Is my 400ex stock carb is too small ?

[Fatal]

Hi guys my 400ex is running with a full rs3 yoshimura exhaust system , a curtis sparks advancer, the second oversized piston 11;1 wiseco on the stock sleeve an hotcam stage 2 and my carb hav a 200 main jet and my spark plug look like white and light grey can help me plz someone told me to put a yfz450 carb on it is it true ?

[400eXr1d3rZ]

200 main is way too rich, get a 165 and 170, and whatever is inbetween, when the plug is white, that means you lean, but i think going too high on the main jet, like waaaaaaaaay super high, could possibly make the plug white.

Try this:

Needle on 3rd notch/clip
2.5 - 3 spins out air/fuel screw
165-170 main
42 pilot

A YFZ-450 carb won't fit, well I don't think it will, but a 450r carb will with some modding.

[2muchquad]

If you have the adaptor flange for the carb the fcr will fit,i have a fcr carb on my 385ex,mine is from a yzf mx bike.A fcr carb will really wake the bike up,its definately a significant gain.:D

[Fatal]

I forgot to say that i hav a k&n filter with the air box lid removed

[Iliketogofast]

White= Hot = lean. Stop riding it, now... Until you get a bigger carb, or try a bigger main or mess with the needle. The mainjet is not the only thing that needs adjusted - with that many mods you need to tinker with the needle jet.

If you can, cut the threads off of the plug so you can see the white part of the plug all the way down, and post a good hi-res picture for us to see it. Some folks can tell you exactly what you need to do - needle, main and pilot adjustments - by looking at the plug like that.

[GPracer2500]

The carb size and the main jet size aren't really related in the way that you're thinking. A carb of any given size will (should) have the ability to deliver an appropriate amount of fuel for as much air as it's able to flow. That's not to say that your EX wouldn't respond to a better carb; but main jet size isn't really a good indicator of when a different carb should be considered.

If you're looking at a new plug that hasn't been ridden around much (at a variety of throttle openings), then a white insulator tip means nothing. If you install a new plug, make a few WOT passes to test the main, pull the plug, look at the end of the insulater (about the only part you can see), and notice its bone white--you could be way to rich. As Iliketogofast was alluding to, the "mixture ring" at the BASE of the insulater (often only visible by cutting away the threads of the plug) is the part of the plug that's giving you good information about air/fuel ratios. Because of that, I'm not a fan of using plug readings. It's more complicated than most people realize and often leads folks down the wrong road. Evaluating how the bike is actually running with different mains will get most people closer to correct in less time than trying to read plugs. HERE'S a thread with a little more info about reading plugs. (http://www.exriders.com/vbb/showthread.php?s=&threadid=237664&perpage=15&pagenumber=2)

BTW, a 200 main sounds way to big.





As a side note: sometimes it's important to realize that fuel delivery through a main jet isn't solely a function of the size of the jet. The vacuum signals in the carb are the other half of the equation. For example, in some cases you might change a intake tube or something and find that a much larger or smaller main is needed. Some of that is because of a different volume of air being handled by the new intake. But equally influential is the change in velocity and differential air pressure in the carb. As the vacuum signal in the carb changes with different settups so does the quantity of fuel delivered by the jets even if the jets don't change in size.

Fuel is pushed up through the main jet because of ambient air pressure (the pressure found in the float bowl) being greater than the pressure in the carb (in the carb's venturi). As air velocity increases through the carb's venturi the air pressure drops. Seems backwards but more velocity = lower pressure. The pressure in the float bowl always stays the same: ambient air pressure. So, more air velocity = lower pressure in the carb's venturi = greater differential pressure between the venturi and the float bowl = more fuel pushed through the main jet (regardless of it's size).

This largely explains why in some cases as you do things like high compression pistons, porting, and other hop-ups that change the velocity of the intake air a smaller main jet is sometimes required vs. the old settup. The engine might very well need more fuel with the new settup. But it is getting more fuel even though the main jet is smaller. The vacuum signals in the carb are what's making up the difference in fuel delivery, not the size of the jet.

Got a little off the subject there, but I'm trying to relay that main jet size doesn't always tell us what we think it does about how much fuel an engine is getting.

[Fatal]

Yes the sparks plug is bone white ! How can i send you a picture of my plug ? Where can i find an hoster if i need it ??

[Iliketogofast]

Yeah, but how old is the plug? Also, a very important question... How does it run?

[jsmith2232]

I have the same thing with the plug on my 416 listed in my sig. I ran a 210 before and it was just starting to look a light tan. I'm back at a 198 now and it looks white but it still hasn't fowled any plugs yet. sounds crazy but that's what i'm gettin.:ermm:

[Fatal]

My spark plug was new and my quad was running rough when turning at high speed when i push gas in strait line he is perfect !