TB Balance

[Westone]

I posted a few weeks ago regarding being unable to get the vacuum on the #3 TB of my K75 in sync with #1 & 2 at idle. Even with brass screw on #3 gently closed and #1 & 2 open 5 turns. I was unable to detect a leak using the propane testing method.

Since then I removed the air plenum and bushings to get a look inside the TBs from the top. They were dirty inside and I cleaned them thoroughly, including the bypass passages and vacuum port passages. I was able to verify the operation of the valves. The TB "bushings" as BMW calls them have been replaced.

While working inside the TBs, I noticed that when the throttle is closed, the valves on #1 & 2 appear to be fully seated, while the valve on #3 is not. #3 is ever so slightly off of the fully seated position. I now think this is the cause of the inability to get enough vacuum on the #3 TB to sync it with the other two. Enough air is passing by the valve in the idle position that even with the bypass screw closed it will not pull enough on the vacuum port to sync with #1 & 2.

I did not change anything. After cleaning everything was reassembled and tested. Still the same inability to sync #3. After doing some reading about the valves and their adjustment I have come to the above conclusion, at least until new information may change that.

I am also suspicious that the adjustment of the valves may have been changed since the bike left the factory. There are only tiny remnants of the original blue paint, and I initially thought this was due to the age of the bike. However only when I was working at cleaning the interior of the TBs did I notice that one half of one of the valve adjustment screws is broken off at the bottom of the slot, like it may have had enough force applied to it with a flat head screw driver that it broke off.

I'm considering going back in there where I can see the valves on all 3 TBs and adjusting the valve on #3 so that seats fully just as #1 & 2 do when the throttle is closed. While I do I am reading anything I can find on the topic.

[bikerboy]

I had problems with syncing mine. The bypass screw would just bottom out and I could not get them in sync. I srewed all the bypass screws all the way in and the using vacuum gauges altered the butterfly adjustments at 2000 rpm (wife holding throttle). I then screwed each bypass screw out by 1 turn and re balanced on the gauges. The bike is transformed from a reasonable smoothness to a sewing machine with much more grunt!

[Westone]

Thanks for that info. That is basically the process I have been reading about, in effect using the engine as a flow bench. I will give it a go and see how it turns out.

[Westone]

I am happy to report that this process has resulted in the vacuum on all 3 throttle bodies being in sync. Once I understood how the valves sync via the linkage, and had visually seen that the ones on my bike were not closing simultaneously, I had no qualms about giving it a try.

However there is one minor hitch in my case: After syncing the butterfly valves, the bike idles too low, around 850, even with the idle adjustment screw turned all the way in. There is no more increase available and the engine is prone to stall. I have listened to the TPS, and it clicks as recommended, just on and off of fully closed throttle.

If I were to re-sync the Idle Air Bypass screws with my homemade device and open them up more, allowing more air in, will that increase the idle speed? I think I may have made a mistake when I didn't open them up the recommended 1 1/2 turns to start the adjustment. After adjusting the butterfly valves at 2,000RPM with the bypass screws closed, when I reduced the throttle, it was close to being in sync then, and I only opened up the bypass screws enough to sync them. Oddly enough it idled at 1,000RPM. At that time though, it had been sitting still and running at 2,000RPM while I adjusted the butterfly valves, so that may have affected the idle at that time.

[Scott_]

Yes, set your idle rpm with the air bypass screws.

[Westone]

Thanks Scott. I re-synced after opening up the bypass screws to the recommended 1 1/2 turns and it idles around 1,000RPM now, and there is a little adjustment available in the idle screw if needed. It is idling and running the smoothest it has in my short 3 month ownership of this bike. I appreciate the help of you folks on this forum.