High RPM's

[iggyw1]

Hi,
I have a 70 H.P. Nissan, model NS70A2. I believe it is a 1991 model. Always runs great. Potential problem today. WOT usually indicates about 5400-5500 RPM's on tach. Today, the RPM's went all the way to 7800 RPM's. Is this possible without the engine blowing up or is it a faulty tachometer. How can I test it out? The RPM's went way up. I did not notice any unusuall running of the motor at the time. I backed her down and shut it off. It started up great again. Went WOT with her again, and RPM's went way up again. I am thinking that it is a faulty tach. but I do not want to blow up my motor. What should I do?

 

[pvanv]

Put a shop tach on it to check.

 

[iggyw1]

Thank you Paul, I will try to research how to hook up a temporary tah to the motor. Does anyone know if my motor would have blown if it actually reached 7800 RPM's as I posted, if the max. should be 5500 at WOT?

 

[pvanv]

Maybe. You would know the second the rod came through the block.

 

[iggyw1]

I think I am going to get a new gage. I went out today and left the throttle alone when I hit about 25 MPH. The RPM's were about 4000. Then the RPM's shot up to 6000, then down to 4500 then up to 6200 then back down to 4200, etc.etc. The RPM's did not sound like they went up at all. The sound of the motor was consistant even though the gage was going kinda hay wire without even touching the throttle. I should mention too that I removed the prop this morning and marked the hub/prop to see if the prop was spinning on the hub. The marks were right where they started out after going on the lake today.

 

[Scott Danforth]

if you didnt hear the motor change RPM's, then it most likely didnt change RPM's

get a laser tach, read the flywheel and verify your dash tach

I bet if you check the connections at the tach, you will find a bit of corrosion.

 

[iggyw1]

if you didnt hear the motor change RPM's, then it most likely didnt change RPM's

get a laser tach, read the flywheel and verify your dash tach

I bet if you check the connections at the tach, you will find a bit of corrosion.

Will check the connections out and clean them real good. Thanks.

 

[raczekp1]

prop spinning on the bushing can do this sytuation

 

[iggyw1]

prop spinning on the bushing can do this sytuation

Thanks for your input. I mentioned in post #5 above that I did mark the prop & hub before I took the boat out and the marks were where they started out when I put the marks there. You must have missed that.

 

[pvanv]

If you end up needing a tach, PM me.

 

[iggyw1]

If you end up needing a tach, PM me.

Thanks. I will try to PM you, but I tried to PM someone on this forum in the past and I just could not do it. I ordered one of the laser tachs to just check mine out, and my vehicle is in the bump shop for the next week (someone hit it when I was parked) so I have a lot of time to work on the boat in my garage and clean up the contacts on the tach, since I cannot tow the boat to the lake. I will get in touch if I need a new tach for the dashboard. Thanks!

 

[iggyw1]

Scaott, I received my laser tach finally. Now another question arises: Where should I place the reflective tape for my laser tach? My flywheel has teeth all around the outside edge of it for the starter to turn the flywheel when it is activated. I can place some tape on top of the flywheel, just inside of the teeth around the edge. Will this give me a pretty accurate reading on the laser tach, or should I put the tape somewhere else on the motor? Thanks!

 

[Sea Rider]

To install the refective tape remove the upper black cover and secure it against the flat surface located above the cogwheel and aim the gun vertically on the tape. That flywheel has a very low border as to secure the reflective tape horizontally. Will need a buddy to hold the gun vertically while you power the combo to its full throttle range. Report if worked ok and the max wot rpm achieved.

Happy Boating

 

[iggyw1]

To install the refective tape remove the upper black cover and secure it against the flat surface located above the cogwheel and aim the gun vertically on the tape. That flywheel has a very low border as to secure the reflective tape horizontally. Will need a buddy to hold the gun vertically while you power the combo to its full throttle range. Report if worked ok and the max wot rpm achieved.

Happy Boating

I'll try that. It will be a few days from now until I can get out on the lake again with a buddy of mine. Thanks!

 

[iggyw1]

I was out today on my boat. There was not a thing wrong with my tach today. at WOT, (or any place lower either) the maximum RPMs went to 5,400. Never went over that and did not go up and down all over the place like it did before. Now, a new question arises: If I had weeds on my prop, could that possible make my RPMs go really high since the prop blades would be full of the weeds and not pushing the boat as though there was no prop at all? Just wondering if this could have been the case when my tach was going crazy???

 

[km1125]

You would easily hear that in the sound of the motor, much like if it was cavitating or ventilating.

 

[iggyw1]

You would easily hear that in the sound of the motor, much like if it was cavitating or ventilating.

So in other words, the RPMs would have actually gone up to the 6700 that I saw on my gage at the time and I know I would have heard the sound change, so it was not weeds then I guess. I still have to get my son over so he can get underneath the dash board and check the wiring on the gage. I'm just too darn big to get down under there.

 

[Sea Rider]

Yes quite possible. So both tachs reads exact same 5400 wot rpm ? at 5400 the motor is 200 rpm over the lugging side and 480 under the max wot rpm range. If wanting to better the hole shot, carry more boaters and increase slightly the top end speed go for an OEM with one les pitch from current prop. How loaded was the combo when achieved max 5400 wot rpm ?

Happy Boating

 

[iggyw1]

There was only two of us in the boat at the time. (Two BIG guys). I only go with two or three in my boat max. My boat is not for pleasure boating, it is just a fishing boat, period. Usually just two of us, and usually the other guy is smaller than me, so we were just about maxed out in weight when I was out for my last report.

By the way: I did not even try the laser tach when my wired tach indicated 5400 RPMs max. my last time out in my boat. That seemed normal to me.