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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => Project Custom Motobricks => Topic started by: dougo on June 07, 2014, 12:31:34 AM
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Hi; I'm new to BMW K bikes-- I bought a 92 k75rs with the intention of replacing the engine (it had 105,000 mi on it and a real mean knock--the previous owner said that he rarely changed the oil.. crazy). I picked up an engine with transmission from a 1990 k75 that had only 22,000 miles on it, a final drive and driveshaft from an 85 K100 with the slightly taller gear ratio, everything needed to overhaul the fuel system from stock pump and filter, all the correct hoses and new (reconditioned) fuel injectors, a fresh starter (the old one was slow--the slightly used one works great) and a myriad of other small things I needed to complete the switch out. Oh yes overhauled the brakes--including the ones on the front end from a yamaha fzr600 that the previous owner had stuck on the bike..don't ask me why.. The guys on the ride44.com forum are calling it "Frankenbeemer"..
Anyway, got the bike together this week, it starts and runs beautifully. However (you knew that was coming didn't you..) the idle is great when it first starts, then as it warms up it climbs to about 2000 rpm.. I was worried about leaks in the air system--but it checks out (I did a good job on it), I verified that both the throttle and choke cables had the correct amount of play and were not binding.. and I started to adjust the idle with the screw you are supposed to use--when I looked closely--there was air in between the bottom of the screw and its strike-plate--the throttle is not closing quite all the way.. I can press it closed with my thumb on the idle screw, and the idle speed goes just about where it is supposed to, but in messing with the cables and anything else I tried, I couldnt get it to stay closed.
My theory is that the previous owner messed with the butterfly valve adjuster screws to get the engine to run (it would barely idle, stall out continuously at a stop, had weak power on the highway, and of course there was that knock..) enough to sell it. There are no paint marks on the screws as there should be. I checked the manual--of course it just says, :DONT TOUCH THOSE SCREWS" big help to me..
So I am kind of scratching my head... I did just purchase a Syncpro Motion Pro carb tuner so that I could balance the airflow in the throttle bodies--that won't get here for a week or 10 days or so...
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The Throttle Position Switch could be maladjusted and holding the throttle open. Back off the two screws holding the TPS and see if the main throttle adjust screw returns to a fully closed position. You could remove the TPS completely and test the bike to see if the idle does not wander upwards, as proof of the symptoms.
When setting the TPS, I found myself absentmindedly pushing down on the unit while tightening. This tends to cause binding. I now make a conscious effort to keep the TPS centered on the shaft when adjusting it, so that this binding does not occur.
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Thanks RBM; it WAS binding; the throttle is now fully closed I believe; Going on a test run to warm up the engine so I can adjust the idle speed..
:clap: :clap: :clap: thanks again!!
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If your test run shows that the bike is idling properly and has good throttle response through the range, don't begin to mess with the inter-body linkage screws. The TBs may be unsynchronized at the moment but possibly NOT due to someone messing with those screws.
The way I'd tackle this situation is:
1. remove the brass coloured air bypass screws, one at a time, and clean them. Also use a pipe cleaner to clean out the associate air bypass hole.
2. replace the air bypass screws and gently bottom them out in their seats. Then, turn each one out 1.5 turns counter clockwise.
3. Remove the TPS
4. Adjust the main throttle adjust screw until it just touches its seat, no further. Blip the hand throttle to make sure that the throttle butterfly plates are in their natural closed position. (P.S. that screw is not used to adjust idle contrary to your statement above; it's to provide a stop for the throttle linkage rotation)
5. attach your Motion Pro and perform a TB sync.
6. At the end of this, if the idle is slightly high, adjust the idle using the air bypass screws ONLY. Turn each clockwise 1/4 turn to lower the idle, 1/4 turn counterclockwise to raise the idle.
7. Reattach the TPS and adjust it.
If I had problems achieving sync in step 6, then I could assume that the inter-linkage screws may have been touched. I'd have to follow Rob Lentini's procedure (http://www.k100-forum.com/t1526-how-to-balance-the-throttle-bodies-solved) for getting them back into line.
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The idle is now good, the engine has good smooth power. .. had the front brakes bled, not. . Front master Cylinder needs rebuilt. ... thats next on the list. ..
The syncpro should be in next week. ..
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So what did it take to correct it?