Jump to content

cr 125 jetting issues Help!

Recommended Posts

Hi my name's Nick and I'm new to this forrum.So ,I just bought a cr 125,and I have a few concerns.There is a bog in first gear when I go full on the throttle.If I ease into it it's fine and hits the power band and all other gears just fine.its just really annoying.So I've tried messing with the needle over and over,tried messing with the air screw,it gets a little better but still bogs bad at full throttle unless I ease into into it.Ive also checked my plug and have had no problems fouling .so my guess is i need a bigger main jet because the person I bought the bike from came from Arizona and I am located in New Mexico about 7,000 asl.I would like an opinion though.what size jet should I through on ????? Help lol

Share this post


Link to post
Share on other sites

If anything, you probably need to go down a size on the main jet.  If the bike came out of the Phoenix area we are about 1200 ft.  If it came out of Flagstaff or other northern Arizona areas they are about at you altitude.

Share this post


Link to post
Share on other sites

So the higher you go up the smaller you want to go on the main.I am not used to jetting at all.ive owned 4 strokes and never had to deal with jetting on them.

Share this post


Link to post
Share on other sites

Two strokes or four strokes, it doesn't matter unless the engine is turbo or super charged.  Higher elevation equates to thinner air (less oxygen) per intake stroke.  The jets in the carb regulate the amount of fuel (or fuel/oil) that gets mixed with the air.  Since the engine is getting less air due to the increased elevation it needs less fuel to keep the fuel/air ratio correct.  A main jet that is a bit too big would also explain why changing the needle around doesn't seem to be having much effect as the needle jet draws its gas through the main jet.

Share this post


Link to post
Share on other sites

Figure out what size main jet is in the carb right now and then go buy two that are 1 and 2 sizes smaller.  Try the 1 size smaller first and if that doesn't make a difference try the 2 size smaller one.  You shouldn't have to go smaller than that.  One thing that makes two strokes more sensitive to altitude than four strokes is the fact that the two stroke is also getting its top end lubrication through the carb.  This means that the ratio of fuel to oil affects the ratio of fuel to air that the engine gets.  The way the engine runs is entirely dependent on the fuel/air ratio.  The amount of lubrication the engine gets is dependent on the fuel/oil ratio.  The more oil there is in you fuel the leaner the bike will run since there is less fuel inhaled with each intake stroke.  So you can also make tuning changes by changing your pre-mix ratio.  Above all keep an eye on the color of the spark plug.  It should be light to medium brown.

  • Like 1

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:


×