Can I get more HP out of my 72 Johnson 100HP?

outlawfox13

Petty Officer 3rd Class
Joined
May 11, 2009
Messages
96
Hi, I just recently picked up a 1972 Johnson 100hp for $100. It has good compression but no spark(im working that issue now, thanks mr reeves). I was just wondering since I am getting it ready for the upcomming crabbing and fishing season, is there a way to get it up to a 125 or 115? Thanks in advance Mike

Oh BTW it's model # 100ESL72R and I am putting it on a 1980 22' Manatee Cuddy Cabin, from what I have read on iboats its max hp is 235 which seems kind of high. I have seen an 18' Manatee with 85hp and the guy said it ran fast.
 

sutor623

Rear Admiral
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May 23, 2011
Messages
4,087
Re: Can I get more HP out of my 72 Johnson 100HP?

Man, I doubt you will get enough power out of that little 100hp to make a big difference. Do you think that motor will even plane that big ol' boat out?
 

sutor623

Rear Admiral
Joined
May 23, 2011
Messages
4,087
Re: Can I get more HP out of my 72 Johnson 100HP?

p.s. you may just want to sell that motor for a few more hundred and get you something bigger. I have heard that you do not want to go less than 80% of the max rated horsepower, or you will be very disappointed.
 

bob johnson

Rear Admiral
Joined
Feb 25, 2009
Messages
4,304
Re: Can I get more HP out of my 72 Johnson 100HP?

run aviation fuel and you may get another 5 hp out of her...... seems like the OMC were hard to squeeze extra HP out of....at least to the point it wasnt financialy worth while....but the mercs seems to be tweakable to get extra power from..

good luck

get a bigger motor made after 1984

bob
 

emdsapmgr

Supreme Mariner
Joined
Dec 9, 2005
Messages
11,551
Re: Can I get more HP out of my 72 Johnson 100HP?

I went down that route many years ago on that exact engine. First of all that is a small cubic inch V4. That year the 85 and 100 hp engines were both 92 cubic inches. The 125 that year was the larger 99.6 cubic inch version. So you are already behind the curve with that engine. Yet, some suggestions: you could install the rubber intake filler blocks which came out in a few of the early 115's and all the early 140 crossflows. part number 322684. You'll need 6 pcs and they are NLA-but you can find them. They are good for 3-5 hp on that engine. You can also get hold of a set of big bore (1 5/16") 125 carbs from a 72 model. You'll have to rejet them, as they (in stock form) will significantly overfuel that small bore block, but they will work. Porting the exhaust and intakes will improve the breathing and get you increased rpm's at top end, where the hp is. In the end, after all your efforts you will pickup at least 300+ rpm's, which will let you swing a prop one size larger. Maybe 3-5 mph depending on the boat. Personally, I liked the performance after those mods. Half the fun was woring on the engine and it ran just great for many years. If that seems like a lot of work for so little gain, get a higher hp engine with more cubes.
 

outlawfox13

Petty Officer 3rd Class
Joined
May 11, 2009
Messages
96
Re: Can I get more HP out of my 72 Johnson 100HP?

Thanks emdsapmgr, I dont think I will invest that much time or money into this motor. I will see how it runs with the 100, and might look for a larger motor later in the year or next.

Well I seriously doubt the max hp for my boat is 235, seems outrageously high. There really is not much info on these boats(atleast from what I can find online) even the I/O's are only rated to 120hp which makes me think the OB max should be somewhere around that. I am pretty sure it will plane the boat as my buddy had a 19' Seaboss center console with a 90hp johnson and his boat out weighs mine by over 200 lbs. And his planes out well and runs like a scalded dog. I was just wondering if my motor could get a few more HP. Oh and I dont think it is actually a 22' I am pretty sure it is 21-20.5 The guy I bought it off of called it a 22'. here is a link to a similar one with a 115.

http://www.boattrader.com/listing/1985-Manatee-207vc-100091482
 

ezmobee

Supreme Mariner
Joined
Mar 26, 2007
Messages
23,767
Re: Can I get more HP out of my 72 Johnson 100HP?

My guess is your boat would be rated around 150-175. Manatees were some stout hulls. That being said, I'd probably run a large 4 cyl motor instead of jumping up to the fuel usage and complexity of a V6. As bob johnson said above, an '84 or newer OMC will be prop rated which will give you a good amount more power than the equivalent older motor. For your 100, just be happy if it's running and shifting! :)
 

boobie

Supreme Mariner
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Nov 5, 2009
Messages
20,826
Re: Can I get more HP out of my 72 Johnson 100HP?

Look at the max hp rating for your boat and go from there. Don't waste your time or money trying to get more out of the 100 hp mtr.
 

SparkieBoat

Captain
Joined
Aug 17, 2009
Messages
3,643
Re: Can I get more HP out of my 72 Johnson 100HP?

I have a 20' well craft with a 200 johnny on it..tops out at about 50 MPH and gets with it nicely..would not dream of wasting my time with a 115 on it. a 22' boat should easily hold a 235, and thats what I would have on it ..it might even handle a 250, check with your manufacturer for limits. anything under a 200 and you will be just trolling.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Can I get more HP out of my 72 Johnson 100HP?

Couldn't find a 22 but the 21 is about 1500. I would think if healthy the 100 should manage about 35.
 

SparkieBoat

Captain
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Aug 17, 2009
Messages
3,643
Re: Can I get more HP out of my 72 Johnson 100HP?

I have a sea pro sv1900, I can barely hit 38 MPH, with a 115 1999. a 1972 100 is only going to be able to get a 21' boat to about 25-30 mph..if that.
 

outlawfox13

Petty Officer 3rd Class
Joined
May 11, 2009
Messages
96
Re: Can I get more HP out of my 72 Johnson 100HP?

Yea all I could find online was a 21' outboard, and it weighs 1520 or somthing like that.

I am looking at a 1996 Johnson 140hp with 75psi in just one cyl, the other 3 are around 125. I was thinking a cyl hone and new rings on all 4 cyls should fix the prob. The owner said that he took the head off and the low cyl has a fine scratch on it. Other than that it looks fine from what he said. I am gonna post this on the board for more responses, but just an idea.
 

dazk14

Ensign
Joined
Jul 22, 2008
Messages
966
Re: Can I get more HP out of my 72 Johnson 100HP?

Just an fyi... They didn't make a 140hp in 1996. Think 1994 or earlier. 125psi is very good for that particular motor.
 

outlawfox13

Petty Officer 3rd Class
Joined
May 11, 2009
Messages
96
Re: Can I get more HP out of my 72 Johnson 100HP?

Thanks Daz model J140TLERR so it is a 94.

yea I meant 94 he posted it as a 96 I called him so I can get the model # to be sure of the year.
 
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