Re: 2003, 9.9 hp Johnson, 4 stroke, high idle
I found this very interesting post on another board.
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RE: 2003 johnson 9.9 4 stroke problems
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That particular motor had idling problems straight from the factory due to some EPA regs. It drove my dad and I completely nuts for over a year until Mr. Mdwstson on this board was kind enougth to offer the solution. It would never keep running no matter how we fussed with the idle screw. We performed the following modification he described and it now idles like a champ and trolls great all day long.
-cranker2
Sun May-29-05 09:58 PM
#183752, "RE: 9.9 Johnson keeps stalling"
In response to Reply # 0
This is a real common issue with these motors, all because of having to meet EPA requirements. Fact is that they are running WAY too lean at idle, especially when the air is cooler and you need a little fatter mix. Truth be known, that is why the '05 4-stroke Johnsons have been re-vamped entirely.
Anyway, here is how to overcome this issue and slow your engine down even more than specs.(spec is around 1200 RPM's) Remove the carb and do these two things;#1 drill out the plug covering the idle mixture screw so that it can be adjusted. #2 drill a 1/16th to a 3/32nd hole in the throttle plate. Reinstall the carb and you will be able to dial in the idle speed and the mixture and your engine will idle slower(around 850-950RPM's), all day without missing a beat.
Okay, for all you nay-sayers and backyard mechanics, here's why this is necessary: By speeding up the idle (ie. opening the throttle plate) they are allowing more air to enter the cylinders and they choke down the mixture screw(leaning it out)and make the EPA happy! Running too lean is why it is stalling. The catch 22 is, if you try to lower the idle speed, the engine will actually run too rich because there is not enough air when the throttle plate is closed, causing excess fuel to be pulled through the idle circuit. By doing this modification, the engine will still get plenty of air/fuel at idle and your idle speed will be significantly lower as well. I'd be willing to bet that the mixture @ this new idle speed would be darn close to meeting the EPA's requirements as well!
I know of several engines with this mod and they are all running great without any issues. A little more time in R&D and those carbs could have been the cats meow. Hope this helps.
