Can I Install a larger power head

[Hugh Dryer]

Hi I have a 1990 johnson (TJ25ELCE) 25 horse.I would like to go higher in horse power.3 people in the boat just does not get it.My Question is can I bolt on a 40 horse power head without any problem.Thanks Hugh

 

[G DANE]

Re: Can I Install a larger power head

No. You could bolt on a 35 HP Powerhead, but you would loose much of its power, since 5-6 HP of it comes from the better exhaust tuning in the 35 HP midsection and lower unit with thru- hub exhaust.<br /><br />You could beef-up your 25 HP by using carb and intake from a 35HP, but wouldnt add much performance.

 

[G DANE]

Re: Can I Install a larger power head

I forgot : <br /><br />Welcome to the forum. What you could do is, check your motors mounting height. Is the antiventilation plate ( just above propeller ) in line with hull buttom or below. If it is, raising your motor a bit on transom will help. Now thinking a bit more about it, i'm not sure the 90 25 hp didnt have thru-hub exhaust. If yours have, the 35 HP powerhead would fit, but adding intake manifold and carb would get your own up to 35 HP then, if you adjust spark advance as per a 35 too.

 

[nordy]

Re: Can I Install a larger power head

Hello Hugh D,<br /><br />I have a 1999 30hp (2-cyl.).<br />I increased engine power significantly by means of the following:<br /><br />1.) Widen up the carburetor's air intake.<br />2.) Rounding the air intake edges.<br />3.) Increasing the narrow spot in the carburetor by 2 mm.<br />4.) Boring jet whole at maximum possible.<br />5.) Widening the air intake in the engine case.<br />6.) Shortening and polishing the screws in the throttle.<br />7.)Increasing compression ratio by taking off 0,7 mm from the head.<br /><br />With work done you have a different, much more powerful engine.<br /><br />Nordy of Germany

 

[G DANE]

Re: Can I Install a larger power head

I could add: replace reeds with boyesens and bend reed stops back.<br /><br />DONT do it with original reeds - I did it, they broke and a leaf travelled into the motor.<br /><br />Be aware changing the intake manifold with one from a 1986 35Hp or later will add 5 HP if your carb has same bore as 35, which than 1986 has. It is not the size of the hole, it is the shape and hight of manifold creating better flow.<br /><br />Does yours have thru-hub exhaust ??<br /><br />Do a search here for 25 35 Hp difference - will get a lot of info.

 

[Hugh Dryer]

Re: Can I Install a larger power head

Yes i have exhaust that comes out behing the prop<br />Hugh D

 

[G DANE]

Re: Can I Install a larger power head

If you do not have the thru-hub exhaust, but exhaust that leaves mid-section up behind the prop, best you can do is to get the intake manifold from a 35 HP and check that you have the biggest bore carb ( compare with a 35HP ) That will add maybe 3-4 HP, but not much. You need the thru-hub exhaust. Best is to try to trade the motor in for a bigger one. You can gain some by checking your motor isnt mounted too low, try to raise it by putting small pieces of wood under the clamp on transom. Be sure it gets cooling water. It can usually be raised till anti-ventilation plate ( the large plate above propeller ) is 1 to 1 1/2" above the boats keel line.

 

[Duck Loads]

Re: Can I Install a larger power head

I had a '95 25hp Johnson and went through the same thing you are going through. I ended up spending an insane amount of money on my upgrades. I bought a brand new surplus 35 hp military block, carb, and exhaust. I used my old crank, rods and pistons with new rings. I also installed Boyesen reeds, richened the main carb jet, and increased timing 5 deg. It was one screaming motor that I ended up HAVING to get a new hull, because it had to much power for the old one. Somehow or anouther all this was politicaly easier to pull off than just buying a new outboard to start with.<br /><br />Be safe,<br />Duck L.

 

[phatmanmike]

Re: Can I Install a larger power head

cheapest AND easiest is to sell that 25 and buy a used 40.. i see em all day long on ebay...