How to reduce power on '79 Evinrude 55hp.

Can'tFishEnough

Petty Officer 3rd Class
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Apr 19, 2011
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86
Not trying to make duplicate posts, but wanted to ask in a different way in this specific J/E outboard part of the forum.

I have a 1979 55hp Evinrude that I want to put on a 1994 16.75 ft aluminum boat rated for 45hp on Coastguard plate. Currently boat is set up as manufactured as a tiller handle control. If I keep it as a tiller handle control, I'd like to be safe and stay in the 45hp area. There are not any laws against overpowering a boat in Texas as long as it doesn't cause obvious handling issues making it unsafe in the eyes of the Game Warden (called Tx Parks and Wildlife to confirm this and that's what the lady told me).
So, I'm not too worried about it being "legal" (since there isn't something here making it illegal) but do want it to be safe. Let's assume buying a 45hp motor isn't an option right now. ( I have been looking for a 40-45 hp tiller for a while and just haven't been able to find one that was able to be titled (something they are picky about in TX) or one that is in running condition that didn't cost near as much as a new one.)

As I stated in my other post, http://forums.iboats.com/forum/boat...t-horse-reduce-horse-power-to-be-within-specs --- I know that asking how to loose HP is probable sacrilegious- around here, but that may be where I am to stay safe.

So my question is what can I do to reduce the horse power at least mathematically to a safe range without knowing the +/- 10% or whatever wiggle room the manufactures are allowed. Also as I understand it the '79 at 55 hp wasn't measured the same way as a '94 45 hp. (and I may be trying to set this one up as a remote steering at some point, but would still like suggestions if you have any incase I can keep it as a tiller).

Guesses would be:
- to retard the timing some? I don't know how much this drops HP per degree of timing.
- to restrict full WOT position - this seems like it is not a good option if motor needs to run at a certain RPM to maintain burning the oil mix or something.
- to restrict airflow at carb gasket or run a little richer? Also doesn't seem like best option for keeping economy or keeping excess oil from building up.

I don't really want to have to try and find different carbs to run it down, but think that may be one of the main difference between the 50 and 55... but I haven't done a total parts list comparison...

Other thoughts, or best thoughts?
Thanks for any help you all can provide.
 

jimmbo

Supreme Mariner
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May 24, 2004
Messages
12,944
Well a 79 55 would be crank rated, it probably puts out about 48 hp if it were prop rated. Your boat is a 94 and was rated when engines were rated at the prop. but trying to explain that to a bureaucrat in a way they could comprehend would/will be next to impossible. If it were me, I would put it on and not worry, however I live in Canada where nobody cares how much motor you put on a boat.
 

Can'tFishEnough

Petty Officer 3rd Class
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Apr 19, 2011
Messages
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Well a 79 55 would be crank rated, it probably puts out about 48 hp if it were prop rated. Your boat is a 94 and was rated when engines were rated at the prop. but trying to explain that to a bureaucrat in a way they could comprehend would/will be next to impossible. If it were me, I would put it on and not worry, however I live in Canada where nobody cares how much motor you put on a boat.

thanks-
well, yes, that is part of what I was wondering, how much difference does the crank versus the prop measurement make... I've been told a lot "follow" the max #s on the plate... so I was hoping that there might be an easy "fix" to lower the HP some until I can get a feel for the boat and know that it's handling ok (and then maybe open it up if I want later...) But, I don't have the means to have the old thing dyno tested :) So I was just looking for a few mechanical ways to maybe lower the HP to make sure I was within the safety range for my own comfort. Let's just say that I'm slowly moving up with my own personal boat driving experience with bigger motors each upgrade from a 4hp/12 foot boat to 15hp/14 ft to this one that came with a 25hp/16.75 foot boat and now I wanted to just about double the hp... So just wanted to check in with folks who I'm sure know more than I do.
 

F_R

Supreme Mariner
Joined
Jul 7, 2006
Messages
28,195
I don't have a motor in front of me to look at, in fact I've never even seen a tiller 50/55. But I believe the following is true: Study your throttle linkage and how it operates (in forward gear).

1. As you increase the throttle, first the spark advance moves without opening the carburetors.

2. Then the carburetors are slowly opened as the spark continues to advance.

3. Then the spark reaches full advance and the carburetors are still only partially open.

4. Continuing to advance the throttle opens the carburetors to WOT position.

5. So what you want to do if you want to reduce maximum horsepower is put a limit on #4 (prevent the carburetors from fully opening). You DO NOT want to change anything that affects 1, 2, or 3. Limiting # 4 will not change anything except that last little bit of available power. And the horsepower is rated at the WOT position.

So, summing up, limit how far the carburetors open WITHOUT making any changes that would affect full spark advance or the synchronization between the spark and carburetors. If you do not affect full spark advance or the synchronization, it would be the same as simply not giving it full throttle at the tiller control. This is perfectly acceptable to do, and will not damage the motor. After all, there is nothing saying it has to run wide open all the time.
 

Tim Frank

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Jul 29, 2008
Messages
5,333
I have a 1960 Orlando Clipper 15' that was rated at max 50HP. We put a 1960 40HP on it, but after a few years wanted a bit more oooomph and swapped out to a 1968 55HP Johnson.
It was a perfect combo....the extra 5 HP were an insignificant excess.
Fast forward 40 years and I replaced the 55HP (old rating system) with a 40HP Nissan. (new rating system)
The performance is indistinguishable.

If your 55HP is running well, why mess with it? I'd suggest that you simply bolt it on and go boating.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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The local LEO will look at the motor, look at the capacity plate, and not care if you limited power or not.

They only care if the motor tag is equal or smaller than boat capacity tag
 
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ondarvr

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Apr 6, 2005
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11,527
So did you call any of the shops in AK like I said in your other thread on the same subject, the kits are available.
 

Can'tFishEnough

Petty Officer 3rd Class
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Apr 19, 2011
Messages
86
The local LEO will look at the motor, look at the capacity plate, and not care if you limited power or not.

They only care if the motor tag is equal or smaller than boat capacity tag


Supposedly there is not a law in TX that they care about - they only care subjectively (about power and overloading weight and general operation) if you are being safe or not. That's what I was told directly from TX parks and wildlife. In the last 6 years that I've had any boat, I've only taken one of them out of state for one weekend (to Arkansas) and don't see going anywhere but maybe to Oklahoma with this one in the foreseeable future. I'll have to check on their laws, but I'm not under the impression now that there are any in either of those two or Louisiana. I very much doubt that I would be pulling my boat further than that.
 

Can'tFishEnough

Petty Officer 3rd Class
Joined
Apr 19, 2011
Messages
86
So did you call any of the shops in AK like I said in your other thread on the same subject, the kits are available.

Was going to work on that next week. I'm not sure I need a "registered" kit, but as things are going this is likely going to be an end of the summer/fall project since I'd rather be using what i have now rather than messing around setting it all up for something different and not being able to be on the water.

Just wanted to ask in this forum section hoping to get some more details about how to set it up as I'm about to pull the carbs and redo them anyway. :) I did look up the 90s era tillers and understand what you're saying. may start scanning used ones - but I have a control box and just picked up a used steering system locally. But obviously being a tiller set up from factory, this one doesn't have a console yet...
 

Can'tFishEnough

Petty Officer 3rd Class
Joined
Apr 19, 2011
Messages
86
I don't have a motor in front of me to look at, in fact I've never even seen a tiller 50/55. But I believe the following is true: Study your throttle linkage and how it operates (in forward gear).

1. As you increase the throttle, first the spark advance moves without opening the carburetors.

2. Then the carburetors are slowly opened as the spark continues to advance.

3. Then the spark reaches full advance and the carburetors are still only partially open.

4. Continuing to advance the throttle opens the carburetors to WOT position.

5. So what you want to do if you want to reduce maximum horsepower is put a limit on #4 (prevent the carburetors from fully opening). You DO NOT want to change anything that affects 1, 2, or 3. Limiting # 4 will not change anything except that last little bit of available power. And the horsepower is rated at the WOT position.

So, summing up, limit how far the carburetors open WITHOUT making any changes that would affect full spark advance or the synchronization between the spark and carburetors. If you do not affect full spark advance or the synchronization, it would be the same as simply not giving it full throttle at the tiller control. This is perfectly acceptable to do, and will not damage the motor. After all, there is nothing saying it has to run wide open all the time.


Great! When I get to the link and sync after doing the carbs I'll look into this. I'd like to keep from having to modify anything permanently. This sounds like it may be the ticket.
 
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