Outdoor Power Equipment (Lawn Mowers, Snow Blowers, Chain Saws and more) Discussions

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.

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

   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. 

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?

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.

trouts2 wrote:
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.

I installed a tach on a  ST824 i found it to be a worth while add on.At max load in different types of snow it really lets you monitor rpm sag.Its a great tool for  giveing you  a good idea of the condition of your engine. If you can manage  to stay at or above your peak torque RPM your engine is in pretty good shape         ving y

Its a easy hook up and was 2 wires in my case !

It's good to know what your engine is up to under varying loads.

I use the same tach to monitor engine rpms on various machines.  On machines where the engine is concealed, I usually just put the required wire(s) on the spark plug and ground then feed them outside of the engine compartment and leave them there.  When I want to take an engine reading, I just attach the tach with a pair of Mar connectors. 

They are a nice little tach for cheap money. Although the refresh rate is a little slow its good enough to get the job done !