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snowstorm


Location: Montreal QC Canada
Joined: Dec 23, 2008
Points: 11

Tachometer & RPM
Original Message   Jan 7, 2009 7:02 pm
Hi all,

I need to adjust the RPM of my Tecumseh L-Head (9 HP / 318 cc) engine that I bought in 2007.

For that purpose, I plan to purchase "digital photo (laser) tachometer".

From my understanding this tool works by sticking a piece of reflective tape on a rotating part of the engine (such at the shaft near the pulley where the belts are located). The tool then counts the number of time this piece of tape passes in front of the laser beam in one second and multiplies by 60 to display the RPM.

Did anyone ever used such a tachometer? If yes how are you satisfied with this tool?

Also, I need to know the RPM values that this engine should be adjusted to at: full throttle and also when idling. Do anyone know these RPM values.

/Snow Storm
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bmwe0692


"Have a great and glorious day"

Location: Iowa
Joined: Dec 4, 2004
Points: 79

Re: Tachometer & RPM
Reply #13   Jan 15, 2011 12:02 am
In using the laser tach, there must be an unobstructed view of

the piece of tape. If the tach gets interupted in "seeing" the tape,

the readings will be inaccurate. BTDT

T.J.

Put it where the Big iron wheel runs!!!
trouts2




Location: Marlboro MA
Joined: Dec 8, 2007
Points: 1328

Re: Tachometer & RPM
Reply #14   Jan 15, 2011 11:05 pm
Can any reflective tape be used?  There's fake chrome tape.  Will that work?  I'm thinking of using it to meter rakes and impellers.  Not sure if tape on am impeller would reflect back in the right way.  I think you would have to have the tape on a surface square to the meter.  That might be tough to do on an impeller.  For a rake it would be easy.
Shryp


Location: Cleveland, OH
Joined: Jul 26, 2010
Points: 532

Re: Tachometer & RPM
Reply #15   Jan 16, 2011 5:42 am
The laser ones usually come with some tape.  I have heard of people cutting the tape into strips, but not peeling the backing off then using scotch clear tape so they can reuse them instead of buying more tape.
edgenet


If you enjoy doing it, It's not work

Location: Toronto
Joined: Nov 27, 2010
Points: 84

Re: Tachometer & RPM
Reply #16   Jan 17, 2011 7:04 pm
trouts2 wrote:
Can any reflective tape be used?  There's fake chrome tape.  Will that work?  I'm thinking of using it to meter rakes and impellers.  Not sure if tape on am impeller would reflect back in the right way.  I think you would have to have the tape on a surface square to the meter.  That might be tough to do on an impeller.  For a rake it would be easy.

I have read that people using  the shinny duct tape works perfect. I believe any shiny material will work.

If you are getting paid for what you are doing No matter how much you Enjoy it,   It's a Job
jrtrebor


Location: Michigan - 3 hours north of Chicago on the lake
Joined: Feb 10, 2010
Points: 539

Re: Tachometer & RPM
Reply #17   Jan 17, 2011 10:25 pm
To check the impeller rpm with a laser tach.  I'm guessing you would probably have to take the reading looking down the chute, or take the chute off.  Either that or I think you could get an unobstructed view shooting in right above the auger gear case.

Amazon has a number of them for sale. With reviews
This message was modified Jan 17, 2011 by jrtrebor
trouts2




Location: Marlboro MA
Joined: Dec 8, 2007
Points: 1328

Re: Tachometer & RPM
Reply #18   Jan 17, 2011 10:29 pm
   I just ordered a $15 lazer tach and an $18 small inductive tach.  I doubt it will come with tape.  I have a roll of very shiny silver back tape which I hope works.  Bottom fishing and hoping for the best. 

  A small pack of tape costs $41. 

JimmyM


Joined: Dec 20, 2009
Points: 82

Re: Tachometer & RPM
Reply #19   Jan 19, 2011 9:20 pm
I installed a TinyTach to adjust RPM and accumulate hours. The inductive connection couldn't have been easier. One thing I didn't expect was the RPM drop under just light loads. Full throttle no load is 3700 RPM. Under a little load it drops to 3500-3550. Under really heavy load it's down to 3000. It seems as though it dials in more throttle the farther it gets away from it's unloaded RPM. But even though it's down to 3500, it should have plenty of power to pick back up to 3600. It doesn't sound like it's at full throttle even when it's running at 3500 rpm and minimal load. Thoughts?
trouts2




Location: Marlboro MA
Joined: Dec 8, 2007
Points: 1328

Re: Tachometer & RPM
Reply #20   Jan 26, 2011 8:39 pm
This message was modified Jan 27, 2011 by trouts2
trouts2




Location: Marlboro MA
Joined: Dec 8, 2007
Points: 1328

Re: Tachometer & RPM
Reply #21   Jan 26, 2011 8:41 pm
JimmyM, 


   What you have described sounds normal but the terms do not pin down specifics.  A governor will keep an engine within a no load ball park of the throttle setting i.e 3/4 or full.

   Under “light” you see a drop to 3500 (200 rpm for you).  You do not know how much the governor influenced things already to keep the engine at 3500 rpm. It was already adjusting the throttle before getting to 3500 trying to sustain 3700 then 3600 then stopped trying and leveled off at 3500.  You see 3500 on the tack and say why can’t it bring it back to your 3700 setting.  It already tried and could not. 

   It’s max torque is back somewhere around 26-2800 (guessing).  The torque curve is falling rapidly after 3200 (again guessing) and a faster rate at 3600 and for you even more  at 3700. 

    The engine will have a tough time trying to sustain 3700 because there is not much torque up there.   A light load drops it.  There will be and increasingly smaller drop (less rate) as you  head into the higher parts of the torque curve.  Light loading does not have too hard a time of lowering rpm.  It has a tougher time each 100 rpm you drop with increasing load as you are climbing back up the torque curve (less rpm).  That will keep going until approx 2800 rpm where you will be at max torque, most governor influence (effective influence given the design – jet port opening). 

So light load and quick drop at the top end and as you go down more resistance to drop as you get into the higher torque regions.

You did not notice the governor’s interactions going from 3700 to 3500.  By the time it got to 3500 and showed up on your tach it was done correcting and could not correct (govern) anymore. 

 

Got in two laser tachs.  One for $8 and the other $25.  Both cheapies matched readings on a variable fan blade.  Also got the $18 induction tach.  It’s looks the same as other tachs with S1 S2 buttons, records hours and rpm, single wire hookup, records hi, lo rpm, and has angled side mount plates.  All from Amazon.  The induction tach tracks with the vibratach from idle to full throttle in a few motors so it seems fine also.  Both lasers seemed to work fine with reflector sections cut from a roll of shiny silver tape.  

 Tried out the tach on a crummy motor.  It sagged under load to 1800 and could not pull itself up.  On a good motor under load the droop was to 2800 where it tossed well and could sustain the load without overly struggling i.e. driven hard but taking it ok.

This message was modified Jan 27, 2011 by trouts2
JimmyM


Joined: Dec 20, 2009
Points: 82

Re: Tachometer & RPM
Reply #22   Jan 31, 2011 2:33 pm
trouts2 wrote:
JimmyM, 


   What you have described sounds normal but the terms do not pin down specifics.  A governor will keep an engine within a no load ball park of the throttle setting i.e 3/4 or full.

   Under “light” you see a drop to 3500 (200 rpm for you).  You do not know how much the governor influenced things already to keep the engine at 3500 rpm. It was already adjusting the throttle before getting to 3500 trying to sustain 3700 then 3600 then stopped trying and leveled off at 3500.  You see 3500 on the tack and say why can’t it bring it back to your 3700 setting.  It already tried and could not. 

   It’s max torque is back somewhere around 26-2800 (guessing).  The torque curve is falling rapidly after 3200 (again guessing) and a faster rate at 3600 and for you even more  at 3700. 

    The engine will have a tough time trying to sustain 3700 because there is not much torque up there.   A light load drops it.  There will be and increasingly smaller drop (less rate) as you  head into the higher parts of the torque curve.  Light loading does not have too hard a time of lowering rpm.  It has a tougher time each 100 rpm you drop with increasing load as you are climbing back up the torque curve (less rpm).  That will keep going until approx 2800 rpm where you will be at max torque, most governor influence (effective influence given the design – jet port opening). 

So light load and quick drop at the top end and as you go down more resistance to drop as you get into the higher torque regions.

You did not notice the governor’s interactions going from 3700 to 3500.  By the time it got to 3500 and showed up on your tach it was done correcting and could not correct (govern) anymore. 


What you say makes perfect sense. I didn't realize that the torque curve fell off so quickly in that RPM range. You would think that having the Torque peak at round 3200-3400 would be more advantageous than having it at ~2800. My blower is really slugging away at 3000 and that may be very near it's peak since I really have to push it to get it under 3000 rpm.
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