Getting accurate tach reading from NS9.9D

[Bronlonius]

I'm trying to figure out my WOT rpm so I can determine if I have the right prop or not.

My motor is an NS9.9D, with the 15HP throttle cam mod. I tried using my automotive multimeter (a MAC tools EM710) to get a good reading when moving.

The EM710 has 2 ways of getting a reading:

1. By probing the coil. I tried this with the yellow wire, and setting it to "2stroke, 2cylinder" and could not get it to give any reading at all.

2. With the induction clamp attachment. I put this clamp around the upper spark plug wire, and it reads okay from idle to about 3500 RPMs, then it just stops working and reads 0. The reading seems off to me though, it seems too high it reads around 1500rpm at low idle, which certainly doesn't sound like 1500rpm to me. There is no setting for number of cylinders in this mode, only 2 stroke or 4 stroke, so I'm not sure if this is just really inaccurate or what.

Anyways, I know the first answer I'll get is probably to buy a genuine tohatsu brand tach, but I'm just wondering if anyone else has gotten a tach like this to work, I really don't want to buy a $75 gauge that I'm only going to use once if I can help it.

 

[pvanv]

Re: Getting accurate tach reading from NS9.9D

We use a good induction tach in the shop. Since you have a siamese coil, both cylinders fire on each revolution (with a "wasted" spark on exhaust), so your tach may read double. Also, the output is very high, and very short, which can fool some tachometers, especially at higher speeds. Odds are your tach is not adequate to work properly in this case. Yes, a Factory tach will work well. If you prefer, you may be able to borrow a good shop tach.

 

[Bronlonius]

Re: Getting accurate tach reading from NS9.9D

Thanks Paul, I guess the siamese coil explains why it seems to be reading double. Interestingly enough, it reads up to WOT when I run it in a tank, but craps out when the boat is actually in the water and moving. I'm going to try it again, and try and secure the induction clamp in place with electrical tape or zip ties, I think it might be moving slightly when the bow of the boat goes up. It's kind of tricky, the clamp just barely fits in there to get the cover on. Unfortunately there's too much splash back there from my depth finder transducer for me to be comofortable running it at WOT on the lake without a cover. I think I have another induction tach with an analog gauge somewhere too, I'll see if I can find it.

 

[Sea Rider]

Re: Getting accurate tach reading from NS9.9D

Buy a Hardline induction hour/tach, easy to install, just set it for 2 sparks per revolution. Go full wot, check max achieved rpm. That engine should rev 4500-5300. Standard delivered props usually achieves very near max rpm if engine is placed on light boats.

Happy Boating

 

[Bronlonius]

Re: Getting accurate tach reading from NS9.9D

Thanks Sea Rider. I think if I end up having to buy a tach, I'll just buy an oem tohatsu, and then re-sell on ebay.

I did find some info here:

http://faria-instruments.com/site_manuals/IS0086p.pdf

Strong alternator interference on some TOHATSU / NISSAN outboards and some pre 2001 Mercury 90HP outboards may require wiring a .1mf, 100 volt non-polarized capacitor between the signal and ground stud terminals.

I have a drawer full of capacitors laying around, so I might try that with the probe method and see if it gets me a reading. Also, wonder if my plugs are resistor plugs or not.

 

[Sea Rider]

Re: Getting accurate tach reading from NS9.9D

Why buy and re-sell, a hour/tach is a must have instrument, you can attach it at front lower pan, plenty of space there. Hardline is smaller in size than TT, and much friendly to install too, including less cable left overs to deal with once installed.

Happy Boating

 

[Bronlonius]

Re: Getting accurate tach reading from NS9.9D

Okay, you sold me. I had assumed that the tohatsu tach was more plug and play, and guaranteed to work. But if they are both induction tachs that require tweaking the number of turns to get a reading... and they both will work, the price of the hardline tach is right, especially if the ones from china on ebay are the same thing and work as well(My guess is they are fakes, but who knows - these days everything is fake!).

 

[pvanv]

Re: Getting accurate tach reading from NS9.9D

The OEM Tohatsu tach is not an induction tach. It is an alternator tach. Yes, it is plug-and-play, and yes, it does work right out of the box.

 

[Sea Rider]

Re: Getting accurate tach reading from NS9.9D

If you have a center console, go for the OEM, if tiller installation Hardline hour/tach. Don't throw money buying $ 15 cheapo China tachs at Ebay, on some can't adjust apark firing, or have non accurate rpm readings. Buy US Hardline tach on line if you like, it's worth the extra price difference including accurate readings.

Happy Boating