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 Post subject: Johnson 25hp question
PostPosted: 25 Aug 2018, 15:44 

Joined: 25 Aug 2018, 14:50
Posts: 20
Folks, I just picked up a new to me Lowe 170 with a Johnson 25 on the back (Model #J25ELSSS). This boat has been modified with three batteries in the back and currently has about 3/4 tank of gas. I've put new plugs in it, added Stabil to the tank since it was sitting for a few weeks before we got it. The motor is tucked back to the transom in the closest hole (manual tilt) to help keep the nose down.

With two humans on board it'll spool up just fine and start to plane but it sounds like it's not reaching full RPM. Now, this is by ear only since the rpm gauge is inop. It shows @15 MPH pretty consistently on the dash, occasionally creeping up to maybe 16-17. If this is what it is, I'm fine with it. But it seems like it wants to rev more but just never gets there. Prop is stock.

Thoughts?

Thanks in advance!


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PostPosted: 25 Aug 2018, 17:04 

Joined: 12 Mar 2017, 12:09
Posts: 564
Is it planing ?


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 Post subject: Johnson 25hp question
PostPosted: 25 Aug 2018, 19:17 

Joined: 25 Aug 2018, 14:50
Posts: 20
I would say so, but it doesn't seem fully on plane if that makes sense. Still runs slightly nose high, but not bad.


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 Post subject: Johnson 25hp question
PostPosted: 25 Aug 2018, 19:24 
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Joined: 12 Dec 2014, 14:32
Posts: 2069
Location: Eastern Mass
Right plugs? Plugs gapped to 0.030”?

With the motor off, and cowling off, twist the throttle all the way open- does the throttle linkage allow it to go FULL up against its stop? Also take off the air silencer, with throttle all the way open, do the throttle plates open all the way up? They should be ‘sideways’ with their edge towards you.

If your speed is being read by an ‘in water’ pitot tube, those are notoriously inaccurate and that speed sounds low for that hole in motor to me.

It’s just me, but I find it hard to do any diagnosing or reasonably accurate ‘predictions’ on a motor’s status without having a good tachometer on board and operating. Proper RPMs and performance are all about having the right prop on the motor for you rig, gear and load you carry.

Check out the propeller calculator that is the sticky under the Motors forum.



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PostPosted: 25 Aug 2018, 19:57 

Joined: 12 Mar 2017, 12:09
Posts: 564
With the three batteries in the back, I have to wonder about weight distribution.


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PostPosted: 25 Aug 2018, 21:29 

Joined: 27 Apr 2012, 21:01
Posts: 227
3 batterys plus gas and driver.. there is too much weight in the back end..
Get rid of the batterys

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PostPosted: 25 Aug 2018, 21:39 

Joined: 04 Feb 2013, 22:11
Posts: 429
GYPSY400 wrote:
3 batterys plus gas and driver.. there is too much weight in the back end..
Get rid of the batterys

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I have three in mine, plus 10 gallons of fuel but it is all up in the front with a 25. Makes a big difference with the weight distributed evenly.


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 Post subject: Johnson 25hp question
PostPosted: 26 Aug 2018, 09:30 

Joined: 05 Sep 2016, 05:50
Posts: 41
My first step would be to shift weight from the rear to the front of the boat. I believe that motor with the stock prop should run at least in the low to mid 20’s to be in its power-band. Getting the weight balanced in the boat will get you more speed, but if it doesn’t get you where you want to be, dropping one pitch, two at the most, on these small motors makes a big difference. But I’d check the rpm’s with a tach first.


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 Post subject: Johnson 25hp question
PostPosted: 26 Aug 2018, 11:43 

Joined: 25 Aug 2018, 14:50
Posts: 20
DaleH wrote:
Right plugs? Plugs gapped to 0.030”?

With the motor off, and cowling off, twist the throttle all the way open- does the throttle linkage allow it to go FULL up against its stop? Also take off the air silencer, with throttle all the way open, do the throttle plates open all the way up? They should be ‘sideways’ with their edge towards you.

If your speed is being read by an ‘in water’ pitot tube, those are notoriously inaccurate and that speed sounds low for that hole in motor to me.

It’s just me, but I find it hard to do any diagnosing or reasonably accurate ‘predictions’ on a motor’s status without having a good tachometer on board and operating. Proper RPMs and performance are all about having the right prop on the motor for you rig, gear and load you carry.

Check out the propeller calculator that is the sticky under the Motors forum.


Correct plugs, NGK called for 040, will regap to 030 and retest. Throttle opens to stop (console boat, so side throttle). Speedo is the plastic finger type that falls off the bottom of the transom. I will take a look at the tach to see if it's unhooked or what.


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 Post subject: Johnson 25hp question
PostPosted: 26 Aug 2018, 11:48 

Joined: 25 Aug 2018, 14:50
Posts: 20
ktoelke54 wrote:
My first step would be to shift weight from the rear to the front of the boat. I believe that motor with the stock prop should run at least in the low to mid 20’s to be in its power-band. Getting the weight balanced in the boat will get you more speed, but if it doesn’t get you where you want to be, dropping one pitch, two at the most, on these small motors makes a big difference. But I’d check the rpm’s with a tach first.


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I would like to but that's going to take a lot of rewiring since it's set up with an on board charger back in the bilge for all three batteries. I'm going to try running some seafoam through it as well. Can't hurt.


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 Post subject: Johnson 25hp question
PostPosted: 26 Aug 2018, 18:28 
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Joined: 15 May 2010, 00:38
Posts: 2635
Location: Central Florida
If your running quality is good at idle then your engine is well sealed and you will be wasting money on Seafoam.
The suggestions on re-distribution of weight are the correct suggestions. With a small boat and limited horsepower it is all about planing surface (area) Your boat is loaded similar to full sized bass boats minus the horsepower. By moving the weight forward you will increase planing surface and your issue will be minimized.
All you have to do is move the batteries on a temporary basis to verify. After that you can make a permanent choice.


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PostPosted: 26 Aug 2018, 21:20 

Joined: 27 Apr 2012, 21:01
Posts: 227
Why 3 batteries anyhow? I run 2 size 24, one in the back for the electric start and all the boat accessories, and 1 in the front for the bow mount TM.

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 Post subject: Johnson 25hp question
PostPosted: 26 Aug 2018, 23:07 

Joined: 12 Jul 2017, 21:42
Posts: 183
Location: Springfield, MO
I'm guessing 170 means 17ft? If so, that's a lot of boat for a 25.

As mentioned, move two batteries up front. You likely have too much weight in the back, it can make more of a difference than you can imagine. I moved one battery from the back to the front of my 1448, it went from plowing at 11mph to planing nicely at 17mph

Next I would get a tach rigged up, see what RPMs you are running WOT. It might be worthwhile to try a lower pitch prop, since you're running a small motor with a heavy load.


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PostPosted: 27 Aug 2018, 16:02 

Joined: 25 Aug 2018, 14:50
Posts: 20
GYPSY400 wrote:
Why 3 batteries anyhow? I run 2 size 24, one in the back for the electric start and all the boat accessories, and 1 in the front for the bow mount TM.

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It was rigged that way, two batteries in series for the 12V trolling motor for increased capacity.


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 Post subject: Johnson 25hp question
PostPosted: 27 Aug 2018, 16:03 

Joined: 25 Aug 2018, 14:50
Posts: 20
Pappy wrote:
If your running quality is good at idle then your engine is well sealed and you will be wasting money on Seafoam.
The suggestions on re-distribution of weight are the correct suggestions. With a small boat and limited horsepower it is all about planing surface (area) Your boat is loaded similar to full sized bass boats minus the horsepower. By moving the weight forward you will increase planing surface and your issue will be minimized.
All you have to do is move the batteries on a temporary basis to verify. After that you can make a permanent choice.


Will give that a shot this week and report back.


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