Yamaha 70HP, max rpm 3200

[PROS]

Hi All,

Boat is 15ft fiberglass Brooker halfcab.

Engine is 70hp Yamaha 2-Stroke, not sure of the exact year, might be 1999-2000.

Although I have been fishing for many years, this is my first boat, owned for less than a year.

Engine always worked without problem and sounded good, no problems however since I bought the boat, max rpm at WOT always shown as around 3000-3200.
I normally cruise at about 2400 rpm at about 40kms.
If WOT at 3000rpm, speed is just above 50kms.
Reading on Yamaha website, max RPM should be 5000-6000.

Recently reading about incorrect propping might be the cause for low rpm, got me thinking.
The prop is stainless steel, it has a 14-K stamp on it.

Spark plugs are new.
There is spark in all 3 cylinders.
I havent cleaned the carburetor, thinking of doing this next.

My aim is to make sure engine is not overworking, reduce fuel consumption and more top speed if possible.

Boat is catching fish, I really want to look after it.

Many thanks for your help

Regards

Max
PROSEAT
Auto Aircraft Marine Interior Centre
www.proseat.com.au

[Fed]

Judging by your boat speed the tacho is reading half what it is actually revving at.
Check the back of the tacho to see if there is a little switch that you turn depending on the # of poles in the alternator.
50+KPH with a 14" prop says you're revving up around 6K.

[STUIE63]

+1 for the tacho reading wrong

[PROS]

Thanks for your assistance.
Wrong Tacho would be my best case scenario,

Checked the back of the tacho, it seems to be a closed unit,
cant see any switch to move,

Tried to move the little round bit on bottom left corner but it doesnt move.
I will open up the tacho box to see if anything can be done inside

[captain rednut]

the selections are behind the little black rubber grommet on the top right hand side above the cable tie in the photo.
cheers cr

[PROS]

underneath the grommet, I can see a switch board with 3 switches,
#1 & #3 is up in ON position, #2 is down.

Should I put the #2 up and try to see the readings next time I will be out

Max
PROSEAT
Auto Aircraft Marine Interior Centre
www.proseat.com.au

[Fed]

Just call your local Yammy dealer he should be able to tell you what to set the switches to.

[PROS]

After bot more research, I found that

Switch 1 - up for two lead trim sensor, down for three lead.
Switch 2 - up for 6 pole generator, down for 12.
Switch 3 - up for 4 stroke, down for 2.

70hp is 6pole as far as I understand.
Switch #2 & #3 should be down.
As I dont know the exact year of the engine, I will try both options for switch #1 and figure out which one is correct.

All I need is some good weather now.

Watching the news tonight, C10 reckons Apr will be another windy/rainy month.......


Max
PROSEAT
Auto Aircraft Marine Interior Centre
www.proseat.com.au