dumb jetting question

scar tissue

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#1
My buddy thumbs( CR125) and I (XR600 & KLX340) were disscussing jetting the other day and the more we talked the more we confused ourselves. How can you tell if your bike is to lean? ( to rich is pretty easy) He rejetted his cr125 per the manual for big bear (5000 or so ft)and is still running the same jetting down here at see level, and I've always just taken mine to a shop and said jet it for this type or area please and never changed it. What kind of damage can result from running to lean. thanks:confused:
 
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#2
If it is running on thelean side , the plug should have a white reading(should show some white). The bike will also be nice and crisp with no bog. If a bike is to lean the motor could(can go), and you dont want that. So do a plug reading.:)
 

scar tissue

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#3
Originally posted by cr125_king
The bike will also be nice and crisp with no bog. If a bike is to lean the motor could(can go), and you dont want that. So do a plug reading.:)
I know this is the case on a 2 stroke (personal experience) What about a four stroke, would you expect the same kind of motor damage?
 
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#4
dunno

I dont got any experience with 4-strokes. But i do know that they are alot more to fix$$$ if something go wrong. Sorry cant help ya on that one
 
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#5
Fuel is a large part of the cooling that a piston recieves. Very lean jetting can melt aluminum pistons, regardless of engine type. A four-stroke is not as sensitive to this as a two-stroke, because a two-stroke piston has less opportunity to cool, due to firing on every revolution, but it can still damage a four-stroke. Contrary to what many people believe, a four-stroke can seize the top-end just like a two-stroke. The difference is that a seized two-stroke usually just requires a new top-end, where a seized four-stroke frequently has the rod sticking through the cases somewhere.To make a long story short, lean jetting can potentially be much more catastrophic on a four-stroke, but the jetting usually has to be so lean that there are obvious signs, such as poor running and severe overheating.
 
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#6
As far as a ‘feeling’ you know that rich tends to ‘blubber’ & lean tends to ‘stutter’.

Gee Dave! -that’s not much help -the symptoms are much the same. Well yeah they are a bit, but lean is a bit more hesitant vicious bitchy & rich is fat.

Try rich first, if you want to get a feel for it try changing your pilot too rich & then too small & you’ll know the feeling. As always remember the pilot only really controls just off idle THROTTLE position.

And please you guys the extreme ‘too’ has 2 ‘o’s. It is too lean or too rich(see?).
If it’s not to much to ask.(sic).
 

Jaybird

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#7
lol, U2?

So this fat, blubbering, rich guy and this skinny stuttering chap walk into a bar....
 

mtngoat

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#8
On 4 strokes, I've seen lean symptoms surface two different ways on two different bikes. On an old DR, crossing the lean threshold caused the bike to stutter/pop at high RPMs. On an XR4, it lost top-end power, but didn't stutter/pop. However, it caused the engine to run so hot that my head gasket started seeping. I figure different degrees of "lean-ness" surface different symptoms.

While "leaner is meaner", I try to sanity-check my jet settings against manufacturer's jet info/correction tables/forum experience, to be safe.

I don't rely on plug reads.

BTW, I just read the link under "Another jetting link" in the parent forum and I thought it was informative.
 
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scar tissue

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#9
Thanks for the tips, Thumbs just changed his jetting back from the high alltitude setting. I guess he doesn't like siezing motors :( Thanks for the 4 stroke tips also. As I get used my new four stroke, I keep them in mind. :)