Tune Johnson -85 20hp to 35hp?

[Andrea Gail]

My engine is a Johnson from 1985 <br />Modelnumber: J20BEL0NB <br />Serialnumber: B007701<br />Two cylinder <br /><br />20HP spark advance 34 degress (and it says 1978?)<br />Manual start<br /><br />A dealer told me that the only things that were different between 20 hp to 35 hp were the carburator.<br />I have changed the carburator, I noticed a nylon limiter at the orginial carburator.<br /><br />BUT, there was no difference in perfomance at all, nothing.<br />The engine ran good at idle but at high speed it was weak, so I pulled out the choke and then it started to give more power and sounded better.<br />This can´t be right?<br /><br />I know that the manifold is really small on this engine, compared to the carburator diameter.<br />I have a identical engine from 1976, this engine has a manifold that is as wide as the carburator.<br />It seems more logical, because a small manifold becomes a limiter?<br />Isn´t that so ?<br /><br />//Jens

 

[rwise]

Re: Tune Johnson -85 20hp to 35hp?

Andrea Gail <br />first welcome to the board!<br />I would think that one would need to change everything from and including the read valve out. just a guess though.<br /><br />Richard

 

[Riverrat43]

Re: Tune Johnson -85 20hp to 35hp?

AG....I have virtually the same question in a post below. You might want to read the reply from Dane. Makes sense. I've also been told by a mechanic that the prop also would need to be changed to a larger diameter/higher pitch to compensate for the increase in RPM's brought on by the horsepower change. On my particular 79, there is just barely enough clearance now for the prop..no way I could add a larger one with out changing out the lower unit, which would mean changing out the mid section ad infinitem...... With that said, I could be very wrong about all of the above.

 

[Riverrat43]

Re: Tune Johnson -85 20hp to 35hp?

By the way, mine is an Evinrude instead of a Johnson and you've gotten my curiosity aroused. I too wonder why the tag would say 1978 spark advance...etc...when it's an 85. My serial number and model number are nowhere to be found..no ID tag, no model number on the welsh plug...I'm simply going by that same spark advance notice to determine mine to be a 79 model.

 

[Andrea Gail]

Re: Tune Johnson -85 20hp to 35hp?

Riverrat43, I understand that we are asking the same question, I´ve reed our topic.<br />The mechanic I talked to said that the only things that were different was the carburator.<br />Well, i will exchange tha manifolder, and then come back with the result. <br /><br />Feels safe to know that someone else is as mad as me!

 

[Chinewalker]

Re: Tune Johnson -85 20hp to 35hp?

Hi JR,<br /> Yes, the intake manifold is considerably smaller on the 20hp than the 35hp. I'm not sure if there's any differences elsewhere, though. A friend of mine happened to have a 20hp and a 35hp apart at the same time and we checked some things out - with the intake manifold being the most obvious difference....<br />- Scott

 

[G DANE]

Re: Tune Johnson -85 20hp to 35hp?

I´ve had a couple of these.<br /><br />The cylinderheads on the 20 Hp later than 84 are different from the 25 and 35 which have the same heads. The rest of the HP comes from midsection and thru-hub exhaust.Reeds are the same on all. 20 HP has restrictor in carb, different venturi and carb body. Changing manifold and carb on a 25 will only add very little performance, I think 3-4 HP.35 HP has a little extra spark advance too, if I remember right.

 

[Andrea Gail]

Re: Tune Johnson -85 20hp to 35hp?

Chinewalker <br />Are you telling me that the engine exploded????

 

[Andrea Gail]

Re: Tune Johnson -85 20hp to 35hp?

I have no thru-hub exhaust, my engine can´t be later then 84

 

[Chinewalker]

Re: Tune Johnson -85 20hp to 35hp?

Hi Jolly Roger,<br /> Not sure what you mean by exploded... A friend of mine happened to have two mid 1980s motors apart at the same time - a 20hp and a 30 or 35 hp and we noticed several obvious differences between them. As GDane pointed out, there are other things to deal with, too - what goes in must come out...<br />- Scott

 

[G DANE]

Re: Tune Johnson -85 20hp to 35hp?

I dont think the 25 HP´s came out with thru-hub exhaust before 1991.

 

[Andrea Gail]

Re: Tune Johnson -85 20hp to 35hp?

Sorry Chinewalker I miss understood you <br />text: "My memory is muddy, What's this river that I'm in?<br />New Orleans is sinking, man, and I don't wanna swim."<br />- The Tragically Hip<br /><br />I understand that what comes in must come out but since the mechanic I talked to looked at blueprints and serialnumbers he said there is no other difference, he even mentioned the exhaust system.<br />Well I will talk to him again and probarly make the mainfold larger, the same diameter as the carburator.

 

[G DANE]

Re: Tune Johnson -85 20hp to 35hp?

It is not only a question of making the manifold hole larger. If you lay the 35HP manifold flat on the table next to yours, you will discover the 35 is higher, creating more room on outside of reeds. Your mechanic cant have looked at midsection and L/U blueprints, they are quite different.

 

[Andrea Gail]

Re: Tune Johnson -85 20hp to 35hp?

But are you really sure about the hole G dane?<br />My old engine from 1976 has a larger hole, and it´s not like a egg, it´s a normal hole, 35,2 mm in diameter, just as the carburator...And the manifolder has same height as the one I´m going to change from.<br /><br />I will try to find a picture on internet so I understand how the hole is supose to look like, or if someone can describe it for me...<br /><br />Thanks for all help!

 

[G DANE]

Re: Tune Johnson -85 20hp to 35hp?

The higher minifold came out in 1984 if remember right, with a different carb on 35HP probably to make prop HP meet the specification after they started proprating HP. The early manifolds are alike, as long as they have the larger circular hole. Not all 20 HP has the small manifold hole, some only restrictor in carb.

 

[Andrea Gail]

Re: Tune Johnson -85 20hp to 35hp?

That makes sens, but I cant´t use the manifold from my older engine becasue there are some smal difference.<br />I will go to the mechanic in two days, and try to fin a solution.<br />When you mentioned that they had to change the carborator and manifold to meet the specefikation I remember the mechanic told me that long ago.<br />He said my 20 hp was only 17-18hp and the 35hp was about 32-33hp.<br />Still, if I succed with this I will gain (with some luck) 15hp.<br />I can tell you, my boat really need those hp, and perhaps the will engine start to meet the specifications.

 

[G DANE]

Re: Tune Johnson -85 20hp to 35hp?

The only thing you will be able to do, is jump to 25 - 28 proprated HP instead of the 20 you have now. If your 20 is Belgium made and 85 it is proprated. Otherwise you will have to change the midsection and lower Unit. I was told by a VERY experienced OMC Guy that these 20 never really held specifikations. BIA rules demands every motor introduced to be within 10 % of the rated HP. As the 20 is build on exact same powerhead as 35 to save production costs, most of them put out 22HP on a dynamometer. Can be seen by RPM with a testprop too. You probably wount gain as much as you dream of. Try to look an old thread up named 25/35 HP difference, I worked with it myselves.

 

[Andrea Gail]

Re: Tune Johnson -85 20hp to 35hp?

OK<br />I tried to find the thread but I couldn´t..<br /><br />My engine is made in Belgium 1985, sorry for my language but what does proprated mean ?<br /><br />I guess it was a dream-thinking that I could gain 15hp, but still, I just need that little extra power, she doesn´t have high RPM enough.<br />The mechanic told me that those engine should have about 5500 RPM, I would say mine has about 2500.<br /><br />Other boats actually goes in circles around me.<br /><br />Probarly I will make the manifold hole larger, but my question is, will I gain anything at all of doing that?<br />Or will the engine start bllblblbl...and don´t work at all?<br /><br />Once again, G Dane, I really appreciate your help!

 

[G DANE]

Re: Tune Johnson -85 20hp to 35hp?

If your wot is 2500 Rpm its probably only making 10 HP. Proprated means the HP is measured on the propellershaft using a dynamometer, the motors true performance. Earlier than 1984 the given HP of a motor was measured at the powerhead cranckshaft, and in the real world the motor would produce less HP´s as there is a loss in the lowerunit gears - in the neighborhood of 10 %. <br /><br />Are you sure your motor runs as it should ? What propeller is on it, and which boat is it on ? As you noticed I´m from Denmark so I need to improve my english too. Enlarging the hole in manifold wouldnt harm, I guess you will gain some from changing carb and enlarging hole. <br /><br />Standard propeller is a 10x10 on a 14 - 15' dighny, should give you 5500 Rpm.<br /><br />Be Absolutely sure the 35 Carb you put on is CLEAN. Dissasemble carb, remove all plastiparts, including the small keyholelike gasket around lowspeed mixture needle, soak carb body overnight in laquer thinner, and blow out all jets and passages next morning with compressed air.<br /><br /> http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=28;t=002921 <br /><br />Sorry for being so longwinded.

 

[G DANE]

Re: Tune Johnson -85 20hp to 35hp?

Also Check your WOT ignition timing and the carb linckage - to see if throttle actually opens all the way to horizontal. These are good motors. I have a 1984 25HP with a new 35 carb sitting, lower unit is shot. Sat for maybe 10 years now, looking for a lower unit. Post back if I can be of any assistance.