Need help with a 450 YFZ Problem.

[teamnutzracing]

Ok, here is the problem!!! I have a YFZ 450, I had the head port and polished, i am running the carb with a 52 pilot and a 190 main. My low end was good, and the top end was sputtering and popping! I have tried everything, that i know of, any help on what the problem could be or what i could try! Please get back to me ASAP!

[GPracer2500]

Does the condition improve if you remove the airbox lid? Try that. If it gets better than you are too rich on your main jet.

[teamnutzracing]

umm, we have to run it with the choke on, but we were told to go with a 200 main. We havent tried it yet. could it be anything else?

[GPracer2500]

It won't run without the choke on? If that's the case then something isn't right. If a 200 main is the correct jet with your setup (a 200 sounds awfully big!!) than I can't imagine running a 190 would require the use of the choke to get it to run. Operating with the choke on can produce all sorts of strange conditions.

What is your complete settup: intake/exhaust/etc.?

If you take the airbox lid off and it gets even worse than your too lean.

It really sounds like a jetting issue.

[teamnutzracing]

ok ill start from the begining i got my head ported and i am tryin to get it jetted i started from a 155 main and a 42 pilot and i had really lean and poppin on the low end and then we put up to a 190 and a 52 pilot and now we arent to lean but were not blowin black smoke yet and we have called a few ppl we no that nows bout carberators and they are all tellin us to go to a 200 main and i think that is big but whatever it taked i guess lol o and i got a uni air filter and a full carbon pro cut in half.

[GPracer2500]

I think your too rich. If you're running your airbox lid I wouldn't think a 200 or even a 190 would be right. Take your airbox lid off and I bet the condition improves.

Jetting is mostly a trial and error process. One you get really good at detecting lean/rich running conditions than it goes faster because your guesses are educated rather than blind.

But blind guesses DO work. Just make one change at a time and move in small steps. A 155 to a 190 is a big step! Make a change and if it runs better than go a little further in that direction until it starts running worse, then back off a bit.

Adding or removing an airbox lid is a great jetting diagnostic tool. It's basically the same as rejetting--you're just changing the amount of air instead of changing the amount of fuel. But it's really fast and easy!

Changing the mains on a YFZ is pretty easy too. Get a range of jets and start experimenting.

[teamnutzracing]

no, we worked our way all the way up to a 190 from the 155. we worked on it all night last night and then started back in this mornign, and at 190 it was good, now it is rinning like **** again. we tried a whole bunch.

[GPracer2500]

Oh, I see. I'm not sure then. :confused:

[teamnutzracing]

yeah lol, no one is haha.. but thats cool, thanks alot, ill try the lid thing, see if it helps.

[GPracer2500]

There is a throttle position sensor on those carbs. I believe they are adjusted by indexing the sensor in it's mount on the carb. I think you measure the resistance of the sensor with a multimeter to get it within specifications. Something like that--really I don't know much about them.

Maybe it got bumped out of wack or something?? I don't think a mal-adjusted TPSs causes dramatic running problems though. Just a though...

[teamnutzracing]

yeah, idk how the hell to even do that lol..id break somthing lol...so yeah i dont think it is that. but it could be, thanks for the idea.