RPM's

B

BA'sboat

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Oct 17, 2011
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came off the lake yesterday, first time with this vessel, 1986 2.3 bayliner capri, AQ131A w/275 stern dr, also came with a 15-19 prop, my question is : is this a correct prop and can I increase the HP?, everything runs ex, temp 185, oil pres 60lbs, but the rpms won't get over 3600-3700rpms @ approx 25mph, I would like more of everthing if possible speed and rpms?????need some good advice. thanks in advance
 
2

2stroke1971

Petty Officer 1st Class
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Nov 8, 2009
Messages
209
Re: RPM's

what size Capri is it? Ive got an '85 1950 (19 feet) with a 2.1 that will do 34 mph topped out...(But I almost NEVER do it) Seems like yours should do at least that unless its much larger than mine.
 
B

BA'sboat

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Oct 17, 2011
Messages
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Re: RPM's

it is a 19.5
2stroke1971 said:
what size Capri is it? Ive got an '85 1950 (19 feet) with a 2.1 that will do 34 mph topped out...(But I almost NEVER do it) Seems like yours should do at least that unless its much larger than mine.
Click to expand...
 
M

Maclin

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May 27, 2007
Messages
6,761
Re: RPM's

I would make sure the engine is up to the task first. Do a compression test to see if it is tired or not. Good tune-up with dwell and timing set accurately (including timing advance in the distributor working properly) using good quality parts such as points, spark plug wires, cap, rotor, condenser and sparkplugs.

Realize that if compression is down a tune-up can only do so much.

A 19p pitch is pretty standard in 120hp 18-19 footers.

It is a good idea to weigh the boat also to see if it has accumulated excess "water weight" thru the years.
 
M

Maclin

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Re: RPM's

and Welcome Aboard! (just noticed your post count :redface:)
 
wellcraft19

wellcraft19

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Jun 21, 2011
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201
Re: RPM's

I would say that you have too high pitch and/or too large diameter on your propeller.
I have a similarly sized boat, with an I-4 AQ171C (that puts out a fair amount of more HP than your AQ131). I run a 14x19 and can top out a bit north of 40 knots. If I would put a 15" inch diameter, I'm likely overproped. If I go to 14x21, I'm clearly overproped (unless I am alone in the boat).

I am assuming that you have gearing in your lower unit that is for the I-4 engines (or 2.15:1).

That said, still undertake all the needed work recommended by Maclin. And make sure your boat is not waterlogged and "obese" by having absorbed too much humidity
 
Don S

Don S

Honorary Moderator Emeritus
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Messages
62,321
Re: RPM's

You are a thousand rpm low on WOT rpm, that is not a prop problem.
Here is a list of possible causes of low WOT (Wide Open Throttle) rpm.

In no particular order, check each one and mark it off the list when ok. You will find the problem.

Engine Won't Reach Operating RPM. Check

1. Fuel condition. Type and Octane possibly old fuel
2. Propeller pitch or diameter, damaged blades
3. Restricted fuel pickup tube or anti siphon valve Fuel System Test
4. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
5. Marine growth on hull and outdrive
6. Wrong gear ratio in outdrive
7. Restricted carburetor air intake (clogged flame arrestor)
8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
9. Poor cylinder compression Compression Test
10. Carburetor defective, or wrong type.
11. Fuel pump pressure and vacuum
12. Boat overloaded, improperly loaded, or improperly trimmed.
13. Engine Overheating
14. Engine timing and ignition system operation
15. Remote control cables and linkage for proper travel to open throttle plates fully.
 
B

BA'sboat

Cadet
Joined
Oct 17, 2011
Messages
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Re: RPM's

I am contemplating going with a 14-17 prop,boat was tuned up new wires plugs points cond, starts great runs and idle great, thanks for all your good info, I'll let you know what happens. yes it is the 2.15.1 gearing,
wellcraft19 said:
I would say that you have too high pitch and/or too large diameter on your propeller.
I have a similarly sized boat, with an I-4 AQ171C (that puts out a fair amount of more HP than your AQ131). I run a 14x19 and can top out a bit north of 40 knots. If I would put a 15" inch diameter, I'm likely overproped. If I go to 14x21, I'm clearly overproped (unless I am alone in the boat).

I am assuming that you have gearing in your lower unit that is for the I-4 engines (or 2.15:1).

That said, still undertake all the needed work recommended by Maclin. And make sure your boat is not waterlogged and "obese" by having absorbed too much humidity
Click to expand...
 
Don S

Don S

Honorary Moderator Emeritus
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Messages
62,321
Re: RPM's

1" of prop pitch is going to change your rpm about 100 rpm, 150 if you are lucky.
Being 1000 rpm low, you have to drop your prop to between a 14-9 and a 14-12 pitch to get a 1000 rpm change.
 
B

BA'sboat

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Joined
Oct 17, 2011
Messages
12
Re: RPM's

I'd be happy if i canrealize approx 500-600 more rpm
Don S said:
1" of prop pitch is going to change your rpm about 100 rpm, 150 if you are lucky.
Being 1000 rpm low, you have to drop your prop to between a 14-9 and a 14-12 pitch to get a 1000 rpm change.
Click to expand...
 
M

Maclin

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Re: RPM's

Has a compression test been performed recently on this engine?
 
Don S

Don S

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Re: RPM's

You need to ask that question over in the prop forum, that's what it's there for.
 
B

BA'sboat

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Re: RPM's

FYI if your enterested: Installed a 15-15 alum prop, rpms increased to 4100, speedo read 30mph, that's exactly were I want to be , thanks everyone for your input.
BA
 
Don S

Don S

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Re: RPM's

BA'sboat said:
FYI if your enterested: Installed a 15-15 alum prop, rpms increased to 4100, speedo read 30mph, that's exactly were I want to be , thanks everyone for your input.
BA
Click to expand...

All you did was find a temporary work around to keep from fixing the real problem. You aren't done yet, it's not going to get better or even stay where it is now.
 
C

captmello

Captain
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Jun 30, 2008
Messages
3,829
Re: RPM's

http://continuouswave.com/cgi-bin/propcalc.pl

I'd have to agree with Don. I checked some specs on your motor.

4800 rpm should be your engine speed at WOT.

Using this prop calculator and the specs of your motor, outdrive and prop, your setup, showed a prop slip of -10%. That also tells me something isn't right.

Have you done a full tune up?

Maybe its your tach and speedo, is 30 mph on gps?

I'd continue to look for issues.

BTW, thanks for updating your thread.:)
 
B

BA'sboat

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Oct 17, 2011
Messages
12
Re: RPM's

Question on motor mounts: 1986 2.3 / 275od
looking at different photos and manuals, my volvo does not have a front motor mount, do I need one or was it designed that way??
 
Don S

Don S

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Re: RPM's

You don't need them, it was designed that way.
 
B

BA'sboat

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Oct 17, 2011
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Re: RPM's

Thanks Don I was really concerned about that, I can rest at ease now.
Thanks
BA
 
B

BA'sboat

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Oct 17, 2011
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Re: RPM's update

Re: RPM's update

BA'sboat said:
Thanks Don I was really concerned about that, I can rest at ease now.
Thanks
BA
Click to expand...
UPDATE!!!
Took my vessel out new years day, after much work on the distributor, complete rebuild, new plugs, reset timing, and dwell, new water fuel seperator, the boat ran like it was designed to, I reinstalled the 15x19 prop, boat planed out in seconds, @4500rpms, we were crusing @31mph as per my GPS, well worth the time and money.
happy new year everyone
BA
 
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