see if you can borrow a tach from someone - some auto parts stores here in the US let customers borrow tools for the day
I have tinkered with ways to measure the RPM on my bike - setup an oscilloscope once with a microphone to catch the exhaust pulses - worked very well, a lot of work to set up
there are also aps for smart phones that create a frequency spectrum of the sound picked up by the microphone - they are fun to play with, the exhaust note shows up as a spike on the graph. 1200 rpm has each cylinder firing at 10Hz, and because the pistons are offset from each other you dont get a 20Hz combined sound from the two cylinders, you get two 10Hz tones that are out of phase. Problem with the phone aps is the phone's microphone is not designed to go down to sub-audible frequencies - but playing around with the settings and filters I was able to get it to work a few times, well enough to set the idle on my VS650. It works better on an engine running at operating speeds - I was sitting in my yard watching the frequency of cars as they drove by, and I could see the RPM of a neighbors lawn tractor as she was mowing her lawn.
A tach that you hook over a sparkplug wire, or clip onto one side of one coil is the easy way.
Going by ear is sketchy, but one way to do it is to put a sound generation ap on your phone, one that creates tones, and set it to 10 Hz (for 1200 rpm). Then you should be able to tell if the bike exhaust note coming from one muffler sounds higher or lower.
Last edited by KCW; 06-23-2019 at 07:26 AM.