Mechanical Johnson 9.9/15 Question

TinBoats.net

Help Support TinBoats.net:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Gotem

Well-known member
Joined
Aug 31, 2012
Messages
122
Reaction score
0
If the 9.9s were detuned 15's, and the only major difference was the carbuetor and on some the tuned exhaust, how dangerous to the motor is it to take a unconverted 9.9 and run it at the rpm level of a 15 by switching to a lesser pitch prop? Has anyone ever intentionally done this on purpose and is it wise to do it.

I would never do this to an outboard because of the potential of blowing the engine, but is this the exception to the rule?
 
Well being someone that has actually done this conversion to an 87' 9.9 I would say it is not a good idea to run it at that RPM by going to a lower pitch prop. See, the larger carburetor allows for more fuel and air to flow through the engine. And as we all know 2 strokes are lubricated by the fuel oil mix. More fuel equals more lubrication at higher RPM. Increasing RPM without that extra fuel/air is a recipe for failures internally from what I can see. Also by going over the recommended RPM of the 9.9 the engine will not be producing 9-10hp anymore....... horsepower curve

I am sure you could probably run your 9.9 at 500rpm higher than recommended for a considerable amount of time (possibly forever) before seeing problems or damaging engine, but that is only due to how solid those motors were made....not recommended though.
 

Latest posts

Top