popping on decel?

[atrdrvr7]

Just picked up a new to me 08' yz450f. No matter what i do with fuel screw i still have popping on decel. I read through the jetting thread but still have a few questions. I was wondering if the stock exhaust could be the cause of this? Seems very choked up with the stock exhaust. Could it be causing this? Also thinking of going to a r&d power pump to avoid seasonal jetting changes. Anyone using one a 450? I used one on my 250f and it made a world of difference.

[BDubb106]

Ive never been on or around a Yz that didnt pop at least a little on decel. IDK why but they are much worse then any other 450 Ive ridden. Check jetting to make sure your not lean (specifically the pilot) and ride it. I have a full White Bros. system on mine and it pops even worse now then with the stock system if I go from WO to closed.

[grayracer513]

The R&D Power Pump won't help avoid seasonal jetting changes because the only thing it offers is adjustability of the leak jet, which once established, really never needs to be adjusted, and has no effect on anything unless the accelerator pump is active at a given moment (the throttle is opening). Waste of money, IMO. The pilot and main are the two that need the most tinkering when the weather changes.

The two main causes of decel popping are a lean idle circuit and an air leak in the exhaust system. If you have an air leak, you can chase jetting until the bike is so rich that it fouls plugs like a two stroke from 1958 and still not get rid of it because jetting didn't cause it.

If the pipe isn't leaking, then you may need to bump the pilot one size:

https://www.thumpertalk.com/forum/showthread.php?t=327405

Bear in mind that a healthy YZ450 with perfect jetting WILL pop through the exhaust a little bit some of the time.

[rhinoracin]

What is a good needle to use in the 2009 carb? Mine is all stock and ran lean when I bought it, so I cleaned the carb and that didn't help. Then I put in bigger pilot and main jets and raised the needle 1 clip and it's still lean up to 3/4 throttle. The fuel screw is about 1.5 turns out. I think I should just get a richer needle instead of having to raise it a bunch. I ride at sea level 70-85 deg, low moisture.

The bike can also be hard to start when hot and it seems to use up gas too quick. This is my first mx 450 so maybe I need to adjust to it, but our 07 wr450 and all other bikes ive owned never ran like this. Any ideas? Is something wrong with the carb? There are no exhaust leaks.

[BDubb106]

Positive there are no leaks in the intake system? Check to make sure you decompression system is working correctly, that will cause the bike to run lean and drink fuel. I had a similar issue with my last 450.

[YZF_MXracer71]

Just picked up a new to me 08' yz450f. No matter what i do with fuel screw i still have popping on decel. I read through the jetting thread but still have a few questions. I was wondering if the stock exhaust could be the cause of this? Seems very choked up with the stock exhaust. Could it be causing this? Also thinking of going to a r&d power pump to avoid seasonal jetting changes. Anyone using one a 450? I used one on my 250f and it made a world of difference.

I also have an 08 yzf 450. i had the same problem. there was a local at my home town track that is really good with carbs. he told me to replace the seal on the plate thats on the slide in your carb. here is a link. http://www.yamaha-motor.com/partviewer/default.aspx?ls=sport#/Yamaha/YZ450F_-_YZ450FXL_-_2008/CARBURETOR/YZ450FXL_(2008_MOTORCYCLE)/CARBURETOR_(YZ450FXL_-_2008) you want number 11. that should take care of your popping on decell. also your very right about the stock exhaust. an after market will turn it into a whole diffrent bike. with some jetting possibly. also on your throttle cable side of the carb if you take the plate off and turn your throttle fast you will see a linkage that runs your fuel pump and its slow to move compared to your throttle. take the carb off and use saftey wire to tie them togather. your can also buy a new link arm made my rapid but its around $100. i have wired all my FCR carbs and friends to with no problems EVER. that wont create power but will aid in much better throttle response. good luck

[YZF_MXracer71]

IM SORRY THE LINK DOES NOT WORK BUT PICK YOU YR AND MODLE AND GOT TO THE CARB PARTS BREAK DOWN AND #11 IS THE SEAL YOU WANT TO REPLACE

[rhinoracin]

Positive there are no leaks in the intake system? Check to make sure you decompression system is working correctly, that will cause the bike to run lean and drink fuel. I had a similar issue with my last 450.

What should i check in the decompression system?

[BDubb106]

Do you have a manual? If you do there is a step by step guide on how to check it. If not let me know and I will see if I can cut and paste and Ill email it to you.

[grayracer513]

Check to make sure you decompression system is working correctly, that will cause the bike to run lean and drink fuel.

Not true.

[BDubb106]

Why is that not true? Not trying to debate, I just always thought that was how it worked, at least thats the way I interpreted what I read. That the decomp lever leaned out the mixture.

[grayracer513]

You are confused. There is no "decomp lever" on any YZ450. All YZ450's, WR450. and YZ250F's from '03 on have automatic decompression systems built into the exhaust cam. The small lever on the left lever perch is for the hot start valve in the carb, which does lean out the mixture.

So, in a way, you are half right, because the hot start plunger can cause the bike to run lean if for any reason it fails to completely close when the lever is released.

[BDubb106]

You are confused. There is no "decomp lever" on any YZ450. All YZ450's, WR450. and YZ250F's from '03 on have automatic decompression systems built into the exhaust cam. The small lever on the left lever perch is for the hot start valve in the carb, which does lean out the mixture.

So, in a way, you are half right, because the hot start plunger can cause the bike to run lean if for any reason it fails to completely close when the lever is released.

Makes perfect sense... thank you for correcting my mistake, and sorry to the OP for any misinformation. But yes I was referring to the hot start.