Motor adjustment question

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Dman23

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Not very experienced with motors. On my 1975 Johnson 25 hp there is a rich or lean dial. It runs great but what will changing that do? Never really known i just have kept it where it is
 
"It runs great"

Old saying .... If it ain't broke....don't fix it. I'd leave it alone.

I can't tell you how many things I've screwed up by messing around with something that was previously running just fine.... and I wanted to "improve" it. Often there was nothing that needed improving.

Half way through trying to get it back to where-ever it was...I always say to myself "why didn't I just leave that alone???"

regards, Rich
 
Haha yeah isnt that the truth. I was just curious more than anything. I have done good not messing with it yet!
 
Awesome, thanks for the info got all i wanted to know off there.
 
Agreed, if it ain't broke don't fix it. That being said, it's good to know what everything on your boat does and how it does it. The lean/rich knob on a carb is used to adjust the fuel/air mix going into the combustion chambers. By twisting the needle in, the relative amount of fuel to air is lessened thereby making a "lean" mix (i.e. not very much fuel to air). This generally results in a hotter, faster burn which in turn generally raises the rpms slightly. Conversely, backing the needle out, increases the relative amount of fuel to air being combusted thereby resulting in a "rich" (a.k.a. "fat") mixture. With more fuel in the mix the combustion is somewhat slower and less violent thereby lowering the rpms slightly. It may seem a little backwards, but imagine a campfire. IF you want it to burn hotter, faster, better you blow on it to increase the air mix and speed up combustion, whereas dumping more gas on the flame may/may not increase the activity and may actually smother the fire if added too quickly. The trick is to dial in a steady idle at low rpms, which means you have to find a balance between the carb setting and the timing (rate of fire) of the motor. This is best achieved with the boat on the open water, allowing for 10-15 second intervals in between adjustments of the needle to allow for it to take effect. You generally only need to make small adjustments on the order of 1/8-1/4 turn. Here's another handy link for dialing in your carb should you need it.

Note: the terms "lean" and "rich" also apply to the fuel/oil premix you use on a 2-stroke motor. A "rich/fat" mix has a relativel high proportion of oil in the gas, whereas a "lean" mix would have relatively less oil in the fuel. It's always better to err on the side of caution when it comes to mixing your fuel. Better a bit too much oil than not enough. Too lean a mix will not provide enough lubrication for the internals of the motor and may lead to an overheat/seizure.

https://forums.iboats.com/showthread.php?t=167352
 

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