Can't lower idle below 1200rpm

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,
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

Zitat

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

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