Tachometer and engine RPM's

[Roc]

Have a question about checking the tachometer with what the engine rpm's actually are.  Is the way to check tachometer calibration is to get another handheld tach and jump the wires in the back of the existing tach and compare what the two are reading?  If the panel tach is off vs. the handheld, then you know how much to adjust it.  Or is it using a light on the spinning pulley with a piece of reflective tape?  I would think the reflective tape is telling you timing (it's advance, etc) and not how many revolutions the engine is running at a particular throttle setting.

Thanks!

[Craig Illman]

Roc - You can get inexpensive, hand held optical tachs that cost maybe $30. Yes, you put a piece of reflective tape on the pully and it bounces a laser off the tape. You have someone vary the throttle to fixed points on the engine panel tach, say, 1000, 1500, 2000, 2500 and compare to what the hand held shows. My panel tach is 10% low, I mentally adjust for it. Next time my panel is off, I'll change the dip switches.

Yes, you're measuring rotations per minute of the crankshaft. It's not the same as a timing light on a gas engine where you were comparing the firing of cylinder one with degrees in advance of top dead center.

Craig

[Roc]

Thanks Craig...so what I need to get is an 'optical tach'.  Thanks!

[Ken Juul]

There should also be a screw accessed from the back side that will adjust the needle's position on the tach.  I believe the dip switches just tell the tach the alternator specifics, once they are set, the screw is used for fine tuning.  Ron, Stu, Others, please correct me if that is not the case.

A florescent light flickers at 60 hz.  You can illuminate the crankshaft pulley with the light to do your adjustment.  I found it easiest to set the engine at about 1200 on the tach (multiple of 60) then adjust the throttle until I got the bolts heads on the pulley face to "stand still".  Adjust the tach to read 1200.  Increase the throttle to 1800, again adjust the throttle to get the bolt heads still, adjust the tach.  Repeated  at 2400 and 3000.  It took me a couple cycles until the tach was close at all throttle settings.

That said, I would never recommend that someone not add a new tool to their collection

[Craig Illman]

Roc - Here's one: http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=41727 It's $40, I don't remember where I got mine. Maybe Amazon? Search on "Photo Tach".

I agree with Ken, "no project is worth doing unless you can buy a tool for it".

Craig

[Ron Hill]

Roc : The laser tach is the only way to go.  You add a narrow reflector strip to the drive pulley (on the bottom) then you point the laser tach at that strip and it'll tell you the actual engine RPM.  Take that RPM and compare it to what your Tach instrument says and you'll know if your instrument error is plus or minus of the actual laser rpm.  Do this at a number of rpm points and then run your engine accordingly.

The dip switches entail a formula that you put in the number of poles on the alternator and the diameter of the alternator drive pulley.  Then you go to a table (from Teleflex) and you set the (12 - I believe) accordingly to the up or down position.  Your M35 BC engine dipswitches were set for the original alternator/pulley that came with the engine by Seaward using Teleflex/Universal data.  

Ken, on the MKI Teleflex tachs ("flat" glass) I wouldn't fool around with any screwdriver adjustment.  I don't know about the later "domed" glass tachs.

[Steve Sayian]

Roc,

I used a laser tach on my boat and it worked great.  You'll have to paint the pully flat black (can of spray paint) before you put the reflector strip on as the M35 grey pain is reflective and will hose up the readings.  Once I painted the pully the readins were rock-solid.

Steve

[Ken Juul]

From the UK Teleflex website. 

If the number of alternator poles is not known, measure the actual RPM with a Master Tachometer. Adjust the dip switchs that puts the tachometer closest to the actual RPM. Final Calibration adjustment requires the use of a Master Tachometer. Connect the Master Tachometer and start the engine. At about 1500-2000 RPM, use a jeweller's screwdriver to carefully adjust the potentiometer in the "CAL" hole (about 10 o'clock on the rear of the tachometer), bringing the tachometer into agreement with the Master Tach. Only a
minimal amount of turning is required. Overtorquing of adjustment will damage gauge mechanism. Turning
clockwise increases tach reading, counter clockwise reduces reading. If the tachometer cannot be brought into
calibration, the dip switchs are probably on the wrong setting.

[Roc]

Thanks everyone for this information