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Jetting.....how low should I go?

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I have seen this kicked around a few times but am in the process of dialing in my 125 for really tight woods and trying to avoid going through the process of rejetting 8 times.

My 125 has always run fat down low.  Other than some spooge, it was not really a problem since I ran more open mountain trails. Now that I have started running really tight and steep trails it is becoming a plug fowling machine.  I have covered the basics (top end, floats and reeds) and am now working my way into the brass.  I just got a 37.5 pilot jet but after doing some homework I am seeing that might not be enough to clean it up enough for some of my steep slow descents (sometimes upwards of two miles) followed by tight trails and a climb.  

I will throw the 37.5 in but am curious how low some of you have gone.  30?  If this does not help much I will order several.  How low should I go?  The stock 40 is obviously nowhere close for technical woods riding.

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I've run a #35 on occasion even for motocross track use,

in your application I don't see any problems going to a #35, #32.5 or even a #30

as long as you don't suddenly shut off the throttle at the end of a long WOT pull (engine will lean surge).

 

You've probably already experimented with it but lowering the float/fuel level in the carb will help,

as with using a low flash point premix (Interceptor, Lucas)

For strictly low/midrange woods type off-roading (no WOT) I'd consider going 40:1 to 50:1

a friend of mine with a woods only CR125R successfully runs it at 45:1

Of course that will require you to re-jet even leaner (less oil = more fuel)

 

Reducing the compression squish height also improves combustion,

going from 1.37mm to 1.17mm on mine made an already great running engine somehow run even crisper.

It also allows for leaner jetting.  I left room for future experimenting as 125's can go with as little as 0.8mm squish height.

Edited by mlatour
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Thanks. I will probably throw all three of those in my next order.

Heck, I may as well do it. I doubt the 37.5 is even close.

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In 50°F to 75°F ambiant temps, a #37.5 is what I use for strictly MX track riding.

For you lower rpm / prolonged idling while off-roading, I can very well imagine using a #32.5 or #35 at most.

Edited by mlatour
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