1988 85hp force

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djohnalyn

Seaman Apprentice
Joined
Apr 23, 2007
Messages
34
on a 17' bayliner Capri...

...I'm running 4800-4900 rpm's at wot (MPH unknown):(

However, I'm yearning for those extra 600 rpm's to get into the 5500 range. :)
1. where do I start?

2. Is this just a prop situation? If so, how do i determine my prop size?

3. What prop is providing the best results for this boat/motor combo?**

**Please, only those with this exact boat/motor combo respond to #3...any variance to this combo is N/A in my book...apples to oranges fellas.

Also, 3 out of 5 times that I kill the motor it won't restart right away. Could this be a symptom of a fuel supply condition, thus leading to the lower rpm condition?:confused:

Thoughts?
 
S

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: 1988 85hp force

While you are in the right section for Force info your prop question would do better in the prop forum be sure to give as many details as possible.Prop numbers usually numbers like -- X --typical load etc.
A gps speed would be helpful.Difficult to evaluate rpm without an accurate speed.
You should varify your tachs accuracy.Borrow a tach or get a tinytach for about 40 dollars.
In the mean time check the compression,where is the antivent plate in relation to the bottom of the boat.Are the throttle plates opening all the way?
Check the boat for absorbed water, could add 100s of pounds to the weight of the boat.Is the bottom of boat clean and true?Does the boat seem to get up on the water and not plow.Have you adjusted the trim and what were the results?
You can't make a proper evaluation without all this info.
 
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djohnalyn

Seaman Apprentice
Joined
Apr 23, 2007
Messages
34
Re: 1988 85hp force

You should varify your tachs accuracy.Borrow a tach or get a tinytach for about 40 dollars.
In the mean time check the compression,where is the antivent plate in relation to the bottom of the boat.Are the throttle plates opening all the way?
Check the boat for absorbed water, could add 100s of pounds to the weight of the boat.Is the bottom of boat clean and true?Does the boat seem to get up on the water and not plow.Have you adjusted the trim and what were the results?
Click to expand...

I dont want to drone on, but I figured I should answer as many of those as I can.

The tach is accurate, I used a known good one for comparison.
I dont have a gps nor do I know anyone with one.
[The antivent plate...I'm not sure what that is..?
Throttle is opening wide
How do i check fo absorbed water? Best method?
Bottom is clean and seemingly true yet I see about a dozen or so 1/2in hairline fractures. They are really small and really hard to notice. Thoughts?
The boat gets up and planes in about6 or 7 seconds..no plowing.
I have only tried one trim setting (second from all the way in with a manual trim setup)
 
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djohnalyn

Seaman Apprentice
Joined
Apr 23, 2007
Messages
34
Re: 1988 85hp force

ok as it turns out I have a michigan 13 1/4 X 15 prop installed on the motor.
 
M

Matthew A.

Petty Officer 1st Class
Joined
Jan 24, 2006
Messages
232
Re: 1988 85hp force

I have the same boat and motor setup as you. You can get the feeling of a whole new boat under your seat just by simply installing a set a trim tabs if you haven't already. Along with tabs you can also gain a few hundred rpm by making sure you trim the motor at wot. The tabs don't really effect the ride at top end speed boat how quickly she gets up on plane. Trimming via the motor [if you are not already aware] alone can add another 3 to 4 mph in top speed. Which also correlates into the extra rpm you maybe wishing to obtain.

During light loads and smooth water conditions and proper trim angle, my 17 foot Capri with 85 Force w/14" x 17" semi-clever 3 blade alum. prop. tops out at 33-34 mph at just smidge over 5200 rpm. Add some gear, a passenger and some wind chop and speed drops to about 29 mph and 4700-4800 rpm at wot.
 
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djohnalyn

Seaman Apprentice
Joined
Apr 23, 2007
Messages
34
Re: 1988 85hp force

Well Matt, I have manual tilt/trim. I have been using the 2nd slot away from the transom thus far, but next time out i am going to test the other 3 positions and see what happens.
I have the same boat and motor setup as you. You can get the feeling of a whole new boat under your seat just by simply installing a set a trim tabs if you haven't already.
Click to expand...
As far as trim tabs, what brand/model did you go with?
I want to buy a set of smart tabs sx 60's, but from the looks of my setup with the swim ladder I am afraid they wont work properly. DO you have a pic of your install?

Also, considering you have a 17 pitch prop and you run at 5200, I would assume if you dropped to 15 you would probably be back up to the 5500 range right? I currently have a 15 and cant break into the 5k's. Hopefully I learn somehting this weekend after I take all the suggestions and do some testing.

Off topic, but I think my spark in #3 may be weak...i wonder if that could effect my rpms to the tune of 600 rpm.
 
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Matthew A.

Petty Officer 1st Class
Joined
Jan 24, 2006
Messages
232
Re: 1988 85hp force

Any of amount of "out of tune" can cause of loss of rpms. As far as my motor not obtaining the 5500 rpm mark, it is within the 5000 to 5500 rpm range listed it's serial plate. The difference in speed gained by going beyond much beyond 5000 rpm is minimal. Less then 1 mph if even that much. In my opinion, the wear and tear and increase in risk of motor damage (espescially an out of tune or lean engine condition) at rpm's over and at 5000 rpms isn't worth the gain of an extra 1 or 2 mph. A slight lean condition at mid rpms that might not even be detectable can within a fraction of a second at WOT be all that it takes to burn a hole clear through the top of the piston.

Prop pitch and the type of prop used and it's effect on engine performance can be huge. Such as cupped vs. non-cupped. I recommend visiting the prop forum. The members there know their stuff.

Concerning trim tabs, no real magic there. I made my own [sorry, no pics] by bending 12 inch by 12 inch by .125" thick stainless steel sheet and cut them at a slight angle to fit the angle of the transome and to clear the ladder brackets. They are not adjustable but considering I made them in less than an hour at a cost of 20 dollars I'm not complaining. And the difference installing them has made in how quickly my boat gets onto plane as opposed to without them is SICK. (in a good way) What took 10 to 15 or more seconds to obtain plane has now become in two or three boat lengths without having to gun the throttle. Myself, gear and one passenger an on plane at 12 mph full plane at 14 to 16 mph.
 
Frank Acampora

Frank Acampora

Supreme Mariner
Joined
Jan 19, 2007
Messages
12,004
Re: 1988 85hp force

If you want to spend the bucks, the same prop in Stainless Steel will give 200 to 400 rpm more with all other conditions the same. This is because the blades are thinner and slice through the water easier. The steel also flexes less for improved performance. Once I went to stainless, I never went back to aluminum. I just use my aluminum props as emergency spares
 
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djohnalyn

Seaman Apprentice
Joined
Apr 23, 2007
Messages
34
Re: 1988 85hp force

Well I have been thinking about an SS prop. I've found a couple used ones for reasonable prices around here.

However, as far as my rpm problem goes, I discovered somthing today.

After I installed my newly acquired tilt/trim with wiring harness :D I decided to do some testing.

I took off the air intake, jammed my control to WOT, went back to the motor and noticed that my throttle plates weren't opening all the way. :eek: If opening all the way = 45 degrees, then my plates are only opening about 40 degrees.

Something tells me ( steelespike ) that if I can figure out how to get the plates to open all the way then my 5000+ rpm will follow.

any thoughts
 
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