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post #1 of 5 (permalink) Old May 11th, 2017, 10:02 pm Thread Starter
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Sapping engines

Can a 1991 BMW K100RS 4 valve engine be swapped for a 1985 engine?
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post #2 of 5 (permalink) Old May 12th, 2017, 9:24 am
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Location: Austin, Texas, USA
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Yes- it will bolt in with no problems. The exhaust mounts slightly forward compared to the '85. I swapped a '93 K1100LT engine, Motronic, airbox and exhaust into mine- but that is not a project for a novice or the electrically dis-inclined.

Frank

'17 R1200RS, 2 yrs. 27,000 mi.
'16 R1200RS, 18 mo. 28,000 mi. (gone)
'03 R1100S Boxer Cup Replika 6/200
'85 K100/1100RS, 34 yrs. 326,000 mi.
'05 R1200ST, 8 yrs. 57,400 mi.-3rd deer killed it.
'11 R1200RT, 2 yrs. 30,000 mi. (gone)
'99 R1100S, 2 yrs. 15,000 mi. (gone)
'96 Ducati 900SS/SP, 12 yrs. (gone)
500,000+ BMW miles
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post #3 of 5 (permalink) Old May 12th, 2017, 9:43 am Thread Starter
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Wiring doesn't bother me. I am making a harness for the bike anyway. Does the header bolt on, just a little bit more forward? And on the controlle.Is the plug the same? What parts of the electronics are you talking about? And last question is it worth t?
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post #4 of 5 (permalink) Old May 12th, 2017, 11:57 am
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Location: Austin, Texas, USA
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I did this swap 17 years and 50,000 miles ago so remembering the details is sketchy. The stock or aftermarket exhaust will still bolt up to the head just fine. The 4-valve design moves the exhaust ports ahead maybe 10mm,3/8 in. the rear mounting is slightly affected. I bought a complete K11 engine, airbox, Motronic computer, O2 sensor and engine wiring loom out of a junkyard for less than $1000- less than I would have spent on rebuilding the original engine.

The hardest task was modifying the '85 wiring harness to power the Motronic system. I remember spending the winter with this project. I took several sets of '85 and '93 wiring diagrams to map out each part of the separate systems to cross over. I remember having trouble with fuses blowing when winding out (hard accelerating) the engine. A whole series of dedicated grounds and increased capacity fuses finally fixed that.

Was it worth it? Riding the bike after made me feel that this is the engine that should have come with the bike to begin with. Better torque all the time. Within the last couple of years I went through the bike mechanically and cosmetically and it looks and runs like a new one- but it is still a 30+ year old scooter and feels like it compared to my '16 R1200RS. I really don't like riding it much anymore. But I'm keeping it since it really isn't worth much to anyone but me.

Good luck with your project!

Frank

'17 R1200RS, 2 yrs. 27,000 mi.
'16 R1200RS, 18 mo. 28,000 mi. (gone)
'03 R1100S Boxer Cup Replika 6/200
'85 K100/1100RS, 34 yrs. 326,000 mi.
'05 R1200ST, 8 yrs. 57,400 mi.-3rd deer killed it.
'11 R1200RT, 2 yrs. 30,000 mi. (gone)
'99 R1100S, 2 yrs. 15,000 mi. (gone)
'96 Ducati 900SS/SP, 12 yrs. (gone)
500,000+ BMW miles
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post #5 of 5 (permalink) Old May 12th, 2017, 12:10 pm Thread Starter
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Thanks for such a through explanation. I am building a bobber for a client. Right now the engine is locked but I hope to take care of that today. Sounds like it is a lot of work and then I would be delivering a bike that is non-standard. If I was keeping the bike I might consider it but I feel that a bike that he can take to any dealer for service is what I should deliver. .
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