BMW R1200RT/R1250RT Forum

Hello knowledgeable experts.

Last February after a 50 mile run to Taunton Dene Services, the starter on my bike would not turn the engine over past top dead centre. It was if the battery did not have enough power to get past compression. After waiting 2 hours for BMW rescue to arrive, I tried starting the bike, and it immediately burst into life. (Apparently BMW rescue eventually turned up around an hour later, despite me calling BMW to cancel my call!)

Further to advice from one of the members, I purchased a new Tayna battery, and fitted it at the end of February 2022.

In December 2022, the same problem occurred - the battery did not seem powerful enough to turn the engine over compression.
In order to test the battery, I measured the no load voltage on the bike - 12.95 volts.

I charged the battery and fitted to the bike. 13.12v.
I then took readings over several days. Readings as follows.
Day 1. 13.03v.
Day 2. 12.99v.
Day 3. 12.95v.

I disconnected the battery and measured the voltage drop over several days. Readings as follows.
Day 3. 12.97v.
Day 11. 12.95v.

This would indicate that the battery is in good condition.
However, on two occasions since fitting the new battery, the same problem occurred - the starter would not turn the engine over compression.
On the first occasion, I got a jump start from a local friend's car.
Yesterday, after a short journey to go food shopping, again the starter would not turn the engine over compression. Strangely enough, after around an hour, I tried starting the bike, and it burst into life.

Is there some "decompressor" device on the RT1250 engine, that enables it to turn over full compression? If so, how can I test to see if this is working?

Tony Fyson wrote: ↑Sat Jan 21, 2023 5:19 pmIs there some "decompressor" device on the RT1250 engine.

Yes, a lengthy thread but scanning through it indicates some sort of automatic de-compression device.

https://www.advrider.com/f/threads/r125 ... m.1418465/

Tony Fyson wrote: ↑Sat Jan 21, 2023 5:19 pmHow can I test to see if this is working.

See page 17 of the Adventure Rider thread, for some thoughts and ideas of others on the decompressor.

Having read the thread myself and viewed the videos below, it would "appear" that when the engine is not running, the decompressor should be "closed". Next time the engine fails to start, remove both valve covers and view the position of the decompressors. If they are "open" which I think is what happens centrifugally, then they may be sticking.

This photo is taken from the A R thread, it's a 1200 but I think it's the same on a 1250 showing the decompressor "closed".



This photo shows the decompressor "open".



"Some people have got the dealer to replace the decompressor, and fixed the issue."

Please note, I have no experience of this myself, just trying to help with my interpretation of the information provided by others.
I could be completely wrong and happy to be corrected.

These two videos by Boxflyer checking the valve clearances on a 1250 show the decompressor on the end of both the left & right cylinder exhaust cam shafts. At 3:18 in the second video, reference is made to the centrifugal device.