BMW K bikes (Bricks)

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Blackthumb

Blackthumb
active member
active member
Have just stripped the k100 frame off of the bike and the  Decided that I need to do a Compression test 🤦🏼‍♂️ I have scoured the internet and cannot find an answer... How can I do this without electronics connected/what minimum electronics do I need to conect? 

Any help would be appreciated 🤙🏼

    

Dai

Dai
Life time member
Life time member
I've done that with a Guzzi but never with a K...

For the Guzzi, I connected up the battery to the starter solenoid as per normal, then wired in an MO (momentary operation) switch that pretended to be the starter relay. For a compression test that's all you need. To do the brick would be similar; something like this:-

1. Dismount the starter relay from the box
2. Using at least 95-amp capable cable, connect the battery positive to pin 30 on the starter relay, then connect pin 87 on the starter relay to the starter motor.
3. Again, using 95-amp cable, connect the standard earthing point on the gearbox (it is still attached to the engine?) to battery negative.
4. Using ordinary wire (11 amp 16/0.2 or similar; this one isn't going to be carrying much current), connect pin 86 on the starter relay to pin 30 on the starter relay (yes, it's a loop across).
5. Using an MO switch (it has to be MO because if you drop it, everything stops) and the ordinary wire, connect one side of the MO switch to pin 85 on the starter relay and the other side of the switch to earth.

Push the switch and the engine will spin - unlike the Guzzi, I don't think it will try to jump off the bench but be careful. Make sure that the battery is fully charged so that you don't damage the starter relay.

Can someone else please cast their eyes over those instructions? I'm pretty sure they're correct but given what Blackthumb is trying to do, confirmation would be nice for him.

One last comment; although you'll probably think it's a PITA, use proper crimps and connectors on the cables and wires. You do want everything to stay secure.


__________________________________________________
1983 K100 upgraded to K100RS spec, 1987 K100RT
Others...
1978 Moto Guzzi 850-T3, 1979 Moto Guzzi 850-T3 California,1993 Moto Guzzi 1100ie California
2020 Royal Enfield Bullet 500
    

Blackthumb

Blackthumb
active member
active member
Thanks dai, that's a great explanation. I will give this a try at the weekend!

    

Laitch

Laitch
Life time member
Life time member
There is also this approach .


__________________________________________________
1995 K75 81,000 miles
How to Compression test with no electricity?  Usa-lo10
    

Dai

Dai
Life time member
Life time member
Back EMF shouldn't be a problem Laitch - Blackthumb did say that the frame is off the engine, which kinda indicates the electronics are all on the bench somewhere.


__________________________________________________
1983 K100 upgraded to K100RS spec, 1987 K100RT
Others...
1978 Moto Guzzi 850-T3, 1979 Moto Guzzi 850-T3 California,1993 Moto Guzzi 1100ie California
2020 Royal Enfield Bullet 500
    

Rickmeister

Rickmeister
Life time member
Life time member
I was always under the impression that the compression test was to be done with the engine at its operating temperature. I did mine with the engine cold, and then compared when done with the engine hot. I forget the result!!

Anyway, I disconnected the wire to the starter motor, used an external battery, +ve wired via a heavy current switch, to the starter motor, and the -ve wire to the frame.

No ignition or internal electronic wizardry was involved, all you are doing is turning the motor over.


__________________________________________________
Assumption is the root of all stuff-ups!
    

Dai

Dai
Life time member
Life time member
Always do a compression test cold because when the engine is warm, it can close up any problem points. I had a severe lesson in that with (what else) a Moto Guzzi 850-T. The owner said it had low compression on the lefthand pot and could I fix it? The bike was left with me; I let it cool off for about an hour, then ran a compression test and both pots were within about six psi of each other. I pulled the head, did the usual parafin leak test to check the valve guides, cleaned the crud off the piston and combustion chamber and reassembled it. By this time the engine was stone cold. More out of idle curiosity than anything else, I ran another compression test. The lefthand pot was now 40 psi below the lowest acceptable pressure (130 psi). This time I stripped everything back to the crankcase and found a broken no.3 ring (850-Ts have four-ring pistons as standard).

Lesson learned. While the engine was still warm, the two halves of the ring had expanded enough to close up the gaps and gave a false reading. That cost me 90 minutes of my time for the second stripdown and the price of a second overhaul gasket set. As Yamaguzzi will tell you, Guzzi parts ain't cheap.

Yes, a big fat 125volt 100amp switch will do the job just fine as you describe - one of those switches that are sold as 'battery isolator switches' is perfect for the job. It's only personal preference to set the thing up so that I have to physically hold the 'trigger' to turn the engine over. It's a safety measure, nothing more.


__________________________________________________
1983 K100 upgraded to K100RS spec, 1987 K100RT
Others...
1978 Moto Guzzi 850-T3, 1979 Moto Guzzi 850-T3 California,1993 Moto Guzzi 1100ie California
2020 Royal Enfield Bullet 500
    

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