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 Post subject: 40 HP prop pitch
PostPosted: 12 Apr 2011, 22:35 
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Well I took the boat for a spin the other day (17 foot Tracker, 40HP Force (2 stroke, 2 cylinder), and something seems off. If the tach is correct, which I'm not sure it is, I'm going to blow this motor. It was reading over 7000 rpm (5200 is normal) at WOT. Anyone know what the idling RPMs should be? I'm going to see what the tach is reading tomorrow at idle and see if its close to a normal range.


So the tach reading was the first thing that got me thinking. Second thing was that I had the prop "runaway" on me one time after throttling down and opening it back up again. That usually means you're under propped right?


The prop that's on it is an 11 pitch. I'd guess that's a little low, especially considering the get up this thing has out of the hole.

I'm not sure of the overall size.


Any suggestions?



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 Post subject: 40 HP prop pitch
PostPosted: 13 Apr 2011, 06:27 
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Joined: 26 May 2009, 05:47
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Location: West Olive Mich
For every pitch change you will change your rpm by 200 as rule of thumb.



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 Post subject: 40 HP prop pitch
PostPosted: 13 Apr 2011, 10:52 
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Runaway as in feeling like it slipped? Could have just been cavitation if it was an abrupt throttle down and rethrottle.

If it jumps right out of the hole, chances are that you may be underpropped. I wouldn't think that it would turn 7 grand without something letting loose but you never know but an 11 pitch does seem kind of small for a 40. Being on a 17 foot boat is probably going to exaggerate the 200 rpm for every inch of prop increase rule, though.



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 Post subject: 40 HP prop pitch
PostPosted: 13 Apr 2011, 17:37 
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I took it out today, and I don't know what to make of it. The first run after letting it warm up read 6800 rpm WOT. Idling at 800 to 1000, which seemed a tad high, but feasible.


After fishing for a while and making a few more runs, it was reading 8000 RPM WOT and 1200-1400 at idle. There's not a chance its idling that fast, nor is it hitting 8000..

At WOT it sounds about right when compared to my last 40 HP, so I'm leaning toward the tach being off and maybe being slightly under propped. I talked to the guy who sold me the boat and he said the 11 pitch is what came on it, and that's why the spare was the same.


I might try to trade the spare for a 15 pitch just to see how it does, but I guess I'm going to leave it be. That motor has been running for years on that prop, so it must be the tach that's bad. I don't see how a motor could last over a decade going 1400 to 2700 rpm over.



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 Post subject: 40 HP prop pitch
PostPosted: 13 Apr 2011, 17:56 
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Is it a normal style marine tach or one of those tiny tachs?

My tach is always goofy the first trip out every spring until I pull the terminals off of it and give them a good cleaning.

Last year it stopped working altogether until I cleaned it and this year it was reading almost 1k rpm too high at WOT.



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 Post subject: 40 HP prop pitch
PostPosted: 13 Apr 2011, 18:05 
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Normal tach.. Probably has never been cleaned, so it'd probably be worth a try. I might just break down a buy a tiny tach, and that way I'll always have something to check with.



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 Post subject: 40 HP prop pitch
PostPosted: 14 Apr 2011, 11:49 

Joined: 25 Feb 2009, 20:44
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i'd like to hear about your findings on this as well. I have the same motor and am curious as to whether my tach is good or not too.

When I'm idling at 800/900 (per the tach) it's rough but if I increase the idle to 1100/1200 it's just right.

Then when I am at full throttle, my tach is reading 5300 and when I trim it a little the RPM will jump up to 5800.

I thought these electronic ignition motors had a rev limiter?


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 Post subject: 40 HP prop pitch
PostPosted: 14 Apr 2011, 12:13 
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crkdltr wrote:
i'd like to hear about your findings on this as well. I have the same motor and am curious as to whether my tach is good or not too.

When I'm idling at 800/900 (per the tach) it's rough but if I increase the idle to 1100/1200 it's just right.

Then when I am at full throttle, my tach is reading 5300 and when I trim it a little the RPM will jump up to 5800.

I thought these electronic ignition motors had a rev limiter?


That motor is always going to feel rough when idling. Those two cylinders have that thing shaking like like crazy.

Not a clue on a rev limiter. I'm trying to get a hold of a service manual.



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 Post subject: 40 HP prop pitch
PostPosted: 14 Apr 2011, 12:19 

Joined: 25 Feb 2009, 20:44
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dyeguy1212 wrote:
crkdltr wrote:
i'd like to hear about your findings on this as well. I have the same motor and am curious as to whether my tach is good or not too.

When I'm idling at 800/900 (per the tach) it's rough but if I increase the idle to 1100/1200 it's just right.

Then when I am at full throttle, my tach is reading 5300 and when I trim it a little the RPM will jump up to 5800.

I thought these electronic ignition motors had a rev limiter?


That motor is always going to feel rough when idling. Those two cylinders have that thing shaking like like crazy.

Not a clue on a rev limiter. I'm trying to get a hold of a service manual.


I've got one, although not with me at work.

Let me know what info you need from it and I can try and get it to you.


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 Post subject: 40 HP prop pitch
PostPosted: 14 Apr 2011, 12:24 
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I appreciate the offer, but its more of a "light reading" for me. I like to know the ins and outs, so when something goes wrong, I can fix it on the water.

Any idea where they're sold online? I haven't been able to find one for a 97.



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 Post subject: 40 HP prop pitch
PostPosted: 14 Apr 2011, 12:29 

Joined: 25 Feb 2009, 20:44
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dyeguy1212 wrote:
I appreciate the offer, but its more of a "light reading" for me. I like to know the ins and outs, so when something goes wrong, I can fix it on the water.

Any idea where they're sold online? I haven't been able to find one for a 97.



http://www.amazon.com/Force-Outboard-Sh ... =8-1-spell


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 Post subject: 40 HP prop pitch
PostPosted: 14 Apr 2011, 12:48 
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Is that the one you have? Does it have a lot of info on our specific motor? I saw that one and thought about buying it, but I've seen ones that were just for 40 horses and a 2 year span. Figured itd have more accurate and less generic info, but if the one you linked me to has solid info I'll pick one up.



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 Post subject: 40 HP prop pitch
PostPosted: 14 Apr 2011, 13:49 

Joined: 25 Feb 2009, 20:44
Posts: 117
Location: .....
dyeguy1212 wrote:
Is that the one you have? Does it have a lot of info on our specific motor? I saw that one and thought about buying it, but I've seen ones that were just for 40 horses and a 2 year span. Figured itd have more accurate and less generic info, but if the one you linked me to has solid info I'll pick one up.



I'm not finding as detailed of information as I'd like to find but all the specifics for each engine is there such as the number of turns for the idle air screw, torque specifications, timing and such.


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 Post subject: 40 HP prop pitch
PostPosted: 15 Apr 2011, 11:48 

Joined: 22 Aug 2010, 10:28
Posts: 72
Location: Central Arkansas
Dyeguy,

Do you know the model number for the OB? If so, it should be pretty easy to find it online and at least get the RPM range and factory prop to give you a baseline.


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