Carb Jetting 101: Terms, Tips and Jetting Theory

[dizasterzrfun69]

Are you running the stock round slide 85 carb? Where exactly in the throttle is it bogging? Will it idle? If it idles you don't need to mess with the pilot. My gut says your needle is too lean (can't trust plug readings in that range in my experience). I'd put the clip in the middle and see what it does.

well i JUST got this project finished AT trikefest, it ran good for the first day or so, then all the sudden one day it started bogging on the bottom end when id press the throttle. Now it wont even hardly start, when it does though it wont hardly idle and when i give it any throttle at all it just bogs out and dies. I Am going to put a new plug in it and see what that does. The plug in it was real dark though so thats why i think the carb itself needs something adjusted. Its the stock round slide carb.

[Dammit!]

You have the needle on the top clip right? That's the leanest setting. What you're describing sounds like it's not getting enough gas.

Do you run it with the airbox lid on or off? If you run with it off, put it back on and see if there's any improvement at all. If there is the slightest improvement, your needle is too lean.

I have doubts that a 145 is rich enough with that pipe and rad valve as well but you won't be able to dial the main in until you at least get the needle good enough that it can run. You should dial in the main first, then the needle, then the pilot and/or air screw and idle.

[dizasterzrfun69]

You have the needle on the top clip right? That's the leanest setting. What you're describing sounds like it's not getting enough gas.

Do you run it with the airbox lid on or off? If you run with it off, put it back on and see if there's any improvement at all. If there is the slightest improvement, your needle is too lean.

I have doubts that a 145 is rich enough with that pipe and rad valve as well but you won't be able to dial the main in until you at least get the needle good enough that it can run. You should dial in the main first, then the needle, then the pilot and/or air screw and idle.

yes the clip is all the way on the top of the needle. The manual calls for it to be on the 2nd clip. The carb right now is how it was when i got it. I have the lid on and i have a K&N filter in the box. Do you think it'd be wise to just put the clip in the 2nd position like the manual calls for, put the stock main in it and go from there?

wouldnt it be too rich though if it bogs and my plug is black? You said you think its not rich enough..

[Dammit!]

If it's bogging like the way they do when you run out of gas, it's far too lean. Too rich usually causes them to sputter like there's more gas going into the combustion chamber than it can burn effectively.

Changing the pipe, reeds and air filter drastically changes the potential flow through the motor. Right now you're flowing way more air than fuel. You have to increase the fuel flow to compensate.

Put it this way, you're way more likely to damage the motor by running it too lean than by running it too rich. When I jet a motor I intentionally set everything WAY too rich and work my way backwards. It's possible to have a motor running lean enough to damage it and not even know by the way it's running. That's how people burn holes in pistons.

[dizasterzrfun69]

Rich: A "rich" condition is what occurs when you have too much fuel in your fuel/air mixture. Symptoms of a rich condition can be a rough running engine (sputtering), a black spark plug, a wet plug or excessive plug fouling.


If the engine does not quickly return to idle after a good rev, it's too lean and needs a bigger pilot jet. If there's bogging when taking off or reving from idle, it's too rich. These problems can often be tuned out with the air screw.



So which is it? I have a wet plug and it wont idle down. I turned the air screw all the way in and it didnt do anything. I put the needle clip in the middle groove and it runs good now. It wont idle down though and i have a wet plug. I think its getting too much gas. It has the stock 52 slow in it and a 145 main like i stated before. After one of my "rev's" I pulled the slide out of the carb and there was like smoke coming out of the carb. Im guessing this was vaporized gas and its getting too much fuel.

[Dammit!]

Turning the air screw in cuts off air flow and makes it richer. What does it do when you turn it out? Does the idle screw or a combination of the idle and air screw have any effect at all? What does it do with the airbox lid on/off?

Unfortunately there's no way around the most unpleasant part of dialing in a bike. Trial and error. Sometimes they do they opposite of what you think they should but just keep trying until it starts improving. If going richer then leaner have no effect, there's a good chance you have another problem (electrical, crank seal, intake leak, etc). Sounds like you're getting close though.

Remember that they changed to a 42 pilot in 86. Different carb but that's still a big drop.

[3WheelsOn2]

A couple days ago I read this thread trying to figure out why my 200x was sputtering at about 1/4-1/2 throttle. I then raised the needle a notch (to the bottem notch) and it got a little better, So then I put a tiny washer under the little snap ring, and now it has amazing top end and mid range (from about 1/4 throttle and up) on a gravel road going about 25 it breaks the tires loose in 3rd... (alot of that is because my tires are almost worn off) also
The throttle responce is amazing. And it idles great

But if I hold it between a idle and 1/4 throttle it almost dies, some times it does. and it misses...

any ideas?

[Dammit!]

You say the needle clip is on the bottom notch now? Where was it before?

[3WheelsOn2]

one before the bottem. if this was the needle then its on this one

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[Dammit!]

I would call that the top ring, not the bottom one. It gets confusing because lowering the clip actually raises the needle position. I like your illustration though.

The reason it's wanting to die at low throttle is because it's starving for gas most likely. That is the leanest needle setting. As soon as you start to open up the throttle, a rush of air comes in through the carb but not enough gas is mixed with it because the needle is still too far down into the main jet. Move the clip down a couple notches to raise the needle out of the main jet earlier in the throttle movement.

With what you're describing, I'd start with the clip in the middle.

[3WheelsOn2]

Sorry but I didn't notice that it moved my arrow back when I posted it, What I ment to post was this

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Now that I have it on the

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could getting gas to early make it miss real bad?

[Dammit!]

Too much gas will make it pop and sputter. I'd still try it in the middle.

[3WheelsOn2]

ok, I'll put it on the middle and see how it runs. Thx for the advise.

[burnoutboy]

I am having some issues with my rm250. When give it 1/3 throttle, black smoke comes out of the exhaust. When I get into the powerband the motor feels very "zingy" but is not producing any power...any idea what is wrong?

[stoney420]

excellent read up here, thx alot dammit!

one question tho..

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.

how would i do a "plug chop" for my main jet on my ytm200 that dont have a clutch so i cant shut it off and coast to a stop?