1989 Force 85..4,000 rpm max ??

[mike 1985]

First off , very nice site. This is my first boat. It's a 17' 1989 Maxum with a Force 85 on the back. I noticed that at full throttle the tack reads 3900-4000. I was told it should be 5100-5300 ? Is this true? The top speed is 30-31 mph.

I have read quite a few post and have this info to give.
compression on all 3 cylinders is 120. I checked for spark and it's good on all 3 cylinders. I checked the carbs are opening all the way and they are.

The current prop reads 13 1/2 R 15 MWL. The previous owner said it ran better with a different prop, but he wasn't sure what it was. Could there be that much difference between props? Should I run a
13P prop on this boat ?

Someone suggested to get the boat full speed and trim it up just till the prop comes about half way out of the water ( just for a second) to see if it goes to the 5200 rpm. Do you guys recommend this test?

there's also a large black fin on the back ( searay), i'm not sure what this does or if it has any affect on it?

Sorry my first post is so long, I tried to give as much info as I knew.

thanks in advance for any help/suggestions.

Mike

 

[MWBROWN]

Re: 1989 Force 85..4,000 rpm max ??

Mike,

With a 15 P prop, I would think you would be at the 5300. I have a 85 HP
16 ft 20 P prop and I get 5200 at 38 MPH.

Trimmimg up will get you more rpm and speed, but trim in small amounts.
Take off with engine all the way down to get on plane, and then trim.
You can watch the rpm and speed pick up as you do. At a certian point most boats will start to porpoise.
The wing on the back was probably put on to stop porpoising. I have one on mine.
You could do that test, but you might have a lot of bouncing to get it out of the water.
Other may jump in with engine problem suggestions for you to look at.

Good Luck,

Mike

 

[mike 1985]

Re: 1989 Force 85..4,000 rpm max ??

thanks for the reply. That lets me know I'm definatly low on RPM. I have trimmed it up as you mentioned and as you mentioned it will porpoise alot before it gets close to 4700. I'll try the test later today when I get time.

Mike