Swapping power heads 9.9 johnson with 15 hp johnson

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Wistex

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in a previous thread i had asked about two 14 ft jons: one a 1448 (with center seat) with a newer (2001) 9.9 johnson manual tiller and an older (1994) 15 hp johnson with remote controls on a 1442 ncs jon.

as we discussed previously, the 15 hp seems to be weak; the 9.9 very strong in comparison.

again the 9.9 is a short shaft manual start tiller motor while the 15 is a long shaft, electric start, remote control version.

could one swap out the motors so as to put the more powerful 9.9 powerhead in place of the 15 hp motor?

this project would be assuming two issues. one is that no rebuit 15 hp powerheads were available and two, that the rebuilding of the old 15 might be cost prohibitive as compared to what is in question in this topic.

the possibility of someone overhauling the old 15 AND it becoming more powerful than the newer 9.9 exists. but there may be no assurance that the 15 would be more powerful.

as regards the 9.9's power, strickly by my observation, that it seemed as though it was a 20 hp the first time it's throttle was opened pushing the 1448 with a huge surge and leaping on plane in a couple of seconds... or so it seemed.

i reckon one would need the advice of someone that has done this before as i have no experience other than changing props on outboards. i can though, turn a wrench and would tackle the conversion once the details are learned.

one would want the 9.9 to have the amenities of the 15 considering the long shaft, electric start, and remote controls for use in a 1442 ncs alumacraft with a 20" transom.

the weak 15 hp motor's power head would then be inplace of the 9.9 in the short shaft motor.

doing all this would enable one to keep the more desireable ncs boat with the stronger motor and i would sell off the 1448 alumacraft jon with the conventional center seat and weaker but quieter "15" which i would sell as an older "9.9".

wistex
 
Ok there are far to many variables going on here to say that the 9.9 is stronger than the 15. You have them on different boats, and the motors most definitely have different props (which can make a huge difference). I can put a high pitch prop on my 15 and make it seem like a dog, and then put a low pitch prop on my 9.9 and make it seem like a race motor when it comes to hole shot. What I am saying is on the exact same boat I can make a 9.9 feel way more powerful just by putting a low pitch prop on. The 15hp may be way over propped and is not getting to the recommended RPM to produce 15hp?

Also have you checked compression on the 15hp?
 
So.....based on Johny25's answer why not just swap props real quick and see what happens! May just be a little easier than the undertaking you are planning. Also the compression check would be very wise. A little "due dilligence" here would be worth your time before going that route.
 
hi guys, i have changed props a couple of times on the 15. with the original prop the motor over-rev'd to the extent the valves floated and the motor seemed to have a rev limiter as it struggled to get on plane. tried a higher pitch prop (sorry don't have the numbers in front of me now) and the 15 worked significantly better. also tried a 4 blade prop and seemed to run out about the same-- better, but still not satisfactory..

the guys at the shop where i bought the 15 advised they did a comp check but did not have the numbers.

the shop owner advised he would do a comp check for me but have not gotten that done yet.

as regards the different boats, i have weighed them (separated from the tow vehicle) and they weigh about the same; the 1448 with the 9.9 is 20# more per the truck scales.

does anyone have an idea what it would cost to do a top end rebuild on the 15?

wistex
 
Hate to be the one that breaks the news to ya' but.......you have no valves in either of your engines being that they are both two stroke engines. No top end rebuild necessary at least!!
As far as the rest of it goes.....you have successfully confused the hell out of me with what you have done. Am thinking I need some GPS numbers at this point and photos.
BTW - the 1448 has alot more hull area or lifting surface under you and always will plane off faster.
 
Pappy said:
Hate to be the one that breaks the news to ya' but.......you have no valves in either of your engines being that they are both two stroke engines. No top end rebuild necessary at least!!
As far as the rest of it goes.....you have successfully confused the hell out of me with what you have done. Am thinking I need some GPS numbers at this point and photos.
BTW - the 1448 has alot more hull area or lifting surface under you and always will plane off faster.


Agreed.. You need to back up your observations with GPS data rather than seat of the pants feel.. Also to compare two motors, they need to be tested on the same boat.
One last thing: will a powerhead from a new motor fit on an old chassis ( and vise versa) Swapping powerheads would be the last thing I would do.
 

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