is there anyway to test if a tach is giving correct reading ?

[greasemonkeyjmj]

My 87 Evinrude 48SPL tach is only showing 2800 RPMs at WOT I am running it n a Grumman 24 foot pontoon, the engine sounds to me like it is turning higher than 2800, my prop is a 13 3/4 x 15 aluminum 3 blade. I am pretty much sure this is to much prop for this engine, but I dont want to start buying and swapping props unless I know the tach is reading correctly

 

[tblshur]

might borrow a freinds or some multimeters have a frequency or hz scale on them you can read the ac voltage coming from the stator to read frequency or rpms , unless i am mistaken hope this helps:joyous:

 

[SparkieBoat]

might be your regulator/rectifier going bad. troubleshooting guide below

 

[Ibl0wstuffup]

You can get a tach that hooks up to a spark plug wire or some timing lights will tell you rpm.

 

[emdsapmgr]

The tach has a setting on the back, depending on what engine it is reading. Make sure the switch on the back is set correctly for your engine. My OMC V4 and V6 engines are set at "6" position. Not sure about yours-may depend on what stator your engine has. Possible your engine is overpropped. I ran a 11 pitch prop on the 35 on my pontoon. When I upgraded to a 70, it runs a 13. That's on a 17 pontoon. Yours is huge compared to mine.

 

[greasemonkeyjmj]

Thanks folks for the comments, I have to look at my multimeter and see what all settings it has on it. I dont know anyone to borrow a different one from.

Sparkle I was looking through some of the troubleshooting guide yesterday, But I was hoping that it was not going to be a bad rectifier/regulator

Emdsapmgr, at first my tack was set on 6, but my engine calls to be run on 5, but when i changed the setting, it only made a difference in what it registered at idle. It was a never installed tach, but it had been sitting in the weather for a couple of years or longer and had some moisture in it when my buddy gave it to me so it could possible be bad also

 

[gm280]

You can always buy a "Tiny Tach" for around $20 dollars and it gets its reading off of one spark plug wire. And then you can use it to compare readings and even use it for other ignition engines like lawn mowers, weed eaters and such... Not a bad investment... JMHO!

 

[SparkieBoat]

The tach has a setting on the back, depending on what engine it is reading. Make sure the switch on the back is set correctly for your engine. My OMC V4 and V6 engines are set at "6" position. Not sure about yours-may depend on what stator your engine has. Possible your engine is overpropped. I ran a 11 pitch prop on the 35 on my pontoon. When I upgraded to a 70, it runs a 13. That's on a 17 pontoon. Yours is huge compared to mine.

I totally missed that he has a 48HP motor on a 24' pontoon boat. even if something is wrong with the tach or rectifier that motor should need a 10 or 11 pitch prop on that boat to reach the recommended RPM range. at least that is what I think.
If you go to the CDI electronics site and look up the regulator/rectifier part number for your motor it should have an install sheet pdf download available on the page with the part. this pdf sheet will have the proper test for your part. you should test your regulator anyway no matter what as the tend to go bad and burn up your stator. If you can catch it in time you save your stator. If my tach ever acts up that is the first thing I do.

 

[greasemonkeyjmj]

I went ahead and ordered a new rectifier, I figured being that old, it wouldnt hurt to replace it, and I also ordered a new tach, since I didnt know what effect sitting with moisture in it had on it. I figured I would start with those 2 items and then look toward the prop.