I see alot of people having issues with jetting and performance issues so I just wanted to post this and hopefully it will help someone out and answer a few of lifes great mysteries on the black magic of carburation. I have been working on bikes for around 6 years and carb jetting is the thing I have to do most from friends leaving motorcycles sit with gas in putting on aftermarket parts etc.
Let me start by saying that one thing to keep in mind is that there is a scientific magic number for perfect combustion in a internal combution engine and that ratio is 12:1 and everything you do will effect that ratio. If you get too far under the ratio then you get a lean condition of the bike sputtering at WOT and back fires / deceleration popping etc.. Too far over and you get lousy performance waste gas and can even get some sputtering due to fouling the plug.
An engine from a tuning standpoint is a basic heat pump. If you want to get more power out you have to be able to flow more air and fuel through it. The most common ways of doing this at home are to add an aftermarket exhast and more free flowing air filter be it a K&N or a pod style makes no difference. Then you get into the more arcane of intakes with less bends to restrict pressure and even port matching and polishing the heads in the case or racers and geek mechanics such as myself. All of these things will get more air into and engine and bring the 12:1 ratio down so you have to compensate by using larger main or if you did all of it likely a larger pilot jet as well to get that ratio number back up.
Now that were past the basic theory I'll try to explain how each jet affects the fuel delivery in the system.
The pilot jet controls fuel delivery from idle to 1/4th or so throttle opening. If you are having problem with idle or hard starting when cold this jet is most likely your culprit. It can also affect take off speed. Think of the air screw as a half jet as turning it in or out will richen or lean the idle mixture of the pilot.
The needle just is a tapered needle that slides through a pressed in tube or jet in the throttle body and due to the taper allows more or less fuel depending on how far the throttle is open this controls fuel delivery from 1/4-3/4 throttle. It is dependant on the main jet as it is pressed in underneath of the main jet. Basically it can only flow as much fuel as the main jet allows past it and too the needle. The main jet is basically a bottle neck to the needle jet. If your stumbleing between 1/4 and 3/4 throttle go up one main jet and try that. For all intents and purpose unless your raceing you really wont need to adjust it at all just the main and pilots.
The main jet controls fuel from 3/4 to WOT. The reason changing it affects the stumble below it in the needle jet is due to the bottle neck effect of you can only push so much gas through a small hole and if more is required to get to the needle you have to get a jet with a bigger hole to get it there. This is the jet you will most likely have to change for the majority of modifications to your engine.
As an example of just how little it takes to warrent a rejet if you look at the honda schematics for the same year Ruckus and Metropolitan you find that they used 1 size larger main jet on the Ruckus due to the silencer on the muffler of the Ruckus being a larger diameter and that is essentially the only change in flow between the 2 exhausts. With and aftermarket exhaust the pipe changes from 1/2"diameter to 3/4" and the muffler is designed to have less baffles to slow the exhaust and keep back pressure down all to create probably 2 or more times the air flow of a stock muffler! So if Honda decided just a 1/4" larger silencer is enough to go up a jet size to compensate for a 1/4" bigger hole to bring the 12:1 ratio back in spec just think how far off you have to be, possibly 2-3 main jet sizes with your new muffler alone!
Once you get the correct jets to bring everything in spec for combustion ratio you will find the engine really comes alive since it is no longer for all intents and purposes in a fuel starved state.
I hope you all find this helpful and maybe it clears up some of the questions and confusions on carburator tuning. Im not sure if admin will turn this into a sticky and as a new member honestly dont even know who to talk to about it but if you enjoy the tutorial and do know who to talk to feel free to put in a word to get it made a sticky.

Best wishes and best of luck on your tuning experiance

Dale

good post -- I have been struggling with jetting and understanding it all and found this rather helpful. Has all the information I have spent hours looking up in one post

If I can ask a question... currently I am having the popping when slowing down and also the engine just shutting off or stalling out after coming to a slow stop after WOT... From what you are saying in the above post, this would be a case of being too lean?

i think that looks pretty good. But, I thought the reason the Met uses a smaller main jet is because of the restrictive plate used in the manifold.

I believe only the Met 2 had the restrictor plate in the intake. I have a Met 1 and unless a previous owner took out the plate its not there and the met runs almost 40 mph. I dont think the previous owner did howerver because he did alot of really scarey things to the bike mechanically(cross threaded the plug overfilled the oil etc.) I just bought it and it needs ALOT of work so i expect after getting an exhaust on it and jetting it since im going to use a Rukus exhaust it will run 40+ with ease.

Ad Bazinga does it sputter at WOT? a larger main may help with decel pop but stalling when your coming down sounds more like the pilot is either partially restricted from being dirty and running lean or you need to adjust the air screw in to make it a bit richer. Also if the idle is set low can cause one to stall when stopping.

Dale

yup, what he said.

I'm pretty sure ALL Mets have the restriction plate in the manifold.......except for the ones that remove it.

Sir I do apologize. You got me curious enough to go look up the schematics and both models do in fact have a plate apperantly the met 2's is just more restricted. Getting ready to go to work but when I get my new muffler in and rejet I will deffinatly pull that see if the one on mine is still there. The reason I figured the one was unrestricted is the jet change is only in the main and went up only one step from 72 to 75.

Dale

Helpful, thanks!

no sputtering at WOT, runs great until decel - then loud popping and stalls out at a stop.
I must admit I have not played with the idle much as I just had time to take if for a test ride on the weekend. I will fiddle with this a bit first, as it is a brand new carb and should not have any dirt restricting the pilot
What are your thoughts on shimming the needle in order to make it more rich ?
I appreciate the advice!

did you dremel a slot in the IMS for easy adjusting?

I have never honestly seen the point in shimming the the needle at idle you can accomplish the same thing by turning up the idle screw and while it might get you a quarter to a half jet richer at WOT it just seems simpler to go up 1 main and turn the idle down to match. Shimming is really something you need to do only if the next jet in line is too rich and the current jet you have is just a hair too lean. Its more something you would see at the track if the day starts out hot and sunny then a storm rolls through and changes the tempature 20 or so degrees cooler then to keep everything just right the mechanic might shim the needle fast to compensate for the atmoshpere. Perfect combustion ratio is great but in the real world you will never carry it all together all the time. You just want to be as close to it as possible. Years ago I had a honda XL250R that was 2 carburators feeding a single cylinder and the only way I got it right was to have an o2 sensor bung welded into the exhaust and hook up an air/fuel ratio guage to tune it. Leaveing the guage hooked up you would be amazed how much your combustion burn changed over the day. you could start off lean due to outside temp then as the day gets warmer it richens up and then is back lean by the time you go home at 11p. The best you can do is Find the jets that make it pull like a freight train for the time of the day you ride and the rough ambient tempature you ride in.

Dale

Just want to toss this out. Most engines today are jetted really lean to meet EPA standards for emissions. This includes 4 stroke and 2 strokes.

Gonna' give this a read later..