I recently bought a NSR250R (2J) MC18 from 1988 with only 17.000Km. There is a problem on the bottom cylinder (no compression) so the engine wont start.

The other issue is that the rc valve control is not working. Bought it from a BMW dealers customer and they had the PGM, control and sub control tested but couldn't remember which one was faulty (year ago). The bike had a cylinder/piston issue before and they drove it 2500Km after repairs with tiewraps on the servomotor @ 50% opening. The owned drove it this way until it stopped and there was no compression once again so the bike was up for sale. Since I love small 2 strokes and searched a NSR250 for ages bought it when it was possible.

This NSR250R (2J) MC18 is the first MC18 model with separate PGM, control and sub control unit. I swapped the PGM without result which makes sense since the motor is powered via the sub control unit and not like the PGM2 in the PGM unit (beginners mistake)....

See the image below for the schematic view. [img]https://ibb.co/dcgWiK[/img]

I have the manual with the J supplement. The voltage on all units is ok. The TPS has the correct reading, the continuity between sub control and control unit B/BU is ok, regulator is fine and so on.

The servo should move @ 2000 RPM to HI and 3000 RPM to LO the manual says while engine is running. But my engine is not running so can't test that. The servo should move to a zero position when put on ignition I think. I only have Aprilia RS125, RS250 and Suzuki RG500 knowledge and searching this forum tells me later models do so. Read the TPS remove trick for a new calibration with no result.

The servo and RC valves move if operated by hand easily. Removed RC valve cables to be sure the servo motor is free. If voltage is applied on W and W/B (as stated in the manual) the shaft moves. If + and - are applied the other way round the shaft moves the other way round. I believe + on W and - on W/B to HI and - on W and + on W/B to LO. So I think the motor is right since it moves if voltage is applied to left and right.

Bought a good sub control unit from red line in the UK and did some measuring with the old (defective?) and the new one. Dry it is the same resistance on the W and W/B.
Disconnected the servo, ignition switch set on and started measuring with negative on the chassis/battery and the + on W (white). With the old one no voltage with the new one I measure 12V on W.
Measured between W and W/B and with old one nothing and with new one 12V. That was with - on W/B and + on W.

Connected servo again and switched ignition on, nothing happened so switched off within 3 seconds. Measured again and no voltage anymore between W and W/B. I am 90% sure I measured it before but maybe I was daydreaming. There is voltage between chassis negative and W as before, but not with W and W/B anymore. With ignition switch on measured between W/B and W/R as manual says and there is voltage (3.8V)

If servo is connected (so everything is connected) and - is chassis/battery I measure voltage on W and on W/B which I don't understand.

My idea was that the W/B should work as a negative but that is strange with voltage on both.... If I put negative from the meter on the LG cable I measure 11.3V on W and 0.7V on W/B which indicates voltage is divided in W and W/B wire and LG is acting as the negative?

My questions are:
1. Should the servo go to a zero position if ignition is switched on.
2. Is it possible I blew my new sub control and if so why?
3. If so there should be a problem with the servo causing the sub control to blow? Or is there a issue with the control unit? What is a common issue with these MC18 J model?
4. What does the control unit do with the LG, W/R and G/BY wires? I guess regulate the angle of the shaft from the servo to put the RC valves between 0% and 100% opening. Is it just a variable resistor?
5. Is it possible that part in the servo is damaged and caused the original sub control to blow the servo circuit and the new one to damage the W/B wire?

What is there I can't see or could be a issue? All help is appreciated since I am stuck on the subject.

Regards Eddy from the Netherlands