9.9/15hp conversion=wont plane out

[FishinDave]

1985 Johnson model J10RC0M. I Changed the carb on this motor with a 15hp. I research and it seems that the carb is the only thing that I need to change on this motor so i did. The boat ran great with me in it. Then me and a buddy went fishing and the thing would not get up on plane. It felt like it was doing 5 miles an hour with us in it and with just me in it, it felt like it was doing 25mph. I messed with the transom swivel bracket pin and adjusted the motor and set it at different positions in the water and still slow slow slow. I let my buddy out at the ramp and did a test run again and she flew. First question is how can i have this much difference with another person in the boat? Next question is does a dolphin fin really help get a 15hp on plane? 3rd question is what pitch/length prop should i use on this motor with my 14ft tracker jon with front and back decks. My buddy has the same setup as me but he has a 9.25x8 prop and mine is a 9.5x10. His boat pops right out of the water but he also has a true 15hp. Last question = Is the carb the only thing that needs to be change on this motor. Everything I found says yes and it goes faster than the 9.9hp did with just me in it. I am almost positive that the tuned exhaust started being used in 1981. I am going to drop the lower tomorrow and make sure that it has the tuned exhaust tomorrow. Thanks for any help in advance.

 

[racerone]

Re: 9.9/15hp conversion=wont plane out

Time to do more research.-------First establish that your 9.9 is indeed in good condition.-------Then order the 15 HP exhaust tuner as shown in the parts catalogue.---------Not sure if you can tell if yours has the tuned exhaust by just removing the lower unit !

 

[Mutt606]

Re: 9.9/15hp conversion=wont plane out

I just did the 9.9hp to 15hp conversion with my 1980 Evinrude. My boat is a 13.5' aluminum v-hull Sea King. It is not decked and is probably on the light side. Just to show my results:

9.9hp - 1 Person = 14.8 mph, 2 People = 8.5 mph
15hp - 1 Person = 21.5 mph, 2 People = 17.5mph, 4 People = (Didn't GPS this one, but we got on plane) 10 mph

It was a striking difference, and I was surprised. It actually had so much power I realized that my prop was slipping in the hub, something I could not do with the 9.9. So I had to buy a new prop. Btw, I only changed the carb for this conversion. For a 1980 model I was also supposed to change some reed valve spacers or something but did not want to get carried away.

 

[pecheux]

Re: 9.9/15hp conversion=wont plane out

From what I read elsewhere on Google, In those years some 9.9 models required exhaust manifold change along with carb to upgrade to 15 hp. (confirmed by my OB mecanic)

In my case I added a doilfin Jr. on the LU of my 9.9 and getting something like a 12 hp performance. But ended up hitting a rock on plane at 27 k/per hour and busted the LU ... (hang my head and cry ... LOL)

Good luck

 

[raczekp1]

Re: 9.9/15hp conversion=wont plane out

also you need reed valvs spacers to add if you want to have full 15 hp motor,
i think lose of speed is becouse of low compresion, or runing on 1 cylinder only

 

[HighTrim]

Re: 9.9/15hp conversion=wont plane out

A slipping prop hub seems logical, and worth checking out. I cannot see how weight would cause a cylinder to drop, unless just bad timing!

 

[FishinDave]

Re: 9.9/15hp conversion=wont plane out

Wow. First, thanks for all of your help. Can anyone tell me what is a good prop size/pitch to help get this boat out of the water and up on plane. Also does a dolphin fin really help on this small of an engine. I feel like if the boat was not pointing towards the sky and could get up on plane with the added weight, it would get better performance. Also I have experienced a hub slip before and generally you get a kicking feeling or a higher rev occasionally. This is not the case. I was told that if a plug is fouled, these engines can run steady on one cylinder with a power decrease and you may not feel a miss fire but just a loss of power and still a steady running engine. Is this true. Thanks again

 

[FishinDave]

Re: 9.9/15hp conversion=wont plane out

Update>I put a different prop in and it helped but I will need to change the exhaust to a tuned exhaust if I want more out of it.