With a téster I would not know how to tell you, but they exist in the market account digital vueltas for gasoline engines, without cables, they work in batteries and are very reliable.
Its operation is very simple, you just have to turn it on and bring it closer to one of the high spark plug cables, it will immediately tell you the exact RPM number with a measurement error of + - 10 laps.
I use one in my work to put Small RPM regime engines of electric generators, motobombs, corteped etc.
Its price is about 35 to 40 euros. You can find it in the workshops where they repair agricultural and gardening machinery and motorcycle workshops.
Another option is the stroboscopic gun, is used to dynamically look at the time of the ignition spark jump (Timming). (or in other words, to put the ignition with the engine running). These guns carry a digital or analog screen where several parameters are shown, ignition advance, RPM, Dwell Factor ... Peeero, the price is not the same or far away.
All the best.
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