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2 stroke jetting for cold temps

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i have a 1997 cr 250 with keihen carb, it runs great in summertime, but living in ohio, its now upper 40’s- 50’s and im planning on heading riding, i ride in the woods, not wot hardly ever. wondering if i have to rejet since its colder or am i ok to just richen it with the airscrew?

i dont know what jets are currently in it, but i do know that it runs good in the summertime..70’s -mid 80’s

thanks

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19 minutes ago, luke0415 said:

 

i have a 1997 cr 250 with keihen carb, it runs great in summertime, but living in ohio, its now upper 40’s- 50’s and im planning on heading riding, i ride in the woods, not wot hardly ever. wondering if i have to rejet since its colder or am i ok to just richen it with the airscrew?

i dont know what jets are currently in it, but i do know that it runs good in the summertime..70’s -mid 80’s

thanks

 

Only riding it will tell you. When it gets colder the air gets more dense so it will need more fuel. All bikes are different especially older ones as things wear. Adjusting the air screw could work just fine, if not then it is easy to pull the needle out of it and raise it by lowering the clip one notch. If you have never done it, just google it.

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Changing the main jet would be the last thing that I would consider, especially considering the OP said that he would be in the woods and hardly ever WOT. 0-1/8 throttle is going to be affected principally through pilot jet and needle position. Once past 1/4 throttle, the main gets more relevant for sure but on CR250 in the woods I can't imagine that much more than a quarter throttle is used very often. If bike runs good in summertime I wouldn't touch the main jet for what he described. Needle clip position first, pilot jet second would be what I was thinking but I am definitely no expert.

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Changing the main jet would be the last thing that I would consider, especially considering the OP said that he would be in the woods and hardly ever WOT. 0-1/8 throttle is going to be affected principally through pilot jet and needle position. Once past 1/4 throttle, the main gets more relevant for sure but on CR250 in the woods I can't imagine that much more than a quarter throttle is used very often. If bike runs good in summertime I wouldn't touch the main jet for what he described. Needle clip position first, pilot jet second would be what I was thinking but I am definitely no expert.
Whoops missed the part about no WOT

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Changing the main jet would be the last thing that I would consider, especially considering the OP said that he would be in the woods and hardly ever WOT. 0-1/8 throttle is going to be affected principally through pilot jet and needle position. Once past 1/4 throttle, the main gets more relevant for sure but on CR250 in the woods I can't imagine that much more than a quarter throttle is used very often. If bike runs good in summertime I wouldn't touch the main jet for what he described. Needle clip position first, pilot jet second would be what I was thinking but I am definitely no expert.

thank you, i will 1st see how it is on the airscrew adjustment, then adjust the needle clip position, since i dont have any richer jets yet anyways. so i dont get confused, higher the clip, the lower it sits, so the richer it is?

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1 minute ago, luke0415 said:


thank you, i will 1st see how it is on the airscrew adjustment, then adjust the needle clip position, since i dont have any richer jets yet anyways. so i dont get confused, higher the clip, the lower it sits, so the richer it is?

Higher the clip the lower the needle sits is leaner, not richer. think of the needle as a tapered plug that is fitting into the orafice. the needle is larger diameter at top so if we used no clip to suspend it then it would plug the orafice totally. The lower you place the clip the more (higher) it suspends the needle out of the orafice so it allows more fuel in (richer). Hopefully that makes sense:)

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Higher the clip the lower the needle sits is leaner, not richer. think of the needle as a tapered plug that is fitting into the orafice. the needle is larger diameter at top so if we used no clip to suspend it then it would plug the orafice totally. The lower you place the clip the more (higher) it suspends the needle out of the orafice so it allows more fuel in (richer). Hopefully that makes sense:)

well summed up, thank you

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If you rode MX you might want to go one bigger on the main.  If you're trail riding and not holding it wide open you might be alright with just an air screw adjustment.  It depends on how crisply you have it jetted at warm temps.  If you're running a little rich in the summer time it will probably be either just right or a little lean in the winter months.

If you find that it bogs when you roll into the throttle I would raise the needle one notch.  Give it what it likes.

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