1987 Force 85 Max RPM issues?

[FAQdaWorld]

Hey Folks,

I have a 1987 Force 85 that I am having some issues with.
First some basic info:
Motor is on a 1987 Bayliner Capri Bowrider 16'.
Prop is 13 1/2 x 15
Compression Hot is 125, 120, 125.
Max speed I am getting is about 30mph.
*****I've hooked the engine up to a timing light with a tach. Verified 30 deg advance at WOT, and I performed the carb sync and tune provided on this site. At WOT i am getting about 3800 rpm according to the tach**********
Motor has a new fuel pump, carbs were rebuilt at the end of last season.

From what I have read Shouldn't I be getting rpm around 5k at the engine with this motor? (Or at least somewhat closer to 4.5k?) 30 mph is fine for some towing and fishing but I can always use some more speed. I'm aware That i can always prop up to get some more speed, but that will make my rpm even lower, and make my holeshot pretty awful from what I understand

If anyone has any ideas why my rpm is so low, or if thats just all I'm gonna be able to get I'm all ears. Thanks folks.

-FAQ / Ryan

 

[foodfisher]

Re: 1987 Force 85 Max RPM issues?

Yes 45-5500 rpms is ideal. You may already be propped too high or the boat load is too heavy. Check below deck for unwelcome passengers. Water, soaked foam. Mid 30's is about right mph. If memory serves on the above.

 

[FAQdaWorld]

Re: 1987 Force 85 Max RPM issues?

Something I have noticed is that when I put the inductive pickup on anything other than plug 1 I sometimes get inconsistent readings on the tach (All over the place) I'm wondering If i'm having electrical issues of some kind that are causing an inconsistent fire at higher speeds, or maybe still just not a strong enough spark or something of the like.
I have replaced some of the forward deck that was beginning to rot, but beyond that I havent found any water damage in the usual susceptible places.

*As an aside question, does the weight of the boat effect the maximum speed of the prop / engine? I know that down pitching in prop gives more RPM on the engine, but does weight effect this as well (I wouldnt think so but I'm guessing)*

I see no reason for this engine not to be getting at least 5k rpm with this prop combo. Anyone else have a similar issue that has been resolved?

 

[jerryjerry05]

Re: 1987 Force 85 Max RPM issues?

Get a Tiny Tach.Install it on the motor.The TT will give the correct RPMs.
That boat/motor combo should get around 40mph.
The 15 pitch is way small it came witha 17/19 pitch.
Do a spark test make sure that all cylinders are firing.
Then go through the fuel system.Starting at the tank vent and working through the carbs again.
Replace ALL the hoses and any inline connectors.Connectors can leak and suck air.
Check fuel pump diaphram etc.J

 

[fucawi]

Re: 1987 Force 85 Max RPM issues?

Dont do anything until you have checked that tacho reading against another...