155 psi) in all 3 cylinders. Besides this idle issue, the engine runs great. Idle is very smooth and engine revs strong. Lots of power, especially at 4500rpm and beyond.

Hi Ty ...

welcome've just read your text, but unfortunately I can not help you ...
but am sure that the specialists will be with you soon Report...

Hi Ty,
check for side air effects. Use Brake clean for checking all parts below throttle bodies. Maybe one butterfly valve is in contact to throttle body housing?

best michael

Thanks for the reply, michael. I have confirmed there are no vacuum leaks using brake cleaner as you suggested.

Since my original post, I have been studying my spare throttle body assembly and it seems that one or more butterfly valves can indeed stick in the bore if they are not absolutely clean, especially where the butterfly valve meets the throttle shaft. Perhaps I should remove my throttle bodies again and just keep spraying them with carb cleaner and working the throttles open and closed, plus blow compressed air into the joints where the throttle shafts enter the bore until I get the proper throttle closure. Maybe I didn't clean them as well as I thought! I'll try that.

Thanks again for the reply,

Ty

A possible air leak could also be in way of the injector sealing rings, the crankcase breather hose or the elbow between the AMM in the airfilter housing and the plenum.

Have a good look at your throttle bodies. If it has been tinkered with it could be that one of the butterfly valves is already closed whilst the others stay a bit open and result in the set screw not touching. This of course if you are really certain it is not hanging on the TPS, gas or choke cable or on other parts.

My just my 2 cents. O-rings of injectors are good point to check for leak. Next thing, hose connecting fuel pressure regulator with TB no. 3. Oil filler o-ring. And throttle position switch can work as additional stop device on TBs shaft, so check it out and adjust.
Maybe not the air leak, but too much fuel for damaged fuel injector. Check how spark plug looks and verify that injector does not have a leak under fuel pressure.

Best regards

Hello,

I had the same problem and does the same controls and adjustments (side air effects, valve etc ... ).
Particularly I removed the throttle cable and clean it ( fuel in the cable sheath to clean and after light oil to lubrificate )
Nothing .... 1200 Rpm ...

In fact, the cable gas showed a very slight resistance to come back . ( Very few ...)
I changed the throttle cable and so be able to adjust properly the idle : 950 rpm .

I hope It could help.

Good day to all

Check the choke cable and TPS. Both can hold the throttle open.

The only times I have had this happen was the TPS was adjusted such that it prevented the throttles from closing fully. The bike will run fine with the TPS removed, it is only a two position switch that tells the ECU that you are either at closed throttle or full throttle. Is the idle still high if the TPS is removed from the bike? If it is, I have another step, but explaining it takes some typing. Try disconnecting the TPS first and tell us what happens.

Btw, idle speed should only be adjusted with the bypass screws in each throttle body. Their base setting should be around 1 turn out from fully seated.

Okay, so I finally figured this out. My problem was that THE BOOK IS WRONG. It says to first sync the throttle bodies with the air screws (without caring how many turns in or out these screws are), then, once all 3 throttle bodies are synced, turn the idle screw until you get 950 rpm. Book = Wrong. Themason = Right.

I derived my own proper procedure from the work I used to do on Audi KE Jetronic fuel injection. The "idle screw" (this is the wrong name for this) is actually intended to act as a range limiter, or "stop" so that the throttle plates don't make contact with the throttle bores. If they close too far, they can gouge the bores over time, which can ultimately have a negative impact on performance and ruin the assembly. This is supposed to be a static setting that is only performed once at the factory, which is why this screw originally had paint on it. If, however, you disturb this setting, you need to reset it before you do anything else.

Ty's Procedure for Resetting the Throttle Plates Stop Screw

1) Slacken or disconnect the throttle cable and fast idle cable (aka "choke" cable, which is another inaccurate name)
2) Loosen the 2 TPS mount screws. The TPS should move freely. (I removed mine just to be sure.)
3) Unscrew the "throttle plate stop screw" (aka "idle screw" - sigh) until a gap exists between the end of the screw and the stop plate.
4) Tighten the screw until it just makes contact with the stop plate.
5) Further tighten the screw 1/4 turn. (This is just enough to keep the throttles from touching the bores, but not introduce any significant effect on the idle setting).
6) Re-paint the screw (that is, if you're a perfectionist like me! A dab of light blue touch-up paint is preferred, but anything that's not screw colored will do nicely, ha ha).
7) With the TPS loosely mounted, lightly rotate it clockwise (looking at it from the rear of the bike) until you just hear the click and lock it down with the 2 mount screws. Verify the clicks by manually moving the throttle lever up and down.
8) Re-attach and/or re-adjust the throttle and fast idle cables.

Ty's Procedure for Setting the Idle

1) Confirm the throttle and fast idle cables are adjusted properly.
2) Confirm the TPS is adjusted properly by opening and closing the throttles and listening for the click when the throttle is just beginning to open, and clicks again just as the throttles completely close.
3) Install a sync gauge on all 3 sync ports. (Note: Be sure and use a tee fitting for #3 body to tap inline with the regulator vacuum hose. This is especially important if you set the CO after setting the idle because it will affect this setting if you don't.)
4) Lightly tighten all 3 air bleed screw until they softly seat and bottom out.
5) Unscrew all 3 air bleed screws 3/4 to 1 full turn each.
4) Start the bike and warm it up until the cooling fan begins to cycle.
5) Perform an initial sync by turning the #1 and #3 bleed screws so those throttle bodies sync with #2
6) If the idle is above 950 rpm, try tightening the air bleed screws about 1/8 turn and re-sync. Conversely, if the idle is below 950 rpm, turn the air bleeds in the other direction.
7) After stair-stepping a bit, using smaller and smaller turns on the air bleeds, you'll get 950 rpm with synced throttle bodies.

Ty's Procedure for Setting the CO
(Note: Setting the CO % is the absolute last thing you do during a tune-up. It assumes that valves, ignition timing, throttle body sync and idle speed are all correct. Confirm you have good spark plugs and a clean air filter. Confirm no intake or exhaust leaks. Confirm fresh fuel and proper fuel pressure.)

1) Confirm the throttle bodies are set up correctly as described above.
2) Confirm the throttle bodies are sync'd the idle is set to 950 rpm as described above.
3) Warm up the bike until the cooling fan cycles on and off.
4) Insert an exhaust gas analyser probe into the end of the exhaust pipe
5) Once the readings stabilize, read the CO %. It should be at or very near 2.0% (+/- 0.5%). If it is, then you're done. If not, continue.
6) Remove the service plug from the top of the air box at the front right side. (You may need to slide the gas tank back a little bit to access this.)
7) Insert a 5mm hex key into the hole so it fully seats into the mixture adjustment valve
8) If the reading is above 2.0%, turn the valve clockwise. If below, turn counter-clockwise (looking at it from the top of the air box)
Note 1: Turning this valve has an immediate effect, but may take about 10 to 15 seconds for the exhaust gas analyser readings to settle down, so be patient and wait for the readings to stabilize before you turn it again.
Note 2: If the cooling fan turns on, wait for it to turn off and take the readings. The extra load from the alternator when it turns on makes the readings change.

Thank you all for your replies, especially Themason, who pointed me in the right direction! Please let me know if I need to amend my procedures. This all worked very well for me. My bike has a very smooth idle now, plus strong, smooth and snappy acceleration, just like a new one probably did!

Ty

Although I appreciate your efforts the official BMW manual refers to the screw to set the idle with. I have copied these pages here before.

That is exactly my point. I believe setting this screw is a mistake, even though the book says to do it. I don't think Bosch meant for this to be used to set the idle.

Here's an example: The book says to adjust the screw until the idle is right, but if it goes past the point where the TPS clicks, you've turned it too far and need to back the screw out until the TPS is clicking again. At that point, what do you do about a high idle? I don't think this is different than many other Bosch fuel injection systems. You don't use the stop screw to set the idle with the throttle plates. You use the air bypass valves. So I disagree with the official BMW book's procedure in this case.

Ty

The TPS is set to the position of the throttle shaft. If you move the throttle shaft with the idle screw in question, then you disturb the setting of the TPS.

When I remove a throttle body rack for cleaning, I remove all the bypass screws and everything. When I reassemble, I install the bypass screws until they seat in their bores, then unscrew them one turn. I take a wild guess at the idle setting on that now infamous screw, disconnect the TPS entirely and loosen it on it's mount so it does not interfere with throttle shaft movement. Once the bike is warm I set the idle speed I desire ( I tend to go for a nice round 1000 prm), then I balance the throttle bodies. If idle speed changes by adjusting the bypass screws during set up, I'll readjust it using the adjustment screw on the throttle shaft to minimize the amount of movement of the bypass screws. If it takes more than about 1/2 turn to bring them into balance, I will consider adjusting the throttle plates themselves.

The TPS is the last thing I set. Once set up, however, I never touch the adjuster screw on the throttle shaft again, but always set both idle and balance using the bypass screws.