Early 80's Merc 1700, more speed?

[-DannyBoy-]

Hey all. Just picked up a new rig, a 78 16ft BlueFin aluminum V. It's powered by a mint early 80's 70hp Merc. I had her out on it's maiden voyage last night and was pretty pleased with it's performance! Turned over with first click and idled well. When I got home I was looking at the service manual that came with it and it states that max RPM for these motors is between 5000-5500. My tach only ever showed about 3200 max @ approximately 30mph (not gps) while it was operating tho. I need to mess with the pin placement, the boat is pushing water bad on plane, that may help. Water was spraying off the splash guards in front of the console, about 5 ft back from the bow. So a few questions.

If I move the pin placement to get the bow out of the water better will this allow the motor to rev more?

Could my tach just be off?

Even if I get another 2000 RPM, will that translate to much more speed?

 

[Dave1027]

Re: Early 80's Merc 1700, more speed?

Sounds like your tach is correct because 30 mph is about right for 3200 rpm with a 19 - 21 pitch prop. You are definitely missing your top end. Just think how fast you'd be going if you had 5500. Maybe 50?

Edit to add, how much hp is that boat rated for?

 

[-DannyBoy-]

Re: Early 80's Merc 1700, more speed?

Boat is rated for 85hp.

Should also add: there is a lot of travel in the throttle before it starts accelerating. Maybe the linkage just needs adjusted?

 

[usskydvr]

Re: Early 80's Merc 1700, more speed?

With the engine off check to see if your carbs are opening all the way when the throttle control is maxxed out. If you are pushing a lot of water that could definitely be limiting yout RPMs.