Dirt Bike Repair & Maintenance Forum

... than you can possibly imagine

Jetting - that elusive mystery that has plagued us all.  We test by the seat of our pants,  look at plugs, heck - some of us even dyno with a 4-gas analyzer.. But there just has to be a higher plane of carburetor existence.

My dear friends and colleagues - I am off to find it!

I believe carburettor salvation ultimitely exists in the realm of EGT's (exaust gas temperatures).   I have installed a k-type thermocouple onto the pipe in my YZ250, along with an AIM EV3 lab-grade datalogger. I have tied into the TPS on the carb, as well as the coil signal to pick up RPM's.  

Now I will boldly go where no Dirt Rider Forum wrench has gone before - into the fire, my friends - into the belly of the beast!

I expect the following:

Pilot baseline temps can be established by the conventional method of tuning the pilot circuit for peak RPM.  
Main jet baseline temp will be established by a series of full throttle runs - the lean main jet should crest in EGT and then come down while the throttle is still WFO.  Increasingly larger main jets should yield a temp that peaks and holds for the duration of WFO to redline (expecting around 1200 degrees ***  From there, incorrect needle root diameter and clip position should show anomolies in the curve where these circuits come into play (imagine a spike or drop in a dyno curve and you'll get the idea), and can be adjusted for until the points along the data curve are near linear or at least show a uniform rise.

I've reached the end of the road with conventional jetting models my friends - I must move on - I must find the next plane of knowledge.  Wish me luck! I'll try to contact you from the other side..

Skipper  
Since when is Dirt blue??!!!

You are the mad carb scientist! Just let me know when I need to change my jetting-yet again.  

Ok, if you say so Skipper...  
- 1992 Suzuki RM 125
- 1979 Yamaha Townmate 80

Are you crazy?  
2006 cr85 big wheel
Long live the 2-stroke
No Fear Factory Honda Racing

Please let us all know what happens. There is a lot of riders out there who would appreciate what you are doing. Teach us Obi Wan Kenobi. Cause I know I need some suggestions on jetting my yz250.  

Ahh - extra1, you are referring to the Jet-I Master, Obi-Woods-Kenobi

Chud - by some stroke of luck or genius, your bike is spot on - touching it again would be like repainting the Mona Lisa!

92RM - I say so.

CRKID - Yes, as a matter of fact I am!  
Since when is Dirt blue??!!!

Good luck. You may need it  

I'll probably blow an engine or two in the process, H125, but if I can get some valuable data it'll be worth the topend rebuilds..  
Since when is Dirt blue??!!!

I have a mental picture of a back to the future YZ250 flying through the rockies. You just need 2.2 jigawatts.  

If I could go back in time, I'd shoot the Honda Jetting Ape while he was still a little tyke  
Since when is Dirt blue??!!!

Speaking of obi woods kenobi; where is the jet-i-master?

Thanks to the ape, I am an up and coming jet-i-master. Just need a few more training sessions w/ yoda.  

There is no more I can teach you, my young apprentice.  But you are not a Jet-I yet.. you must face the CR again - when the sun is at it's highest and the temps are soaring.  Only then will you become a true Jet-I!

Obi Woods is off on some damn fool mission again.. he'll be back though. I heard him mumbling something about the power converters in the vette...  
Since when is Dirt blue??!!!

I sense there is a disturbance in the force; no, it's just my wife. check ya later  

Jet-I. LOL!!!  
2006 cr85 big wheel
Long live the 2-stroke
No Fear Factory Honda Racing

Jet-I Force Bump - for Obi Woods to see

 
Since when is Dirt blue??!!!