I also had the same backplate problem with my 170.

Initially the 175 would only run nice at half throttle. It sounded lean at idle. I ran it on a glow plug and the engine ran perfectly. I changed the complete backplate assembly from my 170 onto the 175 and then it ran fine at full throttle. The idle was still a bit funny so today i changed the regulator from my 170 to the 175 and now it idles perfectly and will transition well to full power.
Will idle at 1600rpm for a few minutes and then transition up to full power without any problems. Max rpm is 6900 on the APC 20.5 X 10PN. My other 175 will pull 7200 on the same set up, but that has more run time.
I flew the complete pattern today without any hint of a problem. On completion of the pattern i was practising a couple of manouevres and the engine quit. I landed and did not switch anything off. I trtied to start it and it wouldn't fire. I run the Tech Aero IBEC and this is switched from the gear switch. I checked the output and it had 5.3 volts. OK. I switched the IBEC off and back on again. The engine started and ran perfectly. Hmmm. Even though i have already changed the ignition module when fault finding the initial problem, i will change it again and see how that goes.
Thanks to all for your advice and feel free to give any more that may be of benefit. I will let you know how the ignition module change works out.

Regards
Scott