How are you checking your jetting? I did not see any mention of what area of jetting you are checking.
There are different circuts in your carb that control different areas of your rev range. There are some overlap of course, but you can be way rich in one area and lean in another.
You need to jet in stages... start from the bottom and go up.
Backfiring is(as others have posted) generally a lean mix.
You may have a lean mix screw causing a backfire but be rich on the main jet

Mix srcrew - controls the idle
Pilot jet - just off idle
Needle - midrange
Main jet- WOT(wide open throttle)

If you just go ride your quad you will get a mix of all areas and you will not know what to change.

start with idle.. adjust mix screw till you get a good tan color... What way you turn your mix screw to get leaner or richer depends on if you have a air mix screw or a fuel mix screw. One adjusts air and other fuel. Fuel screws are the front of the carb towards the intake of the motor. Air screws are toward the back of the carb by the air filter. So depending on which you have turning the screw in(clockwise) decreases air or fuel, and turning the screw out(counterclockwise) increases fuel or air. I just sold my Raptor and am awaiting delivery of my YFZ450 or i'd know which it is.

Pilot jet...(doesn't need to be changed with most jetting applications)... check by seeing if your transition from idle to just starting to give it throttle is smooth. No coughs or hickups--

Needle: Check by riding the quad in varying throttles from low throttle to all but wide open(never pin it and hold it wide open.
Adjust your clip position so you have a clean plug(tan) and no dead spots or hickups. You should be able to go from putting along and whack the throttle open very quickly without any dead spots. Put some time on it so you get a good color change in the plug that represents that area. Do not let the quad go back to idling when checking this area. Ride back to your garage or wherever you are going to check the jetting at and hit the kill switch as you pull up. NO IDLING!

Then wide open throttle to check your main jet... hold it open for a bit in a few gears, then get in 4th gear or so and keep it pinned for a while to get the plug a nice color change for that area... then pull in the clutch and hit the kill switch. Coast to a stop and check the plug right then and there. This way you know you are getting a plug color for wide open throttle(main jet) only. If you slow down then shut it off or stop and let it idle before shutting off you are compromising you plug reading.
Then adjust main jet sizing according to what you read.

When finished put in a good long ride and recheck the plugs overall color to check your work. Perfect jetting takes time and effort!

Beaware... when you change main jets use all one kind(brand... dynojets, K, Mikuni, etc.... of main jet to stay consistant. a 168 in one brand is not always a 168 in another brand.

 

ReaperChoppers;
The YFZ actually comes fronm the factory lean. And it incorporates a fuel screw located in the front of the carb.

The fact that he is letting it breathe even more with aftermarket exhaust, is going to lean it out even more. Thus my recommendations to him, should get him pretty close.

Also, the fuel screw controls more than idle, it also is responsible for off-idle too.
The needle controls from approx 1/4 throttle up to 3/4 throttle, and yes, I agree, there is overlap in the circuits.
Main jet controls from 3/4 to full throttle.

 

Yes my parameters are the general areas to help with jetting. They all overlap each other.
It just helps with jetting to know the general areas otherwise people do not know what to adjust and may end up adjusting the wrong thing.

And yes lean is a common thing from factory machines of any type so they pass emisions.

 

Reference the airbox lid on or off. They do sell pre-filters (or I think that's what they call them). I got a bunch out of the Dennis Kirk Catalog dirt cheap. Mine are 1" in diameter and I put them on the side of the air box (lid is off). I think you can get them in 3" diameters. If ya don't want to remove the lid, that's fine, but turn that air box into one big pre filter so at least you'll keep it breathing. The pre filters are easily cleaned/replaced, so keep an eye on them. If you jet it with several of those and they get plugged up, you'll go rich again.

 

wow i suddenly got really confused when u guys explained all that but ill figure it out sooner or later, but are there any complete jets kits out there? besides dyno jet cuz they suck a$$

 

There are a bunch of jet kits out there. I would recommend the GYT-R jet kit, if you don't want to buy the jets individually.

 

Hey this NCVQ needle is the same thing as the GYTR needle right? If you have the stock needle on the 4th clip what would the NCVQ needle be on? Do you have to move it down to make it closer to the stock?
Thanks for the help! Trinity

 

If your confused you might be better of paying someone to do it for you. They last thing you want to do is burn up a motor learning to jet.

 

The needle in the GYT-R kit is the NCVQ needle. Clip position is 2 positions higher compared to stock. i.e. Stock needle at clip 4 would equal NCVQ needle at clip 2.

 

no ik wut im doing with these things but all that u said wuz just a lil much for me [img]i/expressions/face-icon-small-confused.gif[/img]