How To Convert a 9.9 to a 15hp?

[nancur373]

Hey guys. I heard that you can make a 9.9 into a 15hp a few different ways. The reason I'm asking is because I'd like a little more push without buying another motor. I bought a 1974 9.9 Johnson before I bought a boat cause it was a great deal. Bought a 15.5' tinny later on. Wasn't planning on a boat that big but again it was a great deal. I'm getting my boaters card in the next week so I am not trying to cheat the gov.t (Lord knows they deserve it). Some guys are saying it's just the carb that is different. The 15hp carb allows more air and fuel in thus creating higher rpm. From the research I did on parts, it seems that both the 9.9 and 15 have all the same block, piston, rings, etc. The other method I hear is to change the wieghts on the flywheel. Has anyone here done a successful conversion or can vouch for one? Any help or suggestions would be greatly appreciated.

 

Thanks people,

 

Curt

[danbo]

http://www.mechanicschools.com/mmi.html

[ecmilley]

have seen it done, but usually with the cost of the parts and the time setting it up and getting it right it would be more cost effective to sell the 9.9 and buy a 15

[troutologist]

I've got a similar era motor, the guy I bought it off looked into it extensively and printed copius amounts on info off the net regarding this. I talked to a few mechanics and it can be done but at a substaintial cost. For me it was nearly the same price as the motor.

 

On the other hand a lodge I worked at had converted 30hp Mercs to 40s by switching a jet. The modded 30s had a better holeshot and slightly higher top end but burned gas quicker....supposedly cost $12/ engine

[Zamboni]

1-I heard that you can make a 9.9 into a 15hp a few different ways.

 

2- Some guys are saying it's just the carb that is different. The 15hp carb allows more air and fuel in thus creating higher rpm. From the research I did on parts, it seems that both the 9.9 and 15 have all the same block, piston, rings, etc.

 

3-The other method I hear is to change the wieghts on the flywheel.

 

1-In most cases you can't just dump more fuel into the engine to increase HP, along with bigger carbs you often have different heads, almost always larger exhaust ports....ignition systems

 

2-Your 9.9 Seahorse may be one of the few exceptions to above, only around 80/81 was the exhaust system larger/tuned etc. But its not the carbs that are different, its only the jets in the carbs-its same carb body. I think you just need to change reeds (petal plates) and carbs jets....everything else is ame on those engines I've heard.

 

3-Worries me that you think this could somehow increase HP

 

Search the net for details on rejetting your carbs, the guy who told me those 70's 9.9 Johnsons are are capable of being converted knows his stuff. You definetly will need Reed plates also, online they are half what dealers charge. Put Boysen reeds on my merc and it ran smoother, gained 200 revs.