Johnson 150 Jetting

I have a '99 150 Johnson.  After 2 seasons of headaches we found a loose fuel line union behind the gunwale leaking air at idle.  Now the motor runs strong.. so of COURSE I want to mess with it.  I have read that the 150 can be converted to a 175 by mere jetting and timing changes.  Is this true? What are the implications?

Thanks in advance for your help!




It is not true that the 150 can be converted to a 175 by jetting and timing only. The block has ever so slight port differences, and the carbs have different nozzle sizes and passages to boot. The difference in true horsepower between the two is around 15. On the average boat, this doesn't amount to much more than a couple of mph. For more bang for the buck, experiment with propellers and setup changes and you will get more than this in return. There are some aftermarket cylinder heads out there, but this engine is very picky and some of them are prone to detonation on pump gas. My advice is to shy away unless you are willing to pay the price.




Please submit whatever questions you have to  All questions and answers selected will be displayed on this page as they come in.  Note: though all boaters are welcome, priority may be given to I.H.R. members.  Randy can not be held responsible for any advice given.  Though his information and expertise is second to none, he has no control over what you do with your boat.  It is up to you to boat safely and act responsibly, and his advice is only to be used as a guidance for your high performance boat/motor of which you are the one responsible for the risks involved.

Mark C.


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