Carb Jetting 101: Terms, Tips and Jetting Theory

[Mr. Sandman]

Excellent info ! I'd like to add how to do a proper plug chop as I see that question and misinformation all the time. Just remember, plug chops are done to determine main jetting only.

1. Use a new plug, clean air filter and have your bike, trike or quad warmed up to the proper operating temperature.
2. Make a WOT pass through at least 5th gear. What's WOT? Go back and re-read 's first post.
3. Doing all four at the same time, hit the kill switch, let off the throttle, pull in the clutch and roll to a stop. (Rolling out anytime without pulling in the clutch could cause a seizure as the piston will continue at high RPM through it's stroke without pre-mix lubrication and it'll overheat and seize in a hurry. Running out of gas will do the same thing).
4. Pull the plug and read it. The porcelain insulator should be a nice cocoa brown, not on or near the tip, but down inside the plug. If it's still white or light in color you're running too lean and if it's black and or oily your running too rich. Re-jet, go back to step one and start over.

[chris200x]

Excellent info ! I'd like to add how to do a proper plug chop as I see that question and misinformation all the time.

1. Use a new plug, clean air filter and have your bike, trike or quad warmed up to the proper operating temperature.
2. Make a WOT pass through at least 5th gear. What's WOT? Go back and re-read 's first post.
3. Doing all four at the same time, hit the kill switch, let off the throttle, pull in the clutch and roll to a stop. (Rolling out anytime without pulling in the clutch could cause a seizure as the piston will continue at high RPM through it's stroke without pre-mix lubrication and it'll overheat and seize in a hurry. Running out of gas will do the same thing).
4. Pull the plug and read it. The porcelain insulator should be a nice cocoa brown, not on or near the tip, but down inside the plug. If it's still white or light in color you're running too lean and if it's black and or oily your running too rich. Re-jet, go back to step one and start over.

Thank you!

I'm having a heck of a time right now trying to figure this out. This was the specific info I needed.

[3Razors]

I stumbled upon this page from Duncan racing for Keihin carbs. Has some very useful info. on the needle part of jetting.

http://www.duncanracing.com/techfaq/...-jetting.phtml

[Sargon2112]

If I might add one suggestion: When tweaking / dialing your carb in, make one change at a time... especially if you don't have much jetting experience. That way, if the change you made has an effect (good or bad), you'll know what and most importantly, you'll know 'why'.

Great thread!!

[Billy Golightly]

I also would like to add (From personal experience in the last day and a half) if you've got a 2 stroke motor that just will NOT clean out and smokes like a fiend, check your crank seals. Transmission oil level will also more then likely be going down as well too.

[erectordale]

OK you carb pro's I have a 500cc with a bing 55 runs great and the plug chops show its jetted correctly It's impossible too kick start with out priming shoot a sqirt of fuel in the carb she starts right up. where Am I going wrong here??? we have just been pull starting it. it takes about 30 feet too get it started

[TtownJoeShow]

I have a question too, my base ring doesn't get any color change! it stays looking new...BUT where the threads are gets black and the porcillan does a little too. the ground strap goes half black (maybe alittle less) the other half goes alittle yellow/ gray...the side of the ground strap goes fairly yellow. there is no aluminum specs, nore is the electrode ditch getting filled....the 85 atc 250R runs exellent, maybe alittle too zingy tho (FMF Fatty w/Powercore, BRV and Uni, only mods) running a BR8ES 300ft above sea leval @ 10oC and 40:1 Amsoil synthetic, 91-93 octane
Jets are: Main 140
Pilot 52
Needle 28V
Air Screw think it was 1 3/8 turns out grr
i did 4 chops, played witht he screw after 1st, did mental note the turns but well that didn't take lol DAMN!

I ordered jets and they should b in today or tomorw (142-180)

Would i be right by saying it needs a 145 or 142 main?

[Dammit!]

Ttownjoeshow: it's impossible to jet over the internet. All I can say is a 140 main SEEMS like it would be too small for what you're running. Stock is 142 and you're running an aftermarket pipe and filter which should give you substantially better air flow than stock so 90% of the time you would need to jet richer to compensate.

Dale: Is the plug dry after you try to kickstart it? Might want to try a bigger pilot if it is.

[erectordale]

the plug looks dry when trying too kick it over I have a larger pilot and will try that thanks

[TtownJoeShow]

k so i need something cleared up.... By Soot, is that blck or grey?
Should the ground strap be a 2 tone brown?


all i'm getting is black on the ring, well on one a light black almost dark brown.
The ground strap, some black, then closer to the end grey and on one or two sides some yellow
Thanks a million in advance!

i got it! soot can b black and grey! ground strap can b brown black and grey!

now i think black is rich, grey is atad lean and brown is good...

[don250r]

i've noticed over the years, that when people add multi-stage reeds(thru power reeds, a RAD valve, or ESR valve.etc.).
They don't follow instructions!
9 times out of 10, the trike is over-jetted on the main(due to increased air-flow).
when installing ANY of the above mods, remember to DROP the main jet 1-3 sizes, depending on the plug-chop(its in the instructions).

On a side-bar........
I'm not sure, Mr Sandman, but can plug-chops also help to determine correct needle height after the main is jetted in?
Following plug-chop instructions, but at half throttle.
Hope any of this helps anybody out there.

[Rustytinhorn]

I am not asking what size jet to run...but trying to trouble shoot my bike and was wondering what size jet others run at my elevation cuz mine is sputterin BIG time.
(I pulled plug and it looks rich, but bear with me and answer my question) Thanks
elevation: 5280 feet
main jet: 142
idle 'r pilot: 60

[2sprman2]

How do you raise your needle clip?

[Dammit!]

How do you raise your needle clip?

It's pretty obvious once you get it apart. Pull the clip off the needle and put it in a different slot (there's typically five positions).

[edog]

Well your thread has been very helpfull.

Thanks

Eric