Office of Steam Logo_1

Author Topic: If you’re gonna slow em down, a laser tach is a great piece of kit to have  (Read 180 times)

Adirondack Jack

  • Jr. Member
  • **
  • Posts: 119
Thinking about the quest to slow down hit and miss engines at idle, mucking about with Governor spring length and    Spring rate, I realized there are really two numbers to be concerned with. The first number would be how fast the engine goes when firing, before the lockout engages, and the second would be how slow it has to go to get the lockout to release, letting it fire.  The thinking is it’s maybe a matter of both spring force and Governor responsiveness, which though interacting concepts, aren’t exactly the same thing.  Springs of similar tension can be more or less snappy in responding. Governor mechanisms can be buttery smooth or not, etc.

Attempting to “science” this stuff means quantifying something besides the familiar hits per minute. Measuring the effect of adjustment both on absolute bottom end, and on lockout speed would give clues to further steps.  Conveniently, optical laser tachometers are cheap and work well.

Simply apply a small strip of reflective tape to one spot on the flywheel, and “shoot” that spot with the light, exactly like a timing light.  The digital readout will show you what you’ve got. I just received one today, for use both with the hit and miss, and the throttle controlled four banger I’m waiting for.
A quick experiment simply waving the reflective tape back and forth in front of the unit proved it sees the tape well, and gives numbers that made sense.  Under $20. Includes carrying case.


  • Administrator
  • Hero Member
  • *****
  • Posts: 3695
  • Aussie Steamer always on the boil :)
  • Location: South Coast of New South Wales Australia
What's more important to count the revs or hits per minute?

My YouTube Channel

Adirondack Jack

  • Jr. Member
  • **
  • Posts: 119
I think we can get value from both.

  Example 1,  An engine goes into lockout at 820 rpm, slows to 440 rpm, seemingly coasting forever, and achieves 45 hits per minute.

Example 2, an engine goes into lockout at 800, rpms, coasts seemingly forever, slows to 400 rpms, but fires 59 times a minute.

Which engine is running slower?

Spring selection and tension only tell part of the story.
Hits per minute only tell part of the story, especially if an engine double taps or triple taps to get to lockout. Might be the engine that hangs longest in lockout is actually going slowest before firing, but takes two or three pops to get to lockout. Might be a Governor is a bit sticky on the top or the bottom or both? Might be the engine is running too rich or lean, or at a less than optimal timing to get a strong pop when it comes out of lockout.  Bearing condition can make or break coasting.  (I remember watching a video about prep for a motorcycle jump. The crew chief tried half a dozen front wheel bearing sets before choosing the smoothest and most free running). I think of our Chinese main beatings as middling at best.

Just as with throttle controlled engines, we’ve all seen hit dnd miss engines happily and reliable ticking over slowly with a regular heartbeat like single strong pop, and we’ve also seen engines stuttering on the edge of stalling, firing erratically as they barely hang on. 

A recent first run video of a quarter scale hit snd miss restoration was perhaps the sweetest sorta little engine I’ve ever heard. (About nine inch flywheels I reckon). A long, lingering coast down punctuated by the tapping of the mechanism, and a single, throaty “whump” as it fired with authority before locking out. The SMOOTH of the old bronze bushes was palpable.


  • Jr. Member
  • **
  • Posts: 243
  • Crisis?...What crisis?
  • Location: Gurnee Illinois USA
Using the same tachometer as yours my engine runs between 1254 - 1395 rpm. Not as
slow as I expected. [ Guests cannot view attachments ]
Civilization is the limitless multiplication of unnecessary necessities.

Adirondack Jack

  • Jr. Member
  • **
  • Posts: 119
Sometimes the cold, hard facts defy our expectations.  When I used to fly giant scale RC, pilots would routinely OVERestimate engine performance and airplane speed.  A tach and occasional assist from a local LEO with a speed gun were often very illuminating.


  • Sr. Member
  • ****
  • Posts: 1006
  • Location: Ontario, Canada
I bought one of these laser tachs and it worked fine for a while, then quit. It was so cheap, though, that I bought another.

When I was about to throw the first one out, I decided to poke around it with a multimeter, and found that the problem was in the battery connector.

Now I have two! Both are great, especially when used with my lathe and milling machine.



Earlytimes - “Märklin 4122/9“ c1907