Tachometer setting for 4 stroke

knightscape

knightscape

Member
Sanity check please? I've got what should be a real simple question about a tachometer on a 4 stroke. I put the Trail Tech TTO on my HS142, and it's default setting is 1 spark per RPM, which I thought was the setting for a 2 stroke. It has settings of 0.5, 1.0 and 2.0 for sparks to crank rotations. I thought I'd figured out that the 4 stroke needed to be set at 0.5, but on that setting it's showing my cruising RPM at up in the high 13k to 14K range, which should be double what it is. So is the correct setting actually 1.0 for sparks to RPM on a the Huasheng? Do the HS142's waste an unneeded spark on the exhaust cycle? Now that I'm thinking about it, the magneto/CDI would spark on every rotation, so 1.0 would be correct? Thanks!
 
MotorBicycleRacing said:
Yes, these 4 strokes fire on every rotation.
Click to expand...

Don't 4 strokes fire every other rotation? You know, intake, compression (1 rotation), spark fire/power, exhaust (2 rotations)? Not dissing you - maybe some of the thread went clean over my head...

Separately, my cheapie tach registers exactly half the r/m of my 4 stroke GX 35, and has for years. My little brain has finally learned to compensate.
 
I can confirm what MBR said, I reset my tach to 1.0 and it's been reporting the correct RPMs. Morover, I love this little TTO unit, simple and works great! I guess it's pretty common on small 4 strokes to waste a spark after a little more reading. Ideally they would only fire every other time around, but with the magneto on the flywheel running around each rotation, it's 1:1.
 
My brother bought a TTO years ago for his GXH50 and it ended up requiring the 0.5 setting to work properly.

We tried the tach on my HS142 and it needed the 1 setting. Weird.
 
Most 2cyl ignitions do the same thing, when the cylinder that is not on the stroke to fire gets the spark, due to is point in the cycle its just harmlessly wasted.
 
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