Sign in to follow this  
Followers 0
yz450girl

yz450f sputter at low throttle

46 posts in this topic

i recently got my hands on an 2008 yz450f and when i first rode it i noticed that it sputtered a bit at low throttle but once you got on the gas a bit it leveled out. I figured it was due to the cold. (its like 40 degrees) I talked to the previous owner and he said it did it as well at other times of the year. Could it be that it needs to be properly jetted. He had an after market slip on put on it and jetted but the guy said something about the jet might have been a bit small. I have noticed that if you are on the gas and let off at times it will backfire. This is all greek to me as I am used to fuel injected bikes. Any help or suggestions would be appreciated

Share this post


Link to post
Share on other sites

could be a needle issue. Raise the clip of the needle a position and see if the sputtering declines.

I remember when I had my JD jetting kit on my old bike I would have terrible low-mid throttle sputtering until I played around with the clip.

Share this post


Link to post
Share on other sites

Install A 48 pilot jet. Raising the needle one clip richer along with larger pilot jet is what all of us in Northern Cali. do and they run perfect.

Share this post


Link to post
Share on other sites

For most situations, the '08 was pretty close to jetted right as delivered, although the needle sometimes needed to actually be change to be correct. In the event that you can't find something in the jetting sticky, or Gunner's suggestions don't do it for you, try disconnecting the TPS. If that clears the problem up, it may need adjustment, or it may be faulty.

Share this post


Link to post
Share on other sites

Thats the pilot jet. You can access it thru the 17mm hex plug at the bottom of the carb. The pilot jet is the small one higher inside the carb. Take it out, see what number is stamped on it. Add 5 to it. And get a pilot jet that size. Will fix the low throttle hesitation and the popping. I am running a 55 in my 450, with 185 main. Modified Stock exhaust

Share this post


Link to post
Share on other sites
Thats the pilot jet. ... I am running a 55 in my 450, with 185 main.
You're running a WR carb. Those numbers would be grossly rich in a YZ450.

Share this post


Link to post
Share on other sites

thanks for all the input guys. would that also be why its uber difficult to start when cold or are these bikes just cold natured? right now its 40 degrees here in kentucky. i was chalking it up to the sputter and weather for why its hard to start. I generally have to roll start it.

Share this post


Link to post
Share on other sites

The later 450's tend to be "cold natured" because they are set up to start well when hot. In building a bike in mass production, you can't make it perfectly set up for all situations, so you have to pick out a mid-point average that will work for most of your market most of the time. Yamaha set the YZ450 up for around 1000 feet in summertime weather.

To start the bike in cold weather, the accelerator pump is your best friend. Turn on the gas, pull the choke, twist the throttle open 2-3 times, and push the engine at least a half revolution up to the next compression stroke. If it doesn't go that far before you hit the "hard spot", push it through to the next compression stroke. Then twist the throttle twice more and kick. If it fails to start, prime it yet again and push up to compression, then kick. If you ride fairly often (every couple of weeks or so), this should get you running in under 4 kicks pretty reliably.

If you plan to ride a lot in cold weather this year, something that will help is a larger starter jet. See the manual for details on its location.

Share this post


Link to post
Share on other sites

I am going to have it lowered and the jetting checked this weekend. thanks everyone

any suggestions on a good jet kit? only mod to bike is a white brothers slip on.

Edited by yz450girl

Share this post


Link to post
Share on other sites
The later 450's tend to be "cold natured" because

To start the bike in cold weather, the accelerator pump is your best friend. Turn on the gas, pull the choke, twist the throttle open 2-3 times, and push the engine at least a half revolution up to the next compression stroke. If it doesn't go that far before you hit the "hard spot", push it through to the next compression stroke. Then twist the throttle twice more and kick. If it fails to start, prime it yet again and push up to compression, then kick. If you ride fairly often (every couple of weeks or so), this should get you running in under 4 kicks pretty reliably.

I tried your suggestion today. It was cold and snowing outside. Bike started on like the second kick. WOOOT!!! let it run a bit. shut her down and she fired up again on about the third kick. My lil next door neighbor wanted to give it a shot. He has never kick started a bike before. First kick he wasnt ready and it threw him almost off the bike. LOL. i then showed him the compression release and it fired up. SO thanks...!!!!

Share this post


Link to post
Share on other sites
The later 450's tend to be "cold natured" because

To start the bike in cold weather, the accelerator pump is your best friend. Turn on the gas, pull the choke, twist the throttle open 2-3 times, and push the engine at least a half revolution up to the next compression stroke. If it doesn't go that far before you hit the "hard spot", push it through to the next compression stroke. Then twist the throttle twice more and kick. If it fails to start, prime it yet again and push up to compression, then kick. If you ride fairly often (every couple of weeks or so), this should get you running in under 4 kicks pretty reliably.

I tried your suggestion today. It was cold and snowing outside. Bike started on like the second kick. WOOOT!!! let it run a bit. shut her down and she fired up again on about the third kick. My lil next door neighbor wanted to give it a shot. He has never kick started a bike before. First kick he wasnt ready and it threw him almost off the bike. LOL. i then showed him the compression release and it fired up. SO thanks...!!!!

Compression release????? Only other lever on a YZ450F is the hot start.

Share this post


Link to post
Share on other sites
thats what i thought too but the guy swore thats what it was. LOL.

That's a hot start, not a compression release.

As for jet kits, a lot of people run the JD Jetting kits and are quite happy with them.

Share this post


Link to post
Share on other sites

i was looking at those myself but not had any experience with jetting. Last bike was fuel injected.

Share this post


Link to post
Share on other sites
You're running a WR carb. Those numbers would be grossly rich in a YZ450.

YZ450s run a differnt carb? I was not aware.

Share this post


Link to post
Share on other sites

The pilot circuit in particular is very different because the WR has the air cut valve built in.

Share this post


Link to post
Share on other sites

The ACV is a device designed to cut off air flow through the air injection system to prevent monster decel backfires that otherwise would occur because of air in the exhaust on decel. The carb has a diaphragm built in that modifies the air flow to the pilot circuit on deceleration to help reduce the fuel passed through during decel.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now
Sign in to follow this  
Followers 0