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Discussion Starter #1
I've been fighting an issue for a while and feel like I'm close to the answer but need some help.

My Stock 07 YFZ 450 (stock exhaust, removed airbox lid) runs perfect in all throttle ranges, but I can not get the bike to stop fluttering when riding on rough terrain. I've also discovered that I can mimic the fluttering issue by taking hard Right hand circles at relatively low speeds. The bike will start to bog and lose all power until it's going slow enough that the roughness of the terrain (or these hard right hand turns) don't affect the fuel flow in the carb any more.

I have found that decreasing the float level in the carb helps the issue a lot, but it never goes away. My last change was setting the float level at the lowest level possible. The yfz acted fine on a short trail ride and the issue was almost completely gone except for one obvious issue- At WOT throttle for more than 5 seconds the bowl would drain and the bike would die until it sat for a few seconds and the bowl had time to fill up again. So this can't be my permanent solution.

In searching the forum I found this in the Mother FCR Tuning Thread:
Mother FCR snip.jpg

Is there more to this plastic guard inside the bowl? Is there something else I can adjust in the Carb to help this issue? I plan on buying an new OEM plastic guard to see if it helps unless there are some other suggestions.

Here is what the inside of my bowl looks like:
Carb 1.jpg
Carb 3.jpg
Float Bowl.jpg
 

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Pull the petcock out of the tank. Make sure it is clean.
Replace the fuel line.
Check your gas cap breather. If you have that little antenna hose throw it out.
Clean the overflows/breathers on the carb.
Pull the float and needle and clean it with a q tip and some cleaning solution, blow air through the the opening both ways.

Sent from my SM-G970U using Tapatalk
 

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I had a similar problem with a banshee that would keep bogging down and losing power especially on incline. Make sure the needle and seat aren't worn out. If it is, it could be letting fuel past it, resulting in fuel overflowing into the carburetor body. That's what was happening in my case.

I also agree with Rob, clean the fuel tank and the fuel valve's screen. Never hurts to do this and it's pretty easy to do.

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Discussion Starter #4
It's been a busy week, not mention some big temperature swings here in Ky. Makes it hard to keep motivated in the shop.

I have a plastic guard for the jets in the bowl in the mail just in case. The yfz started leaking coolant after the last ride so I have the water pump seals on the way too. When those parts arrive I will dig into it again.

I don't have a reason to think the tank or petcock is dirty, but that should be easy enough to drain and check. The last time I tuned the bike I installed a new oem NCVQ needle from a yz450 so it should be in good shape. I hope that needle isn't part of the problem, it helped a lot when it came to sorting out the mid-range tune!

We'll see what further cleaning and inspection does
 

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I have the 2006 YFZ450 and the YZ450F. I had a similar problem with sputtering cutting out on my YZ. On flat terrain it would run great, over whoops or sharp bumps it would sputter. I could even bounce up and down sitting still and make it sputter. I changed all the wiring, stator, computer, cleaned the carb over and over. Paid a local shop to find the problem. It wasn’t until I did the All Balls mid body carb rebuild kit that fixed it. Coincidentally I was running a K&N filter prior (not anymore).
 

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Discussion Starter #6
Well it's been a while, but the pandemic gave me the time I needed to finally dig back into the yfz. As Jmakx suggested I bought a mid body carb rebuild kit and went through the carb again. I didn't honestly know that the center section of carb separated in half. It was nasty in that section and the O-rings were all flat. Aflter that rebuild kit the bike runs perfect. It's taken me months to track this issue down.

So hopefully someone else in the future finds this info useful,
 

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Dear c10vortec: glad you got your yfz running right. I know first hand that the mid-body cleaning and rebuild are critical and have recommended this step to others in other posts. A lot of folks (and even some shops) skip this and problems linger.
 
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