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.

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 !

Relics,

   Could you cut and resize your pictures to 640 to they don't blow out the forum line formatting?

I think Relics tach is like mine.  His was $16 and mine $18.  That body tach with the S1 and S2 and mounting flanges is all over the net with various names and prices from 14 to 60 dollars.  Most don't have a maker name.  Mine don't.   They all seem to have the same functions but various lead and wire connects.  Some have potted non replaceable bats and some replaceable.  I'm beginning to think they are all the same.

Mine came with a one sheet "manual' which says exactly this (below) for setting the rpm sensor type.  Does anyone have exactly that on their how to set rpm sheet?

TO SET THE TACHOMETER (SPARK PLUG FIRING REVOLUTION):Press and hold the S1 button for 4 seconds.  The LCS will display 01(spark plug fires twice per revolution), 02(once per revoution), 02(every other revolution), 04(hour only, will not display RPM).  Once you find your desired spark plug firing pattern, release the S1 button. The LCD will blink for 8 to 10 seconds and return to total hour meter mode.  The default for the standard model is 02(once per revolution).

The tack is ok and sync's for readings with a versatach which is one of the vibrating wire tachs.  I have three of them an they are all the same and match reading across all three.  The update is slow taking 1/2 second to 3 seconds at times.  This gets to be a problem when trying to read sag.  Often with speed change the thing will post way out of range numbers but come back in over time.  Sometimes when starting it takes the gage three to several seconds to get the rpm right.  I'm wondering if all these tachs respond the same and in fact are all the same internals with a few minor changes i.e. the guts of the reader are all alike. 

Borat, what is your's like?  Does it get instantanous readings and always on the money or does it goof up and sometimes hunt for a few seconds for the right rpm especially with speed changes? 

"Borat, what is your's like?  Does it get instantanous readings and always on the money or does it goof up and sometimes hunt for a few seconds for the right rpm especially with speed changes? "

Normally, refresh rate is probably between .5 to 1 second and is generally accurate.   Mine are getting old already and the batteries might be starting to lose some power.  On really cold days ( colder than below 0F, the lcd is a bit slow.  The tach is refreshing the readout but the lcd cannot seem to respond quickly enough thus causing partial number drop outs.  Overall, I can still get enough of the readout number to know  see the engine speed.   

   Some guys claim they can get much longer than 5 years but possibly they were tractor guys posting from Arizona.  The general spec is 5 years and probably true for a mild temp place or heated garage. 

   Mine were advertised as able to change the bats but their all potted and no instructions on how to change the bat.  Some guys on the net have changed the bat on one that looks like mine with the S1 S2 buttons.  They say it's fairly easy but the soldering of the bat delicate. 

   I'll keep fiddling with the wire hookup placement and number of turns.  Maybe this is the time to switch to an R plug to reduce noise.  Some of them come with shielded hookup wire.  It might help to cut the sensor wire close to the reader and put on thin shielded wire which I have.  Yours and relics have a ground wire and probably a better setup.  The choise was replaceable bat and no ground clip.   

UPDATE: 3/29/2011.  The three cheap tachs (not the lazer tachs) for $18 came without a ground wire and worked but were fuzzy.  After installing a ground wire inside the gage they worked fine.  The response is good and they are on the money compared to the vibratach which is very accurate.  I've quick disconnect wires on all three so can plant them on any machine I put a wire on perminantly.  An aligator clip is used on the wire from the gage to ground.  They all read the same when put on a machine. 

There are many variations of these tachs which are like the one posted by relics.  They all have the same basic board inside but there are 4 board revisions that are around.  Also, some come with a ground wire some don't.  Some are listed as replaceable battery and some not.  Some say they are potted and some not but all the same design.  Mine were listed as potted but not potted.  Mine were also listed as battery replaceable which they somewhat are.  There is no battery compartment and the case is sealed.  You have to open the case with a knive to get at the bat.  The bat is soldered along with having some silicon type glue.  Not a big deal to replace the bat but not exactly what I expected.  Still for $18 they are a good deal and accurate.