Jump to content
Sign in to follow this  

How you learn to Jet?

Recommended Posts

Welp I'm learning a little bit every time. I the symptoms of rich vs lean but learning to how to fine tune jet. Wondering how you fellers fine tune your ears to jetting your bike. How many times you jet a year? List your altitude n temp range too! I'm trying to learn about how to jet the top vs. bottom. Generally you want the bottom rich n the top lean right??

I'm hoping I ain't gotta jet but 2 times a year we got real moderate temp changes here in Memphis @ 300 ft. above sea level. I've got stock jetting (175mj/55pj/42power jet) n my air screw is a little more enrich than should be but the bike runs so great but tryin to make sure it aint too lean!!

Share this post


Link to post
Share on other sites

You learn it from doing it. If your bike is blubbering or cutting out then your just going to have to go up or down a jet size and see if it gets better. One way will make it worse and one way will make it better.

Share this post


Link to post
Share on other sites

I just use the easy-to-read and easy-to-understand chart in the back of my KTM manual. :busted:

Gets you very close and then usually just have to make minor adjustments to float height and/or air screw and then just ride. :busted:

Share this post


Link to post
Share on other sites

Yea I realize it's a pretty straight forward learning process starting with EXPERIENCE but I've been following my tuning chart in my HONDA cr250 manual and everything is right but I get confused with the fine tuned jetting! Eventually I will know this carb like I know my chevy 350!

Share this post


Link to post
Share on other sites

It sure seems a lot easier these days! Every bike I've had in the last ten years has had a table in the manual that's been pretty close (except for my 01 Honda CR 250, but JD solved that one). I just use the table to set for temperature and altitude, and then fine tune (clip, screw, and maybe swap pilots for woods) from there. Our temperatures are not that extreme here in GA, so screw adjustments are normally sufficient for winter vs. summer.

Share this post


Link to post
Share on other sites

Hell yeah rp that's exactly what I needed to hear. I've been trying to get that bit of information out of everywhere and no one has said it but basically the fine tuning IS the screw and clip and what not...Most ppl were making it so much more difficult than it needed to be. I have the stock jets in mine and my table basically tells me not to change the jetting for our winter vs. summer but the air screw amounts change. Okay well maybe it will have me bump up 1 size or so on the main but I need to do a plug chop and just make sure I'm all good!! I pulled her out and started her up but didn't ride. She ran so damn good.

Share this post


Link to post
Share on other sites

Fine tuning is not the screw. The screw fine tunes the pilot, which is a fixed jet.

Basically, your order of adjustment should be:

#1. Float height- this affects which main jet to use; float height needs to be correct to keep from dumping fuel on a slight tilt (high) vs. sucking air on a jump, g-out, etc. (low). Basically, just set the float to the manf. specifications. You can make a bike run good with a high float level and a leaner main and vice- versa, so this step sets up everything that comes after.

#2. Now you can jet for the main- bike running clean at mostly-to-wide open throttle with max power and a dark tan/brown plug, it's about the easiest step.

#3. The needle has always been the trickiest for me, it's kinda subjective. The needle controls jetting at mid throttle, say 1/8 to 2/3 but affects basically everthing from idle to wide open in reality. It unplugs the main as it opens, so you can see how a wrong-size main affects the needle position- If the bike runs perfect at half throttle but terrible on top, you'll need to change the main and now the needle will be wrong too- that's why we're working our way down from the float after all. So play with clip positions until the bike runs its best, that's all. JD gives you 2 needles in the kit, a blue and a red, the red being a step leaner than the blue for fine adjustment.

Lot's of guys have experimented on KX needles for Hondas as well for example and there are many different profiles available from the dealer and aftermarket, it may be worth a try playing with different needle shapes if you are into that stuff.

#4 is the pilot jet, this is idle to off idle and a bit up. Basically it's a rough adjustment to make, and a general thing for most sizes of motor- 125s like a 42 (Keihin), 250s like a 45 for instance.

#5 pilot screw- this is the fine adjustment of the pilot jet, range is typically 3/4 to 2 1/2 turns out. If the bike won't run clean and idle decently for at least a short while within this range, then the pilot jet is wrong. ie: if the bike runs best with the pilot screw bottomed then the pilot jet is too big.

This pretty much my understanding of BASIC 2 stroke jetting, anyone fee free to correct any mistakes- I'm learning this stuff as well.

Share this post


Link to post
Share on other sites

See my bike runs okay and everything but it may be too bogged down low and sometimes it feels like it should have more to go in fifth gear...Like It hits the powerband and tops out too soon in 5th...Obviously I notice this most whenever I'm on the street on the way back from the trails

98 cr250

kehin carb PJ28L

175/55/42

I have a complete carb rebuild kit I'm waiting to put in it with float, gaskets, needle, jets, EVERYTHING

The spark plug is 4 years old, and I'm sure it could use some silencer re-pack....

My tuning chart says to bump up 1 size on the main for the weather we got right now so I was thinking between my main, silencer pack, spark plug those could all be attributing to why the bike doesn't seem to go as fast as she should...

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

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×