lower RPM 70 yamaha

[cj413]

Greetings,
It's been a while since I've posted, however, I haven't had many issues up until recently. My 170 cc with a 70 hp yamaha recently went in to have a new tach put on since the old one stopped working. I took about a month and a week but I finally got the boat back. I installed a stabilizer plate from Bob's and took the boat onto the water. In looking at the RPM it only goes up to about 4000 and not the 5000-6000 at WOT that it is supposed to have. I seem to remember that the boat felt a bit sluggish before the installation of the plate but I don't know if it would take that much off of the top end. I am taking it back to the dealer but I am curious as to whether or not the plate would drop the RPMs that much. Also, when I bought the boat in 2006 the dealer told me that he put a different prop on it because the original Yamaha props didn't perform well with these boats. I tried to get the #'s off the prop but they have been hit with a sander of some sort so I can't read them. Any help or comments would be appreciated.

Thanks,
CJ

[fridaysoff]

CJ,
The stabilizer plate that I put on my 170CC did not drop the RPM's at all. Do you have it installed correctly?.If you do then just take off the four bolts and run the boat again without the plate to see if you get your RPMs back. If you take your boat back to your dealer you'll miss the rest of the summer, since it took him 5 weeks to install a Tach. try some fresh fuel and see if that helps.Good luck.
John D.

[BarryB]

Hi CJ,

I have the same motor. When I originally purchased the boat, the motor had a Yamaha aluminum 13 1/2 x 15 pitch prop. The motor would top out at 6,000 rpm. Too high for my taste. I replace the prop with a Yamaha aluminum 13 1/4 x 17 pitch prop. The change dropped the WOT RPM to 5,600.

By the way, what happened with the Tach? I get condensation in mine, but, Yamaha claims that is okay.

[cj413]

Thanks for the response. The gas that i was running was fairly new and only a couple of days old. I filled up on my way home from the dealer. The problem (so I was told) was that the dealer had a difficult time finding a tach that matched the speedometer from the company which is Faria. I don't know what happened to the tach only that all of a sudden it stopped working for no apparent reason. The dealer promised that the boat would only be gone for a few days as he had to test drive it to figure out the problem. We'll see. I' will be on my boat fishing a week from now one way or another even if I have to use oars. I'll post again when I find the result. I am just afraid that the poor performance will immediately be blamed on the plate but to be honest I don't want to go through the hassle of removing the plate and putting it back on again. Thanks again.

CJ

[cj413]

Got the boat back today after the dealer told me that nothing was wrong and charged $200.00. The dealer said it was running at 5000 rpm and speed of 38 mph with two people in the boat. I was told that it could go higher, however, the plate was making it porpoise too much to trim it all the way out. I then put it in the water and found that it ran about 35 mph and 5000 rpm with me in the boat. If I move forward to keep the bow down the speed and rpm's increase. I think that I will keep the plate even if it may shave off a couple of mph at WOT. I know that I wasn't imagining the first problem but it seems ok now....go figure.

CJ

[BarryB]

For $200, I would have gone for the test drive with the dealer. Oh well, the motor seems to be working.
The optimum trim angle for a motor is to trim up until the boat begins to porpoise, then trim down a small bit.

[mtty]

I think the guy dinged you for the $200. On the back of the tach there is a switch related to the "number of poles" your alternator has. This is so the same tach can be used for different motors of varing sizes. When my boat was new it was set in the wrong position and I couldn't get manufacturers RPMs out of it. After much research, I discovered this switch and adjusted it. No problems in the last 6 years with my Faria gauges.
JT

By the way, I'm with BarryB spinning a 13 1/4 x 17 at 5600 rpms WOT on a 70 hp 2 strk Yamaha.

[glehav]

mtty that is some good info on the tach can you get more info on the motors and what adjustment that need to be made for the tech and evenrude motors. i am haveing trouble getting the right rpms out of my motor might just be the tach..... thanks

[mtty]

I found the right position by trial and error. Check your owners manual and see what WOT rpms are. I think my tach had four settings. Next time on the water move the switch and run it up to WOT and see how close you are. If you're off try again. Each position will vary the reading by 800 to 1200 rpms possibly more. If you're running the average or commonly used prop for you boat /motor combo it'll be obvious when you have the right setting. By the way I'm clueless what a "pole" on an alternator is but apparently the tach is fed based on the number you have.

I reread the above post and must point out that prop diameter and pitch have a lot of affect on rpms. I wouldn't use the method I have described unless the rpms are off WOT rpms by a 1000 or more unless you're sure the boats propped right.

Oh yeah. Be sure the motor is off when adjusting the setting for fear of electronic damage to the tach.

[cagrove]

You can go to the Faria web site and download the information for the tach settings. To save you the trouble, I've attached the file below. Find your engine type and it will point you to the number of polls your tach should be set to.

If you have a different guage, just go to your manufacturer's web site...I'm sure they have something similar.

Tight lines.